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Automated
Surface Observing
System
(ASOS)
User’s Guide

National Oceanic and Atmospheric Administration
Department of Defense
Federal Aviation Administration
United States Navy

March 1998

Foreward
The 1990s have witnessed a carefully planned and executed modernization of the nation’s
weather services. The Automated Surface Observing System (ASOS) is the first system to be
operationally deployed as part of theis modernization. ASOS is therefore in the forefront of system
deployments and associated service improvements that will require most of this decade to
complete. In this sense, ASOS is the harbinger of 21st century weather services.
In the end state, ASOS will be operational at about 1,000 airports across the United States.
This system is the primary surface weather observing system in the United States, which supports
the essential aviation observation programs of the National Weather Service (NWS), the Federal
Aviation Administration (FAA), and the Department of Defense (DOD).
The implementation of ASOS brings with it many opportunities and challenges. The
opportunities include the unprecedented availability of timely, continuous and objective
observations from many more locations. The challenges generally related to institutional learning
needed to fully understand and adjust operation to take the greatest advantage of this new
technological resource. The potential applications of the ASOS data go beyond that of providing
basic weather information for aviation and forecasting; ASOS also will provide enhanced support to
vital national programs such as public safety, hydrology, climatology, agriculture, and
environmental protection, just to name a few. The ASOS User’s Guide is intended as basic
reference and introduction to ASOS for a broad range of users.
As of this writing (March 1998), there are about 500 commissioned ASOS’s nationwide. An
additional 500 are coming on-line in the next few years. This deployment fulfills the commitment
of the Government made over a decade ago to provide the nation a highly cost-effective, capable
and reliable automated weather observing system for safe, efficient aviation operations and other
applications. This achievement is made possible by the dedicated effort of many people throughout
the government and private industry working together as a team to conceive, plan, develop, test
and evaluate, implement, commission, monitor, maintain and operate a system.
This ASOS User’s Guide is gratefully dedicated to all who have worked so hard to make
ASOS a reality. Special thanks are extended to Dr. Jim Bradley for mentoring this program from
the very beginning. Finally I wish to thank Dave Mannarano for coordinating the writing and
production of this ASOS User’s Guide.

Vickie L. Nadolski
ASOS Program Manager

Executive Summary
Since the last Automated Surface Observing System (ASOS) User’s Guide was published
in June 1992, numerous changes have occurred. These changes have, to the maximum practical
extent, been incorporated into this updated version of the ASOS User’s Guide. These changes
include the transition of observing code format from the Surface Aviation Observation (SAO)
code to the Aviation Routine Weather Report (METAR) code in 1996; the implementation of new
software loads into ASOS up to and including software Version 2.6; the incorporation of various
sensor enhancements and improvements, including modification to the Heated Tipping Bucket
precipitation accumulation gauge, the hygrothermometer, and anemometer; and incorporation of
the Freezing Rain and Lightning Sensors into the ASOS sensor suite. Additional product improvement efforts are underway to further expand and improve the capabilities of ASOS. These efforts
are also described in the ASOS User’s Guide.
As of this writing (March 1998), there are about 500 commissioned ASOS’s nationwide. An
additional 400 + are coming on-line in the next few years. This deployment fulfills the commitment
the Government made over a decade ago to provide the nation a highly cost-effective, capable
and reliable automated weather observing system for safe, efficient aviation operations and other
applications. This achievement is made possible by the dedicated efforts of many people throughout the government and private industry working together as a team to conceive, plan, develop,
test and evaluate, implement, commission, monitor, maintain and operate the system.
This ASOS User’s Guide is gratefully dedicated to all who have worked so hard to make
ASOS a reality. Special thanks are extended to Dr. Jim Bradley for mentoring this program from
the very beginning. Finally I wish to thank Dave Mannarano for coordinating the writing and
production of this ASOS User’s Guide.

Vickie L. Nadolski
ASOS Program Manager

Table of Contents
CHAPTER ONE ..................................................................................................................................1
1.0 Introduction ................................................................................................................................ 1
1.1 Purpose and Scope .................................................................................................................... 1
1.2 Background ................................................................................................................................ 1
1.3 Total Surface Observation Concept........................................................................................... 2
1.4 Quality Control ........................................................................................................................... 2
1.5 General Conventions .................................................................................................................. 3

CHAPTER TWO .................................................................................................................................5
2.0 System Description .................................................................................................................... 5
2.1 System Components .................................................................................................................. 5
2.1.1 ASOS Sensor Groups ..................................................................................................... 5
2.1.2 Acquisition Control Unit .................................................................................................. 5
2.1.3 Operator Interface Device.............................................................................................. 9
2.2 ASOS Data Outlets ................................................................................................................... 9
2.3 ASOS Data Types ..................................................................................................................... 9
2.4 METAR Elements ..................................................................................................................... 9
2.5 Automated METAR vs. Manual METAR .............................................................................. 10

CHAPTER THREE .......................................................................................................................... 11
3.0 Automating the Objective Weather Elements.......................................................................... 11
3.1 Ambient and Dew Point Temperature ..................................................................................... 11
3.1.1 Ambient/Dew Point Temperature Sensor ..................................................................... 11
3.1.2 Ambient Temperature/Dew Point Temperature Algorithm .......................................... 12
3.1.3 Ambient Temperature/Dew Point Temperature Strengths and Limitations .................. 13
3.2 Wind ......................................................................................................................................... 14
3.2.1 Wind Sensor .................................................................................................................. 15
3.2.2 Wind Algorithm ............................................................................................................. 15
3.2.2.1 Wind Direction and Speed ................................................................................ 15
3.2.2.2 Wind Character ................................................................................................. 16
3.2.2.2a Gusts ................................................................................................. 16
3.2.2.2b Variable Wind ................................................................................... 16
3.2.2.2c Squalls............................................................................................... 17
3.2.2.3 Wind Remarks .................................................................................................. 17
3.2.2.3a Wind Shift ......................................................................................... 17
3.2.2.3b Peak Wind ........................................................................................ 18
3.2.3 Wind Strengths and Limitations ..................................................................................... 18
3.3 Pressure ................................................................................................................................... 18
3.3.1 Pressure Sensor ............................................................................................................ 18
3.3.2 Pressure Algorithm ....................................................................................................... 18
3.3.3 Pressure Strengths and Limitations ............................................................................... 19
3.4 Precipitation Accumulation ...................................................................................................... 19

Table of Contents
3.4.1 Heated Tipping Bucket Precipitation Gauge ................................................................. 20
3.4.2 Precipitation Accumulation Algorithm .......................................................................... 20
3.4.3 Precipitation Accumulation Strengths and Limitations .................................................. 22

CHAPTER FOUR ............................................................................................................................ 23
4.0 Automating the Subjective Weather Elements ........................................................................ 23
4.1 Automating Sky Condition ....................................................................................................... 23
4.1.1 Cloud Height Indicator Sensor ...................................................................................... 23
4.1.2 Sky Condition Algorithm ............................................................................................... 24
4.1.2a
Meteorological Discontinuity Sensors ............................................................... 25
4.1.2b
Back-Up Sensors .............................................................................................. 26
4.1.3 Sky Condition Strengths and Limitations ....................................................................... 26
4.2 Automating Surface Visibility .................................................................................................. 27
4.2.1 Principles in Visibility Automation ................................................................................. 28
4.2.2 Forward Scatter Sensor ................................................................................................ 29
4.2.3 Visibility Algorithm ........................................................................................................ 29
4.2.3a
Meteorological Discontinuity Visibility Sensor .................................................. 30
4.2.3b
Back-Up Visibility Sensor ................................................................................ 31
4.2.4 Visibility Strengths and Limitations ................................................................................ 31
4.3 Automating Present Weather and Obscurations ..................................................................... 32
4.3.1 Single Site Lightning Sensor .......................................................................................... 32
4.3.2 Single Site Lightning Sensor Algorithm ......................................................................... 32
4.3.3 Automated Lightning Detection and Reporting System (ALDARS) ............................ 33
4.3.4 ASOS Lightning Sensor Strengths and Limitations ....................................................... 33
4.3.5 Precipitation Identification Sensor ................................................................................. 34
4.3.6 PI Algorithm .................................................................................................................. 35
4.3.7 PI Strengths and Limitations ......................................................................................... 36
4.3.8 Freezing Rain Sensor .................................................................................................... 36
4.3.9 FZRA Algorithm ........................................................................................................... 37
4.3.10 FZRA Strengths and Limitations................................................................................... 38
4.3.11 Obscuration Algorithm .................................................................................................. 38
4.3.12 Obscuration Algorithm Strengths and Limitations ......................................................... 39

CHAPTER FIVE .............................................................................................................................. 41
5.0 Explanations/Examples of ASOS Observations ...................................................................... 41
5.1 5-Second Wind Updates .......................................................................................................... 41
5.2 One-Minute Observations ........................................................................................................ 41
5.3 ASOS Aviation Routine Weather Reports ............................................................................... 41
5.3.1 Backup and Augmentation ............................................................................................ 42
5.3.2 Missing vs. Non-Event Data ......................................................................................... 42
5.3.3 Estimated Data.............................................................................................................. 43
5.3.4 Examples of METAR/SPECI Reports .......................................................................... 43

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Table of Contents
5.4 Auxiliary Data .......................................................................................................................... 46
5.5 Standard Hydrometeorological Exchange Format (SHEF) Messages .................................... 46
5.5.1 SHEF 15-Minute Precipitation Criteria Message ......................................................... 48
5.5.2 SHEF Hourly Routine Precipitation Message ............................................................... 48
5.6 Daily and Monthly Summaries ................................................................................................. 48
5.6.1 Daily Summary Message .............................................................................................. 48
5.6.2 Monthly Summary Message ......................................................................................... 50
5.7 ASOS High-Resolution Sensor Data ....................................................................................... 50

CHAPTER SIX ................................................................................................................................. 53
6.0 ASOS Data Outlets ................................................................................................................. 53
6.1 Interactive Video Screen Displays .......................................................................................... 53
6.2 Non-Interactive Screen Displays ............................................................................................. 53
6.3 ASOS Printer ........................................................................................................................... 53
6.4 Long-Line Dissemination ......................................................................................................... 54
6.5 Computer-Generated Voice Messages .................................................................................... 54

CHAPTER SEVEN .......................................................................................................................... 59
7.0 Introduction .............................................................................................................................. 59
7.1 Data Not Provided by ASOS .................................................................................................. 59
7.2 Planned Product Improvement ................................................................................................ 59
7.2.1 Ice-Free Wind Sensor ................................................................................................... 60
7.2.2 Dew Point Sensor ......................................................................................................... 60
7.2.3 Ceilometer/Sky Condition .............................................................................................. 60
7.2.4 All-Weather Precipitation Accumulation Gauge ........................................................... 60
7.2.5 Enhanced Precipitation Identification ............................................................................ 61
7.2.6 Sunshine ........................................................................................................................ 61
7.2.7 Schedule ........................................................................................................................ 61
7.3 Summary .................................................................................................................................. 61

Appendices
Appendix A: ASOS Video Screens ................................................................................................ A1-11
Appendix B: ASOS Initial Operation Capabilities ............................................................................. B1-2
Appendix C: Content of ASOS-Generated METAR: A Quick Reference Guide ............................ C1-2
Appendix D: ASOS Voice Vocabulary ................................................................................................ D1
Appendix E: Acronyms ........................................................................................................................ E1

Table of Contents
Figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.

ASOS Quality Control Concept .......................................................................................... 4
ASOS Combined Sensor Group .......................................................................................... 6
Additional Sensor Group ..................................................................................................... 7
Availability of ASOS Data .................................................................................................. 8
ASOS Hygrothermometer ................................................................................................ 11
ASOS Anemometer .......................................................................................................... 14
ASOS Pressure Sensor ..................................................................................................... 19
ASOS Heated Tipping Bucket .......................................................................................... 20
Laser Beam Ceilometer .................................................................................................... 23
Example Of Cloud “Hit” At 4,500 Feet ............................................................................ 24
Example Of “Unknown Hit” ............................................................................................. 24
Example of Packing Effect ............................................................................................... 27
Forward Scatter Visibility Sensor ...................................................................................... 28
Visibility Sensor—Top View ............................................................................................. 29
Lightning Sensor ................................................................................................................ 32
The Precipitation Identification Sensor ............................................................................. 34
Freezing Rain Sensor ........................................................................................................ 37
ASOS Network Data Flow—Initial Configuration (1997) ................................................ 55
ASOS Network Data Flow––Final Configuration ............................................................ 56

Tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Table 9.
Table 10.

Temperature Sensor—Range, Accuracy Resolution ........................................................ 12
Wind Sensor—Range, Accuracy, Resolution .................................................................... 15
Cover ASOS Cloud Amount Report—Percent of Sky ..................................................... 25
Criteria for Reporting a Meteorological Discountinuity Ceiling Remark ........................... 26
Visibility Sensor—Accuracy in Statute Miles ................................................................... 30
Present Weather Reporting Hierarchy .............................................................................. 38
ASOS Report of Freezing/Frozen Phenomena ................................................................. 38
ASOS Report of Present Weather .................................................................................... 39
ASOS High-Resolution Data ............................................................................................ 51
Weather Elements Not Reported by ASOS ...................................................................... 59

CHAPTER ONE
1.0

Introduction

This User’s Guide provides information applicable to
all ASOS units, whether they are sponsored by the NWS,
FAA, or DOD. The Guide covers all essential aspects of
system operation, including data acquisition, processing,
and report formatting and dissemination. This Guide is by
no means exhaustive and additional information is
referenced.

Since the first aircraft took flight, weather observations have been essential for safe aviation operations. Over
the years, the National Airspace System (NAS) has expanded to thousands of flights and millions of passenger
and cargo miles a day. Paralleling this expansion has been
the demand for increased weather observations. In recent
years the National Weather Service (NWS), the Federal
Aviation Administration (FAA), and the Department of
Defense (DOD) have collectively expended over 1,000
staff-years annually to create and disseminate manual
weather observations. This expanding demand on human
resources spurred the development of automated sensors and
reporting systems to provide surface weather observations.

The ASOS User’s Guide is organized into four topical
areas. Each topical area includes one or more chapters.
The information within these topical areas flows from a
general introduction to specific core detail and finally to a
conclusion or summary.
The first topical area provides a general overview
(Chapters 1-2). The second topical area discusses the automation of the objective weather elements (Chapter 3) and
automation of the subjective weather elements (Chapter
4). These chapters describe the sensor operation principles,
the algorithms, and the strengths and limitations of ASOS
in reporting each element. The next topical area provides
specific examples and explanations of ASOS output (Chapters 5-6). Finally Chapter 7 provides a look to the future
and a conclusion. Appendices provide additional detail.
This includes examples of ASOS video screens, performance specifications, and a quick reference guide to the
content of the ASOS generated METAR/SPECI reports.

With the advent of new reliable and sophisticated sensors and computer technology in the 1970s, it became increasingly practical to automate many observing functions.
This potential came to fruition with development and testing of the Automated Surface Observing System (ASOS)
in the 1980s, and its deployment and implementation in
the 1990s. By the turn of the century, over 900 airports
will have ASOS.
The primary function of the ASOS is to provide
minute-by-minute observations and generate the basic
Aviation Routine Weather Report (METAR) and Aviation
Selected Special Weather (SPECI) report. This information is essential for safe and efficient aviation operations,
and is used by the public to plan day-to-day activities.
ASOS also provides valuable information for the
hydrometeorologic, climatologic, and meteorologic research communities.

1.1

1.2

Background

The earliest fielded automated systems, the Automated
Meteorological Observing System (AMOS) and the Remote
Automated Meteorological Observing System (RAMOS),
were deployed in the 1960s and 1970s. These systems reported only the objective elements of ambient and dew point
temperature, wind (speed and direction), and pressure.

Purpose and Scope

The more complex, spatially observed elements of sky
condition and visibility had to await advances in sensor
technology and improvements in computer processing.
These goals were initially achieved by developing and fielding the Automated Observation (AUTOB) in the early
1970s. It was only when these more subjective elements
could be automated that the opportunity arose to develop a
fully automated observation system. The first such system
was developed during the joint FAA-NWS Aviation Automated Weather Observing System (AV-AWOS) experi-

The purpose of the ASOS User’s Guide is to familiarize users with the unique characteristics of ASOS data, how
to interpret it, and optimize its use. The primary audience
for the ASOS User’s Guide includes pilots, air traffic controllers, meteorologists, hydrologists, climatologists, and
other users of surface weather observations. It is assumed
that readers are familiar with the current Federal Meteorological Handbook Number 1 (FMH-1), which describes
the observing practices, coding, and reporting standards
for surface weather observations.

ments performed between 1973 and 1978. In the 1980s,
further advances in technology finally permitted the NWS,
FAA, and DOD to develop a practical, fully automated
observing system for nationwide use.

There are three cascading levels of quality control for
ASOS. Each level focuses on a different temporal and
spatial scale.
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1.3

Total Surface Observation
Concept

ASOS provides the basic surface weather observation
at many airports. At selected airports, an observer may add
information to the ASOS observation. These additions are
considered important for safe and efficient operations in
the airport/terminal area and provide backup observations
for those elements that ASOS normally reports.

Level 1—At the Site
Built into each ASOS are automated self-diagnostics
and QC algorithms. These QC algorithms operate on raw
sensor data; they prevent questionable data from being included in the One-Minute-Observation (OMO) or the transmitted METAR/SPECI. When the ASOS detects either a
qualifying degree of system degradation, component failure, or data error, the relevant data are excluded from report processing and a Maintenance Check Indicator ($) is
appended to the METAR/SPECI report. If sufficient raw
data are not available for report processing, the element is
not included in the OMO or METAR/SPECI report. The $
is used to indicate that maintenance may be needed but
does not necessarily mean that the data are erroneous. Onsite observers may also augment and/or backup the ASOS
METAR/SPECI data at selected locations. These observers provide an immediate data check and often catch problems before the observation is transmitted.

Although ASOS is the primary source of surface observational data in the United States, other surface observing networks, distinct from ASOS, provide supplementary
data for forecasting and other specific interests. These networks include severe weather spotter networks, hydrological reporting networks, synoptic and climatological
observing networks, and cooperative observing networks.
Data from these sources are not derived from ASOS and
are not provided as part of the ASOS observation. This
information is included in separate data sets or products.
Examples of this information are severe weather reports
(tornado/funnel cloud sightings, etc.), snow depth, water
equivalent of snow on the ground, and middle and high
cloud information (height, amount, and type).
In the modernized weather service, new and improved
technologies such as satellite, Doppler Weather Surveillance Radar (WSR-88D), and lightning detection networks
provide valuable weather information separate from data
reported by the ASOS. By integrating these data, meteorologists can now obtain a more accurate and complete
depiction of the weather than what can be obtained only
from a single source.

1.4

Level 1 is performed on-site, in real-time before an
observations is transmitted.
Level 2 is performed at a Weather Forecast Office
(WFO) for a designated area, about the size of a state,
usually within 2 hours after the scheduled observation transmission time.
Level 3 is performed centrally on all ASOS METARs
nationwide, usually about 2 hours after the scheduled
transmission time.

Level 2—Area
WFO personnel routinely monitor and assess the availability and meteorological quality of long-line transmitted
METARs and SPECIs from all ASOS locations in their
county warning area (CWA). This quality control usually
occurs within 1- 2 hours after the data are transmitted. If
data are suspect, the WFO staff investigates the problem,
informs points of contact (either the on-site observer, maintenance technician, or associated FAA Flight Service Station) and coordinates corrective action to ensure that a
quality observation is provided, or suspends the questionable observation pending resolution of a problem. The goal
at the area level is to correct a problem or prevent any additional erroneous data from being transmitted. When a
system problem is detected, a maintenance technician is
notified. Maintenance technicians are based at selected
NWS offices and service all NWS and FAA ASOS’s within
their area of responsibility. The Navy and Air Force perform maintenance on their own ASOSs.

Quality Control

The automation of surface observations reduces or
eliminates direct human involvement in acquiring
(sensing, collecting), processing (assessing, formatting,
documenting), and disseminating (transmission, display,
broadcast) surface observations. Even though the ASOS is
highly automated and reliable, effective Quality Control
(QC) of ASOS products is critical for ensuring the high
level of trust in the automated output (Figure 1).

Level 3—National

Those METAR elements that differ from the corresponding OI value by more than the allowable criteria are
flagged as suspect and included in the NCEP CQ message.
Other automated QC monitoring programs are operational
at the local WFO and alert the staff when data elements or
whole observations are missing, usually within 1 hour.

National QC operations are centered at the ASOS Operations and Monitoring Center (AOMC). It is staffed 24hours a day and provides an 800 phone-in number for
trouble calls. AOMC technicians routinely monitor the longline transmitted ASOS hourly METAR. To help with this
monitoring, two types of automated messages are routinely
provided to the AOMC. These messages identify: (1) those
METAR/SPECI reports with a $ appended and (2) those
METAR not received within the standard Time-Of-Transmission window. These messages are provided by the NWS
Telecommunication Gateway (NWSTG), which is collocated with the AOMC in the NWS Systems Operations
Center (SOC).

The National Climatic Data Center (NCDC) performs
additional quality control prior to archive.

1.5

The time convention used in this document to
describe the valid times and schedules used by ASOS is:

When a problem is encountered, AOMC technicians
open a trouble report, alert the responsible ASOS Electronic Technician and/or NWS office to take appropriate
action, and monitor progress toward resolution. The AOMC
may also coordinate with the appropriate FAA Weather
Message Switching Center Replacement (WMSCR) facility to resolve FAA National Airspace Data Interchange Network (NADIN) communications problems that affect
long-line transmission of ASOS reports. AOMC QC action is usually initiated after hourly METAR data are missing for 2 hours.

DD:HH:MM:SS
Where:
DD = Date; 01< DD< 31
HH = Hour (24-hour clock); 00 < HH < 23
MM = Minute; 00 < MM < 59
SS = Second; 00 < SS < 59

The AOMC performs other vital functions, such as
downloading critical operational information to the ASOS,
keeping accurate clock synchronization, and maintaining
data for system reinitialization, such as field elevation,
magnetic declination, phone numbers, etc.

H+MM = Minutes past the current hour.
M+SS = Seconds past the current minute.
For example, a time period from 56 minutes past the
previous hour to 50 minutes past the current hour would be
referenced as: “-H+56 to H+50.”

The goal at the national level is to maintain uniform
system integrity and prevent problems in data from continuing for extended periods of time.

The times specified in this document are in either Local Standard Time (LST) or Universal Coordinated Time
(UTC), alternately referred to as Zulu Time (Z).

Automated QC messages are generated at the National
Center For Environmental Prediction (NCEP) hourly and
made available to WFOs. These messages identify horizontally inconsistent and possibly unrepresentative observations through comparison of selected elements in the
hourly METAR with a corresponding computer-generated
Optimal Interpolation (OI) analysis field. The evaluated
elements are:
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General Conventions

In the descriptions of the algorithms, if less than 75
percent of the maximum amount of data used in the computation of any parameter is available, the parameter is not
reported. Unless otherwise noted, all midpoint fractional
values are rounded down to the nearest appropriate value.
All other values are rounded in accordance with normal
rounding procedures.

Wind direction and speed
Potential temperature (used as a surrogate for ambient temperature)
Dewpoint temperature
Sea-level pressure.

Figure 1.

ASOS Quality Control Concept

CHAPTER TWO
2.0

System Description

Sensors are sited in accordance with guidance stated
in the Federal Standards for Siting Meteorological Sensors
at Airports (FCM-S4-1987), published by the Office of the
Federal Coordinator for Meteorological Services and Supporting Research (OFCM). At virtually all locations, the
pressure sensors are located indoors within the ACU.

The ASOS performs all the basic observing functions,
including the acquisition, processing, distribution, transmission, and documentation of data.

2.1

System Components

The field complement of ASOS sensors are typically
located near the Touchdown Zone (TDZ) of the primary
designated instrument runway. If the TDZ site was found
unacceptable, the Center Field (CF) location is the most
likely alternate location. The field sensor array is referred
to as the “ASOS Combined Sensor Group.” At larger airports or airports where the operational need is justified,
additional sensors may be strategically located at an other
location to provide additional weather information (“Meteorological Discontinuity Sensor Group”) or backup sensor capabilities (“Backup Sensor Group”). These additional
sensor groups generally consist of a ceilometer, a visibility
sensor, and a collocated DCP2 . Figure 2 shows a typical
ASOS Combined Sensor Group. Figure 3 shows an optional ASOS “Meteorological Discontinuity” or “Backup”
Sensor Group.

The ASOS consists of three main components. The
first two components are at all ASOS locations; the third
component is found only at airports at which observer augmentation/backup support is provided.
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Sensor group(s), consisting of individual weather sensors and a Data Collection Package (DCP)
The Acquisition Control Unit (ACU)
The Operator Interface Device (OID)

2.1.1

ASOS Sensor Groups

The ASOS sensors continuously sample and measure
the ambient environment, derive raw sensor data and make
them available to the collocated DCP. These raw sensor
data include visibility extinction coefficients, ceilometer
cloud hits, freezing precipitation resonant frequencies and
other sensor data. These data are processed as preliminary
input into the observation algorithms. The ASOS consists
of the following basic complement of sensors1 :
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A DCP is located with each sensor group. It continually gathers and processes raw data from the adjacent sensors (e.g., voltages, extinction coefficients, data counts) and
conditions these data before transmission to the ACU. Data
conditioning may include such processes as sampling, formatting, and scaling.

Ceilometer, Cloud Height Indicator [CHI] Sensor (one
to three sensors per site)
Visibility Sensor (one to three sensors per site)
Precipitation Identification (PI) Sensor
Freezing Rain (ZR) Sensor (not planned to be included
where ZR potential is nil)
Lightning Sensor (only at selected sites)
Pressure Sensors (two sensors at small airports; three
sensors at larger airports)
Ambient/Dew Point Temperature Sensor
Anemometer (wind direction and speed sensor)
Precipitation Accumulation Sensor (Heated Tipping
Bucket [HTB] Gauge)

2.1.2

Acquisition Control Unit

The ACU, which is the central processing unit for the
ASOS, is usually located inside a climate controlled structure, such as an observing office or control tower building.
It ingests data from the DCP(s) and pressure sensors, and
is capable of accepting information from the FAA New
Generation Runway Visual Range (NGRVR) system.
The ACU performs final processing, formatting, quality control, storage and retrieval of the data, and makes
ASOS data available to users through various outlets. A
brief description of the various ASOS data outlets and data

Other sensors are under development and may be
added at a later time.

At the Hub airports, the PI sensor will be included in
the Touchdown Sensor Group if it is located at the TDZ of
the primary instrument runway.
5

Figure 2.

ASOS Combined Sensor Group
6

Figure 3.

Additional Sensor Group
7

Figure 4.

Availability of ASOS Data

types is summarized in Figure 4. Additional details and
examples may be found in Chapters 6 and 7.

2.1.3

2.4

Operator Interface Device

Daily and Monthly Summary messages (described in
Section 5.6)

METAR Elements

The Operator Interface Device (OID) is the primary
means through which an on-site observer enters back-up
or augmentation observations into the ASOS METAR/
SPECI report. It consists of a keyboard, and video screen
interfaced directly with the ACU either through hardwire
or telephone line. Various OID screen displays are available; see Appendix A for examples.

The ASOS will automatically report the following surface weather elements in the METAR:

2.2

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ASOS Data Outlets

The ASOS Peripheral Data Outlets include:
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The ASOS Operator Interface Device (OID) and remote dial-in user interactive video screen displays. Examples of various OID video screen displays are shown
in Appendix A
On-site video display screens. These include the Video
Display Unit (VDU) screens, and user-provided video
monitors
On-site printer hard copy (at OID equipped locations),
when required
Long-line dissemination of coded messages through
NWS and FAA communications networks. Long-line
networks are described in Section 6.3
Computer-generated voice (available through FAA
radio broadcast to pilots, and general aviation dial-in
telephone lines). Examples of computer-generated
voice messages are given in Section 6.4

2.3

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n

ASOS Data Types

The ASOS Capability to report these elements is summarized in Appendix B. The content of the ASOS METAR/
SPECI is described in the following chapters and is summarized in Appendix C. In the future, ASOS may also provide additional information on snowfall, hail, drizzle, and
sunshine duration (see Chapter 7 for further details).

The various ASOS data types available through these
outlets include:
n
n

n
n
n

Wind: Direction (tens of degrees - true), Speed
(knots), and Character (gusts)
Visibility up to and including 10 statute miles
Runway Visual Range (RVR) at selected sites
Basic Present Weather Information (type and
intensity): Rain, Snow, Freezing Rain, Squalls
Obstructions: Fog, Mist, Haze, and Freezing Fog
Sky Condition: Cloud Height and Amount (CLR,
FEW, SCT, BKN, OVC)3 up to 12,000 feet above
ground level
Ambient Temperature, Dew Point Temperature
(degrees Celsius)
Pressure: Altimeter Setting in inches of mercury
(Hg), and Sea-level Pressure (SLP) in Hectopascals
(hPa)4 in Remarks
Automated, Manual, and Plain Language Remarks
(depending on service level) including: Volcanic
Eruption (plain language), Tornadic Activity (plain
language), Wind Shift, Tower Visibility, Beginning
and Ending of Precipitation, Virga (plain language),
Significant Cloud Types (plain language), SLP, and
Other Significant (Plain Language) Information
Additive and Automated Maintenance Data including: 3, 6, 24-hour Precipitation Amount, Hourly
Temperature and Dew Point, 6-hour Maximum and
Minimum Temperatures, 3-hour Pressure Tendency,
various sensor status indicators, and maintenance
check indicator ($).

One-Minute Observation (OMO) data (content same
as METAR/SPECI data)
Aviation Routine Weather Reports (METAR) and
Aviation Selected Special Weather Reports (SPECI).
At staffed locations, SPECI messages for tornadic or
volcanic activity are manually composed and disseminated through ASOS when these conditions are observed (described in Section 5.3)
Auxiliary data display (described in Section 5.4)
Standard Hydrometeorological Exchange Format
(SHEF) messages (described in Section 5.5)
Maintenance Data (raw sensor data, system diagnostics, system status) (examples given in Appendix A)

In the METAR code cloud amounts are depicted as
either Clear (CLR), Few (FEW), Scattered (SCT), Broken
(BKN), or Overcast (OVC)
4

A Hectopascal (hPa) value is equivalent to a millibar
(mb), i.e., 1012 hPa = 1012 mb.

2.5

Automated METAR vs.
Manual METAR

added information in the Remarks section about cloud type
and location which ASOS cannot provide. This is just one
example. Other detailed examples are given in Chapter 5.

In form, the unattended ASOS METAR and the attended ASOS (Observer oversight) METAR look very
much alike. For example, under the same circumstances,
the ASOS would report:

To fully appreciate these examples, the ASOS User’s
Guide first examines each of the weather elements reported
by the ASOS and compares the manual and automated observations in terms of sensors used, the observing procedures employed, and reporting capabilities. Automated
observing concepts are also discussed for greater clarity
and understanding. The specific chapter-by chapter breakdown is as follows:

KDEN 281950Z AUTO 11006KT 6SM HZ SCT080
15/12 A3013 RMK AO2 SLP123
while the observer who is editing the ASOS might report:
KDEN 281950Z 11006KT 5SM HZ SCT080
BKN140 15/12 A3013 RMK AO2 FEW ACC W SLP123
Notice that both METARs contain the station ID, observation type, time, wind, visibility, obstructions, sky condition, ambient temperature, dew point temperature,
altimeter setting, and sea-level pressure (in remarks).
The automated observation indicates the station type
as “AUTO” which signifies an unattended observation (i.e.,
Observer not logged onto ASOS for back-up or augmentation), and identifies the system as one capable of reporting
present weather (AO2).
The second observation indicates that the Observer
logged onto the system (AUTO is missing). The Observer
augmented sky condition for clouds above 12,000 feet and

Chapter 3 discusses the more objective and directly
measured (i.e., non-visual) elements such as ambient
temperature, dew point temperature, wind, pressure,
and precipitation accumulation.

Chapter 4 discusses the more subjective, and to some
extent, indirectly measured (i.e., visual) elements of
sky condition, visibility, present weather (phenomena
and obstructions).

Chapter 5 provides examples of ASOS weather reports, messages, and summaries.

Chapter 6 discusses the outlets through which the
ASOS data are available.

Chapter 7 describes those elements not currently provided by ASOS, future product improvement plans,
and alternate means through which these data are now
available or may be available in the future.

CHAPTER THREE
3.0

Automating the Objective
Weather Elements

The “objective” weather elements are defined as ambient and dew point temperature, wind, pressure, and precipitation accumulation. These elements are classified as
“objective” because they are more simply and directly
measured and are easier to automate than other elements.
This chapter describes how ASOS reports objective elements.

3.1

Ambient and Dew Point
Temperature

Figure 5.

Ambient and dew point temperature reports are among
the most widely disseminated of all the weather elements
in the surface observation. Because of keen public interest, nearly all radio and most television stations report temperature and humidity at least once an hour. Ambient and
dew point temperature are vital in determining aircraft performance and loading characteristics and are critical for
accurate weather forecasts. To meet these needs, ASOS
provides a 5-minute average ambient air and dew point
temperature every minute.

3.1.1

ASOS Hygrothermometer

When this condition occurs, the mirror’s surface is in vapor pressure equilibrium with the surrounding air (i.e., has
reached the saturation vapor pressure). The temperature
required to maintain this equilibrium is, by definition, the
dew point temperature.
Optical techniques are used to detect the presence of
surface condensation. Within the hygrothermometer, a
beam of light from a small Light Emitting Diode (LED) is
directed at the surface of the mirror at a 45 degree angle.
Two photo-resistors are mounted to receive the reflected
light. The “direct” sensor is placed at the reflection angle
and receives a high degree of light when the mirror is clear.
The indirect sensor is placed to receive light scattered when
the mirror is clouded with visible condensation, (i.e., dew
or frost formation).

Ambient/Dew Point
Temperature Sensor

Both the manual and automated temperature sensors
directly measure the ambient dry-bulb and the dew point
temperatures. The hygrothermometer used in the ASOS is
a modern version of the fully automated “HO-83”
hygrothermometer, first used operationally in 1985. This
instrument uses a platinum wire Resistive Temperature
Device (RTD) to measure ambient temperature and a chilled
mirror to determine dew point temperature.

In normal operation, a feedback loop controls an electric heat pump running through a cooling-heating cycle,
which cools the mirror until dew or frost is formed; it then
heats the mirror until the condensate (dew or frost) is evaporated or sublimed. This cycle nominally takes about 1
minute to complete.

The RTD operates on the principle that electric resistance in a wire varies with temperature. This RTD is located in the stream of aspirated air entering the sensing
unit and assumes the ambient air temperature.

As the mirror’s cloudiness increases, the “direct” sensor receives less light and the “indirect” sensor receives
more light. When the ratio of indirect to direct light reaches
an adaptive criterion value, the mirror is considered to be
at the dew point temperature. The adaptive criterion value
(ratio of indirect to direct light) is adjusted once a day to
compensate for residual contamination on the mirror due
to dust and other airborne particulates.

To determine dew point temperature, a mirror is cooled by a thermoelectric or Peltier cooler until dew or frost
begins to condense on the mirror surface. The body of the
mirror contains a platinum wire RTD, similar to that used
for ambient temperature. This RTD assumes the mirror’s
temperature, which is held at the dew point temperature.

Table 1.

Temperature Sensor—Range, Accuracy Resolution

Parameter

Range

RMSE

Max Error

Resolution

Ambient
Temperature

-80°F to -58°F
-58°F
to +122°F
+122°F to +130°F

1.8°F
0.9°F
1.8°F

± 3.6°F
± 1.8°F
± 3.6°F

0.1°F

Dew Point
Temperature

-80°F
-0.4°F
+32°F

3.1°F to 7.9°F
2.0°F to 7.9°F
1.1°F to 4.7°F

4.5°F to 13.9°F
3.4°F to 13.9°F
2.0°F to 7.9°F

0.1°F

to -0.4°F
to +32°F
to +86°F

Since a clean mirror needs relatively less indirect light
to determine when dew has formed than a dirty mirror, the
mirror is heated once a day to recalibrate the reference reflection expected from a dry mirror. This procedure compensates for a possible dirty or contaminated mirror and
redefines adaptive criterion value used to determine when
dew or frost has occurred. This once per day recalibration
nominally takes about 15 minutes.

Once each minute the ACU calculates the 5-minute
average ambient temperature and dew point temperature
from the 1-minute average observations (provided at least
4 valid 1-minute averages are available). These 5-minute
averages are rounded to the nearest degree Fahrenheit, converted to the nearest 0.1 degree Celsius, and reported once
each minute as the 5-minute average ambient and dew point
temperatures. All mid-point temperature values are rounded
up (e.g., +3.5°F rounds up to +4.0°F; -3.5°F rounds up to 3.0°F; while -3.6 °F rounds to -4.0 °F).

The ASOS hygrothermometer meets all NWS specifications for measuring range, accuracy, and resolution. The
specifications for accuracy are given in Root Mean Square
Error (RMSE) and Maximum (MAX) Error. Specifications
are listed in Table 1.

The ACU performs all temperature averaging functions. It also performs a number of data quality tests on the
ambient and dew point temperatures, including upper and
lower limit checks, a rate of change check, and a cross
comparison. The current 1-minute ambient and dew point
temperature are compared against these limits. The upper
and lower limits are +130°F to -80°F for ambient temperature and +86°F to -80°F for dew point temperature.

The RMSE for Dew Point Temperature is given as a
range of values and is dependent on the Ambient Temperature minus the Dew Point Temperature value (i.e., Dew
Point Depression [DD]). The low end of the RMSE and
MAX Error range is for a DD of 0°F; the high end of the
Error range is for a DD of 63°F.

3.1.2

If the current 1-minute ambient or dew point temperature differs from the last respective, non-missing, 1-minute
reading in the previous 2 minutes by more than 10°F, it is
marked as “missing.” The current 1-minute ambient temperature is also compared against the current 1-minute dew
point temperature to ensure the dew point is not higher. If
the dew point temperature exceeds the ambient temperature by 2°F or less, the dew point temperature is set equal
to the ambient temperature. If the dew point temperature
exceeds the ambient temperature by more than 2°F, the 1minute dew point temperature is set to “missing.”

Ambient Temperature/Dew Point
Temperature Algorithm

Both ambient temperature and dew point temperature
are considered conservative elements (i.e., continuous in
space, and slowly and smoothly changing in time). Based
on this characteristic, time-averaging over a short period is
the preferred method of measurement.

If, within the past 5 minutes, there are at least four
valid (i.e., non-missing) 1-minute ambient and dew point
temperatures, the respective 5-minute averages are computed and reported in degrees Celsius in the OMO and
METAR. If there are less than four valid 1-minute average
ambient or dew point temperatures within the past 5 minutes, ASOS does not compute the current 5-minute average for ambient or dew point temperature. When this occurs,
ASOS uses the most recent 5-minute average value calculated within the past 15 minutes.

The ASOS hygrothermometer continually measures
the ambient temperature and dew point temperature and
provides sample values approximately six times per minute.
Processing algorithms in the hygrothermometer use these
samples to determine a 1-minute average temperature and
dew point valid for a 60-second period ending at M+00.
These data are passed to the ACU for further processing.

If the ASOS has not recorded a valid 5-minute average ambient or dew point temperature within the past
15-minutes, no ambient or dew point temperature is reported and the sensor failure notation is entered into the
ASOS system maintenance log (SYSLOG). This
15-minute “hold-off” allows the daily 15-minute calibration heat cycle to occur without adverse affect. The current 1-minute average ambient temperature and the
5-minute average ambient temperature are updated once
each minute and stored in memory for 12 hours. These
stored data are used in further computation:
n

Additional temperature parameters are derived from
the Calendar Day and Monthly Maximum and Minimum
Temperature data. These data may be reported in the daily
and/or monthly summary messages, as appropriate.

Once each minute, ASOS uses the running 5-minute
average ambient temperature to update the hourly
maximum and minimum ambient temperatures. At the
end of the hour (H+59), the cumulative maximum and
minimum ambient temperatures for the hour and the
minute(s) they occurred are stored in memory for 24
hours.

The daily data include: calendar day average ambient
temperature, the latest daytime maximum temperature
(LDT), the latest nighttime minimum temperature (LNT),
departure of calendar day average ambient temperature
from normal, and heating degree days (HDD) or cooling
degree days (CDD).

At synoptic hour (00, 06, 12, and 18 UTC) reporting
times, the current 6-hour maximum and minimum ambient temperatures are computed from the hourly maximum and minimum ambient temperatures in tenths of
degrees Celsius, and included as remarks (“1snTxTxTx”
for maximum temperature, and “2snT nT nT n”for minimum temperature) in the current synoptic hourly
METAR.

The monthly data include: average monthly temperature, average monthly maximum and minimum temperature, number of days maximum temperature exceeded a
set maximum temperature threshold (90°F in the 48 contiguous United States and Hawaii, and 70°F in Alaska),
number of days maximum temperature < 32°F, number of
days minimum temperature < 32°F, number of days minimum temperature < 0°F, monthly heating degree days, and
monthly cooling degree days.

The current 12-hour average ambient temperature is
also computed once each minute from the current and
previous 12-hour reported 5-minute average temperature (this value is used for calculating current sea-level
pressure).

Once each hour (at the hourly METAR report time)
the current hourly ambient temperature and dew point
temperature are reported in the METAR Remarks section, to the nearest tenth of a degree Celsius, in the
form “TsnT’T’T’snT’dT’dT’d.”

Once each minute, ASOS computes the highest and
lowest ambient temperatures, so far for the current calendar day, in tenths of degrees Celsius. The calendar
day maximum and minimum ambient temperatures are
reported in the midnight, LST hourly METAR remarks
(4snT xT xT x snTnTnTn), and are stored in memory for 31
days as part of the Daily Summary Product. The “midnight, LST hourly METAR” is usually transmitted before 23:59 LST. Therefore, there may occasionally be
a discrepancy between the maximum and minimum
temperatures reported in the “4” group and the daily
maximum and minimum temperatures reported in the
ASOS Daily Summary Message (DSM) which runs
from 00:00 to 23:59 LST (see Section 5.6 for details).

Once each day (at 23:59 LST), the highest and lowest
ambient temperatures for the current month, along with
the date(s) of occurrence, are computed and stored in
memory until the end of the following month. On the
first day of the following month, ASOS outputs the
Monthly Maximum Temperature and date(s) of occurrence, plus the Monthly Minimum Temperature and
date(s) of occurrence7 .

Relative humidity is calculated using the 5-minute average ambient (dry-bulb) temperature and dew point temperature. The 5-minute average temperature is also used to
process other algorithms, specifically sky condition (obscuration determination), present weather (freezing rain determination, and snow - rain discrimination), obscurations
(freezing fog, fog, mist, haze discrimination), and pressure
(sea-level pressure, pressure altitude, and density altitude
calculation).

3.1.3

Ambient Temperature/
Dew Point Temperature
Strengths and Limitations

Although the methodology of determining the ambient temperature and dew point temperature is not new, substantial improvements have been achieved by ASOS in
measuring temperature and dew point by increasing the

These data are contained in the Monthly Summary message issued by ASOS.

sensor aspiration and siting the sensors away from buildings and heat islands. Furthermore, ASOS’s continuous
monitoring, self-diagnostics, and application of quality
control algorithms ensure that any ambient temperature/
dew point temperature degradation trend is reported before sensor performance falls below performance standards.
At times, however, the reported dew point temperature may become stuck at around zero degrees Celsius. At
other times, it may not be representative because of excessive mirror contamination due to dust or other atmospheric
aerosols. An aggressive preventative maintenance program
is conducted that includes periodic cleaning of the mirror
surface. Furthermore, a planned product improvement effort is underway to find a more reliable alternative for measuring Dew Point temperature (see Section 7.2.2 for details).

3.2

Wind

The rotating cup anemometer and the simple wind vane
are the principal indicators of wind speed and direction.
Until the mid 1940s, the electrical contacting anemometer
was the standard wind measuring instrument. Since then,
the “F420” series of instruments have become the standard for wind measurement in the U.S. A basic system of
this series consists of a cup-driven Direct Current (DC)
generator with an output calibrated in knots and a vane
coupled to an indicator by means of a DC synchro-system.
The ASOS uses a modern automated version of the F420,
in which electro-magnetic signals generated by the rotating cup and wind vane are directly converted into reportable values by ASOS.

Figure 6.

ASOS Anemometer

grees). Wind direction is reported relative to true north in
the METAR/SPECI message, in the daily/monthly summaries, and on all video displays. Wind direction is reported relative to magnetic north in the computer-generated
voice messages, and on the OID “AUX” data display screen.
See section 3.2.2.1 for details.

Before ASOS, airport wind sensors were generally
exposed 20 feet above ground level. With modern, highperformance aircraft, this standard no longer applies. Now,
current federal standards for siting meteorological equipment specify (with some variance permitted) a height
of 10 meters (32.8 feet). Typical ASOS wind sensor heights
are 33 feet or 27 feet, depending on local site-specific
restrictions or requirements. Figure 6 shows the ASOS
anemometer. The ASOS will report the following wind
related parameters.

Wind Speed: A 2-minute average is updated once every 5 seconds and is reported once every minute in the
OMO and computer-generated voice messages, and included in the METAR/SPECI message and various OID
screen displays. See Section 3.2.2.1 for details.
Wind Gust: This is a basic component of wind character and is updated every 5 seconds. It is appended to and
reported with the basic wind observation only when appropriate conditions for reporting wind gust exist (see definition in Section 3.2.2.2). Wind gust information is included
in the current OMO, computer-generated voice messages,
the METAR/SPECI, and OID displays.

Wind Direction: ASOS reports a 2-minute average
of 5-second average wind directions once a minute (i.e.,
24 samples each minute) for distribution through the OMO
and computer-generated voice messages. The current 2minute average wind direction is updated on selected OID
screen displays once every minute and included in the transmitted METAR/SPECI messages. The direction from
which the wind is blowing is reported to the nearest 10
degree increment (e.g., 274 degrees is reported as 270 de-

Wind Shift: This remark is reported in the OMO and
the METAR/SPECI when appropriate. See Section 3.2.2.2a
for details.
Variable Wind Direction: This data element is reported in the OMO and the METAR/SPECI when appropriate. See Section 3.2.2.2b for details.

Table 2.

Wind Sensor—Range, Accuracy, Resolution

Parameter

Range

Accuracy

Wind Speed

0 to 125 knots

± 2 knots
1 knot
- or 5% (whichever is greater)

Wind Direction

0 to 359 degrees

± 5 degrees
when wind speed
is > 5 knots

3.2.2

Squall: Although squall is a basic component of wind
character and, under appropriate conditions is updated every 5 seconds, it is reported in the present weather section
of the METAR/SPECI observation; direction and speed values are indicated in the wind group. See Section 3.2.2.2c
for details.

nearest
degree

Wind Algorithm

The basic method of observing wind direction and
speed is to take a fixed point, time averaged measurement.
The ASOS algorithm uses a 2-minute period to obtain the
current average wind direction and speed. In both cases,
ASOS obtains the wind character (i.e., gusts and present
weather squalls) and the peak wind by comparing the difference between the average wind speed with the maximum “instantaneous” wind speed observed over a specified
time interval. When this difference exceeds a prescribed
value, the appropriate additional wind information is included in the observation. The ASOS wind shift remark is
determined from the difference between the current 10minute average (of 2-minute average winds) and the 10minute average (of 2-minute average winds) from 15
minutes ago, provided the intervening 2-minute average
wind speeds are greater than 9 knots. Within ASOS, all
average wind directions and speeds are rounded up to the
nearest degree and knot, respectively.

Peak Wind: This remark is reported in the scheduled
hourly METAR, as appropriate. It is the greatest 5-second
average wind exceeding 25 knots which has been observed
since the previously scheduled hourly METAR.
Daily Peak Wind: This value is reported in the Daily
Summary message (see Section 5.6.1). It is the greatest
5-second average wind speed observed (converted to miles
per hour) during the 24-hour calendar day beginning at
5-seconds past midnight, Local Standard Time (LST),
(00:00:05 LST) and ending at midnight (00:00:00 LST)
the next day.
Fastest 2-Minute Wind: This value is reported in the
Daily Summary message. It is the fastest 2-minute average
wind speed (in miles per hour) observed over the 24-hour
calendar day.

3.2.1

Resolution

3.2.2.1

Wind Direction and Speed

In the past, observers monitored an analog or digital
wind dial over a short period to determine the average wind
direction and speed for the observation. Most sites also had
a wind recorder device to provide a continuous documented
record of measured wind direction and speed. The observer
often used this device to determine the maximum instantaneous wind speed over the 10-minute period before completing the observation. The observer used visual/mental
averaging and ultimate human judgement to create an observation of wind. This method was not always consistent
from site to site or from one observer to another.

Wind Sensor

The ASOS wind sensor (Figure 6) employs a “light
chopper,” electro-optical method to determine wind speed
and convert it to appropriate electro-magnetic signals. Wind
sensor measurements conform to the Range, Accuracy,
Resolution specifications described in Table 2. In addition,
the sensor’s starting threshold for response to wind direction and wind speed is 2 knots. Winds measured at 2-knots
or less are reported as calm.

ASOS continuously and objectively measures wind direction and speed once every second, far more frequently,
consistently, and accurately than an observer could. Every

ASOS processes data identically, which provides site-tosite consistency unknown in past records. Five-second wind
direction and wind speed averages are computed from the
1-second measurements. These 5-second averages are
rounded to the nearest degree and nearest knot and are retained for 2 minutes. These five-second averages are the
fundamental units used to compute reportable wind values and are, in effect, the ASOS equivalent to the manual
“instantaneous” wind observation.

utes). Observations of 5 knots with gusts of 10 to 15 knots,
however, are the more common minimum values reported.

Every 5 seconds a running 2-minute average wind (direction and speed) is computed and used to further compute wind character. If the computed 2-minute average wind
speed is 2 knots or less, the 2-minute average wind direction and speed is reported as “calm” (00000KT).

Once every 5 seconds the ASOS computes the current
2-minute average wind speed and compares it with the
greatest 5-second average wind speed during the past
minute. If the current 2-minute average wind speed is equal
to or greater than 9 knots and the greatest 5-second average wind speed (during the past minute) exceeds the current 2-minute average speed by 5-knots or more, then the
greatest 5-second average speed observed during the past
minute is stored in memory as a gust for 10 minutes.

The ASOS algorithm also relies on a 10-minute observation period to determine gusts, but uses it in a different way. Once every 5 seconds, the ASOS computes the
greatest 5-second average wind speed (and corresponding
direction) during the past minute, and once each minute
stores this information in memory for 12 hours.

Once each minute the current 2-minute average wind
is stored in memory for 12 hours and made available for
reporting in the OMO, the computer generated voice messages (i.e., the ground-to-air radio and telephone dial-in
message), the METAR/SPECI reports, and OID displays.

3.2.2.2

Once every 5 seconds, the ASOS compares the highest gust stored in memory for the past 10 minutes with the
current 2-minute average wind speed. If the difference between the two is 3 knots or more, the current reported wind
speed is greater than 2 knots, and the highest gust exceeds
the minimum 5-second wind speed in the past 10 minutes
by 10 knots or more, then the highest gust stored in memory
is designated as the reportable gust. This value is appended
to the current wind direction and speed reported in the
OMO, computer-generated voice messages, and the
METAR/SPECI reports. The minimum gust speed reported
by ASOS is 14 knots. Wind speeds from 3 knots and 11
knots may be reported with gusts to 14 knots. For example,
a 2-minute average wind of 240 degrees at 10 knots with
gusts to 20 knots is reported as: “24010G20KT.”

Wind Character

Wind Character information is added to the METAR
after the Wind Direction and Speed data when the variability in the steady state wind exceeds threshold criteria. Wind
Character components include Wind Gusts and Variable
Wind. Although Wind Squalls are reported as a Present
Weather phenomena in METAR, they are also discussed
here for comparison and contrast.
Both the manual procedure and the ASOS algorithm
determine Wind Character by examining the maximum “instantaneous” wind speed over the 10-minute period immediately preceding the observation. The manual procedure
requires a visual examination and interpretation of the dial
readings or recorder to determine “instantaneous” wind
speed. The ASOS algorithm, by contrast, relies on objective 5-second averages of 1-second wind measurements.

3.2.2.2b Variable Wind
Both the manual procedure and the ASOS algorithm
use the same definition for determining a variable wind
but use different methods for reporting it. In both cases, a
variable wind is reported when the wind direction varies
by 60 degrees or more during the 2-minute evaluation period before the observation. If the 2-minute wind speed is
6 knots or less, than a variable wind direction indicator
(VRB) is included in the basic wind group; if the 2-minute
wind speed is greater than 6 knots, then a variable wind
direction group is appended to the basic wind group in the
body of the METAR/SPECI report.

3.2.2.2a Gusts
In the manual procedure, a gust is reported when an
observer sees rapid fluctuations in sensor wind speed indications with a variation of 10 knots or more between peaks
and lulls during the 10-minutes before the observation. The
reported gust is taken from the maximum “instantaneous”
wind speed observed during this period. The average 2minute wind is used to report wind direction and wind
speed. Conceivably, an average 2-minute wind speed as
low as 3 knots (observed in the last minute) may be reported with a gust of 10 knots (observed in the last 10 min-

The basis for the manual determination of a variable
wind is simply a visual interpretation of the wind instrument reading during the 2-minute evaluation period. The
ASOS algorithm by contrast compares the range of 5-sec-

3.2.2.3

ond average wind directions during the past 2 minutes (24
samples).

Wind Remarks

The ASOS will include Wind Shift and Peak Wind
remarks in the METAR/SPECI reports when appropriate.

In either case, if the current 2-minute average wind
speed is 6 knots or less, the wind direction and speed is
reported as “VRBff,” where “ff” is the current 2-minute
average wind speed in knots. For example, a variable wind
at 3 knots is encoded as “VRB03.”

3.2.2.3a Wind Shift
Both the manual procedure and the ASOS algorithm use
the same definition of a wind shift as described in the FMH1: “A wind shift is indicated by a change in wind direction of
45 degrees or more in less than 15 minutes with sustained
winds of 10 knots or more throughout the wind shift.”

If the current 2-minute average wind speed is greater
than 6 knots, then the current wind direction and speed are
placed in the body of the report and followed by a variability indicator in the form “dndndnVdxdxdx ,” where dndndn is
the minimum, and dxdxdx is the maximum variable wind
direction during the past 2-minutes. For example, a current
2-minute wind of 270 degrees at 10 knots that varies from
240 degrees to 300 degrees is coded as: 27010 240V300.

The observer relies on his alertness and a visual estimate of the 2-minute average wind to determine the onset
of a wind shift. A Frontal Passage (FROPA) remark may
be appended to the wind shift remark when the wind shift
is associated with a frontal passage. This determination of
course is based on human judgement.

3.2.2.2c Squalls
In the manual procedure, observers report a squall
when wind speed suddenly increases by at least 16 knots
and speed is sustained at 22 knots or more for at least
1 minute. Observers manually determine a squall by visually examining the indicated or recorded “instantaneous”
wind speed. The reported squall value is taken from the
maximum “instantaneous” wind speed sustained for at least
1 minute. Although squalls are measured as a parameter of
wind, they are reported as an element in the present weather
field of the METAR/SPECI report.

The ASOS, on the other hand, determines a wind shift
by first making sure that minimum wind speed and direction change criteria are met. These checks are made to ensure that light, variable winds are not erroneously reported
as a wind shift.
The wind speed criterion requires that all 2-minute average wind speeds computed each minute over the past 15
minutes are greater than 9 knots. If this criterion is met,
then the current 10-minute average wind direction derived
from ten one-minute-observations is compared to a similar
10-minute average wind direction from 15 minutes ago. If
the wind directions differ by 45 degrees or more, then a
wind shift is encoded.

ASOS algorithm by contrast, computes a potential
squall value once every 5 seconds. If the current
2-minute wind speed (measured every five seconds) is
greater than or equal to 22 knots and exceeds the 2-minute
average wind speed computed two minutes ago by 16-knots
or more, then the highest 5-second average wind speed during the last 2-minutes is stored in memory as a squall for
10 minutes. Only the current squall or non-squall default
value is stored in memory. The stored squall value is reported as “SQ” in the present weather field if the current
2-minute average wind speed is greater than 2 knots, and if
the squall value exceeds the current 2-minute average wind
speed by more than 3 knots.

The wind shift remark generated by the ASOS in the
METAR report is: “WSHFT hhmm,” where “hhmm” is the
Universal Coordinated Time (UTC) of when the shift began (15 minutes ago). Once the wind shift remark is reported, it will continue to be included in all subsequent
reports (including long-line dissemination of ASOS generated SPECI) through the next scheduled hourly METAR.
The ASOS, of course, is unaware of distant phenomena
and synoptic scale weather patterns and consequently cannot confidently determine if a frontal passage (FROPA)
remark should be appended to the wind shift remark. Therefore, FROPA is not reported by ASOS. At attended sites
however, this remark may be added by the Observer in
accordance with agency reporting policy.

According to this algorithm, a squall may continue to
be reported by the ASOS for up to 10 minutes after the
squall is written to memory, provided the above minimum
wind-speed, squall reporting conditions are met. The minimum wind speed - squall combination reported by ASOS
is a wind speed of 3 knots, with a squall of 22 knots.

3.2.2.3b Peak Wind

the ground. It is the only element that cannot be directly
observed or qualitatively sensed by the observer or pilot.
As a result, pressure has always been carefully measured
and the operational sensor routinely compared to some reference standard.

The Peak Wind, by definition, is the highest instantaneous wind speed observed or recorded since the last scheduled hourly observation (METAR). The Peak Wind
direction, speed and time of occurrence are reported in
METAR remarks as: “PK WND dddff(f)/(hh)mm,” where
ddd = direction (true) in tens of degrees, ff(f) = wind speed
in knots, and (hh)mm = (hour) minutes past current hour
of most recent occurrence of the reported peak wind. The
“hh” indicator is included only when the peak wind occurred in the previous hour since the last METAR.

All the currently computed pressure elements will continue to be reported by the ASOS with the same or higher
level of precision as the human report. The pressure parameters available from ASOS are:
n
n
n
n
n
n
n
n

The manual procedure requires a Peak Wind to be reported when the maximum instantaneous wind speed since
the last METAR exceeds 25 knots.
In ASOS, the Peak Wind is determined from the highest observed 5-second average wind speed which exceeds
25 knots since the last generated METAR, whether transmitted or not (FIBI).

3.2.3

Sensor Pressure
Altimeter Setting
Pressure Remarks
Sea-Level Pressure
Density Altitude
Pressure Altitude
Pressure Change/Tendency
Station Pressure

Because accurate pressure is critical, three separate and
independent pressure sensors are used at towered airport
locations. At other locations, two pressure sensors are used.
The ASOS algorithm compares the pressure sensors’ readings and issues a pressure report only when there is acceptable agreement between at least two sensors.

Wind Strengths and Limitations

The major strength of the ASOS in reporting winds is
the consistency of measurements. While Observers rely on
perception and human judgement to interpret wind instruments, automated systems rely on digital second-by-second measurements that are processed identically from
time-to-time and place-to-place.

3.3.1

The ASOS pressure measurement instrument consists
of redundant digital pressure transducers, which use capacitive sensors, one side of which is permanently evacuated to a vacuum to make it a barometric pressure sensor.
Advanced microcomputer electronics and sophisticated
firmware provide reliable performance. The barometers are
located on a tray at the bottom of the ACU and are exposed
to the ambient air pressure. In cases when the ACU is installed in pressurized buildings, this exposure is through a
port connected to an outside static pressure vent. Figure 7
shows the pressure sensors in the ACU. The specified operational characteristics for these sensors are:

One limitation in the automated observation is a lag in
reporting wind shifts (in METAR remarks). The wind shift
algorithm cannot rely on external clues used by the observer (like thunder or snow showers) for early collateral
assurance of a wind shift - frontal passage occurrence. It
must therefore wait the full 15-minutes required in the definition of wind shift before outputting a remark. Although
highly unlikely, a wind shift and variable wind remark may
both be generated and included on the same METAR under conditions of light and variable winds which just barely
meet the wind shift reporting criteria, or when a FROPA
has occurred in the preceding 10-15 minutes. To be more
responsive to operational needs, the wind reporting algorithm has been tuned to prevent excessive, frequent reporting of a wind shift once a report is issued and when a
variable wind condition exists.

n
n
n

Range:
16.9 - 31.5 inches of mercury
Accuracy: ± 0.02 inches of mercury
Resolution: 0.003 inches of mercury (measurement);
0.005 inches of mercury (reporting)

3.3.2

3.3

Pressure Sensor

Pressure Algorithm

Pressure
A sophisticated algorithm routinely computes and updates the pressure report for each pressure sensor once a
minute from readings obtained every 10 seconds from each

Atmospheric pressure is the most important surface
weather element for aircraft operations since it provides
the means of establishing the height of an aircraft above

3.3.3

Pressure Strengths
and Limitations

The pressure sensors are the most reliable and accurate sensor in ASOS. The only limitation (if one can call it
that) is that pressure remarks will be reported more often
in ASOS METAR messages than in manual METAR messages simply because of the continuous weather watch
which ASOS provides.

3.4

Precipitation Accumulation

Accurate liquid-equivalent precipitation accumulation
measurements are essential for hydrological, flood forecasting, and agriculture applications. For aviation purposes,
freezing or frozen precipitation accumulation measurements
provide a quantitative dimension to the qualitative detection and reporting of freezing or frozen precipitation by
other ASOS sensors.

Figure 7.

Basic manual measurements of precipitation accumulation in the U.S. have traditionally relied on the Standard
8-inch Gauge. This consists of an 8-inch cylinder with an
inverted funnel orifice leading to a graduated inner cylinder at the base of the funnel neck. The inner cylinder is
used to measure liquid precipitation accumulation. When
freezing or frozen precipitation is expected or is occurring,
the funnel and inner cylinder are removed. Frozen precipitation captured in the outer cylinder are periodically melted
indoors to measure the liquid-equivalent of frozen precipitation (LEFP).

ASOS Pressure Sensor

sensor. If one or more of the 6 pressure readings obtained
from each sensor in the past minute is missing, then the 1minute pressure value for that sensor is marked as “missing” and the sensor is logged as “inoperative.” The current
1-minute pressure values from each sensor are then compared against each other and absolute differences computed.
The lowest 1-minute sensor pressure value obtained from
a pair of sensors, whose pressure difference is 0.04 inch or
less, is the designated ASOS pressure to be reported at the
end of the minute. This pressure value is then used to compute an altimeter setting and other derived pressure values.

In the early development of an automated precipitation accumulation gauge, it was recognized that automated
measurement of liquid and LEFP each presented a unique
challenge, so, a separate specification was written for each
type of precipitation. The automated Heated Tipping Bucket
(HTB) technology from the 1970s was adopted and modified to meet these needs. Over the years, many improvements were made and incorporated into ASOS. Early
versions of the heated gauge applied excessive heat creating excessive evaporation and the under-reporting of the
liquid-equivalent mass.

A sensor whose 1-minute sensor value differs by more
than 0.04 inch from another operational sensor is automatically logged as “inoperative” and the sensor pressure value
set to “missing.” This will cause a maintenance check indicator to be appended to all subsequent ASOS METAR/
SPECI reports until the sensor is returned to an “operational” status. Once a pressure sensor is logged as inoperative, it can only be returned to an “operational” status by a
maintenance technician. If one of the sensors (at two-sensor locations), or if two of the sensors (at three-sensor locations) are logged as “inoperative,” then the designated
ASOS pressure elements are all omitted in the METAR/
SPECI reports.

The current version of the HTB gauge applies less heat
over a longer heating cycle, thus yielding a more accurate
mass measurement of frozen precipitation. Changes in the
tipping bucket inner design also have improved overall performance in liquid precipitation events. These improvements have resulted in the ASOS HTB becoming a very
capable liquid precipitation accumulation gauge in all but
the most extreme heavy rainfall events. However, some

deficiencies still remain in its ability to fully measure precipitation accumulation during the cold-season LEFP
events. Consequently, the ASOS HTB is primarily used to
measure liquid accumulation. Alternative solutions are
being pursued to provide LEFP information. These solutions include: (1) Provision of separate LEFP reports
through existing manual supplementary observing networks
from event-driven Supplementary Data Observations and
schedule-driven Supplementary Climate Data reports, and
(2) Development of a follow-on All-Weather Precipitation
Accumulation Gauge for ASOS (see Section 7.2.4).

3.4.1

The HTB has 2 heating elements. One heating element
is wrapped around the underside of the collector funnel,
and the other around the drain tube. Each heater is separately thermostatically controlled to maintain a temperature of 40°F. A master thermostat regulates electric power
to both heating elements. Power is turned on when the temperature falls below 40°F and is turned off when the temperature falls below -20°F. Power is not turned on again
until the temperature rises above -12°F. Power is turned
off when the temperature is at or above 40°F.
The HTB has a precipitation accumulation range of 0
to 10.00 inches per hour, a resolution of 0.01 inch (i.e., one
tip) and an accuracy of ± 0.02 inch or 4% of the hourly
total (whichever is greater).

Heated Tipping Bucket (HTB)
Precipitation Gauge

The ASOS HTB shown in Figure 8 consists of 6 main
components:

3.4.2

Precipitation Accumulation
Algorithm

The precipitation accumulation algorithm obtains precipitation accumulation data from the HTB precipitation
gauge once each minute. These data are valid for discrete
60-second periods ending at M+00. The ASOS algorithm
corrects the tipping bucket measurement, particularly during periods of high rainfall rates. Each minute the measured rainfall is adjusted using the following equation:
C = A (1 + .60A)

Figure 8.

n
n

n
n
n

In this equation, C = the calculated rainfall amount
and A = the measured amount from the tipping bucket. All
calculations are performed internally using floating point
until rounded for final output each minute.

ASOS Heated Tipping Bucket

If a 1-minute precipitation accumulation output is missing and the precipitation identifier sensor (discussed in Section 4.3) is either inoperative or concurrently reports
precipitation, then the associated METAR precipitation
remarks/messages will either be omitted (Prrrr remark is
deleted), or contain a “/ ” (e.g., 6////, 7////). Furthermore,
when the precipitation accumulation sensor is inoperative,
a “PNO” remark will be appended to the METAR to indicate that precipitation accumulation information is not available. In SHEF, missing precipitation accumulation data will
be reported as “M” in place of the missing value. When
available, the output data are used each minute by the algorithm to compile a variety of cumulative precipitation
remarks/messages. These include:

A wind shield that surrounds the HTB and protects it
against blowing snow from falling into the HTB collector funnel (the wind shield is installed on the ASOS
HTB in climates where the snowfall is > 20% of the
annual precipitation accumulation)
A 12-inch diameter collector funnel
A pivoting dual chamber tipping bucket. This bucket
tips when one chamber is filled with 0.01 inch of liquid precipitation, thus emptying the contents into a
drain pan and exposing the other chamber to the precipitation gathered by the collector funnel
An electronic switch which counts the number of tips
per minute
A drain pan and a drain tube
Heating elements to prevent freeze-up during cold
weather.

METAR hourly message, “Prrrr” remark: In this
message “rrrr” is the liquid equivalent of all precipitation
(in hundredths of an inch) which has occurred since computation of the Prrrr remark for the last scheduled hourly

METAR message. The “rrrr” is the sum of all 1-minute
precipitation accumulations calculated during this period.
If any of the required constituent 1-minute precipitation
accumulation calculations are missing, the remark is omitted and “PNO” is appended to the remarks section. If no
precipitation has occurred since the last scheduled hourly
METAR, the Prrrr remark will not be reported. If only a
trace of precipitation has occurred, the Prrrr remark will
report “P0000.”

messages are identified within the NWS by their RR6
AFOS/AWIPS header: “CCCRR6XXX,” where “CCC” is
the originating AFOS node, “RR6” is the designation for
SHEF 15-minute messages originating from ASOS and
“XXX” is the ASOS location identifier (see Section 5.5.1
for details).
All SHEF data from FAA sites are contained in a “collective” (a group of individual messages bundled together)
and are identified by the collective header
“NMCRR7NKA.” The precipitation accumulation for each
15-minute period is derived from the sum of the 1-minute
precipitation accumulation calculations within each discrete
15-minute period. If any of the constituent 1-minute precipitation accumulation calculations are missing, the value
encoded in the SHEF 15-minute precipitation criteria message for the entire 15-minute period is “M.”

METAR 3- and 6-hourly report, “6RRRR” precipitation accumulation remark: “RRRR” is the amount of
precipitation, in hundredths of an inch, which has accumulated in the past 3- or 6-hours. Three-hourly amounts are
reported in the 03, 09, 15, and 21 UTC METAR reports.
Six-hourly amounts are reported in the 00, 06, 12, and 18
UTC METAR reports.
If any of the constituent hourly Prrrr remarks are missing, the 6RRRR remark is encoded as 6////.

SHEF Hourly Routine Precipitation message: The
precipitation accumulation period for this message is 60
minutes. The end time (in minutes past the hour) is set at
H+00. These messages are issued once an hour at the time
specified to support calibration of the precipitation processing at the designated WSR-88D Radar Product Generator.

METAR 24-Hour “7RRRR” precipitation accumulation remark: “RRRR” in this message is the amount of
liquid equivalent in hundredths of an inch accumulated over
the last 24-hours. This remark is reported with the 1200
UTC METAR provided there has been at least 0.01 inch of
precipitation in the past 24-hours (i.e., since the last 1200
UTC METAR). The precipitation accumulation reported
in the 7RRRR remarks is compiled from the hourly precipitation computations. If any hourly amount is missing
during the 24-hour period, the 7RRRR remark is encoded
as 7////.

These SHEF messages contain an hourly precipitation
accumulation. They are issued in “.A” SHEF message format and are made available and distributed in the same
manner as the SHEF 15-Minute Precipitation Criteria messages. The messages are identified within the NWS by their
RR7 AFOS/AWIPS header: “CCCRR7XXX.” All SHEF
data from FAA sites are contained in a collective and are
identified by the collective header “NMCRR7NKA.” These
messages are further distinguished from the 15-Minute
Precipitation Criteria messages by message type identification within the body of the message (see Section 5.5.2
for further details).

SHEF 15-Minute Precipitation Criteria message:
These messages are generated when the current 15-minute
period accumulation exceeds the locally established precipitation accumulation onset threshold and will cease when
the accumulation for the current interval falls below the
termination threshold. The precipitation accumulations for
the four most recent discrete 15-minute periods (ending at
H+00, H+15, H+30 and H+45) are chronologically listed
in these messages.

The SHEF Hourly Routine Precipitation accumulation
is derived from the sum of the 1-minute precipitation accumulation calculations during the latest discrete 60-minute
period. If any of the constituent 1-minute precipitation accumulation calculations are missing, the SHEF Hourly Precipitation message will report the precipitation
accumulation as “PPH M” (see Section 5.5.2).

These messages are issued in “.E” SHEF message format, and are initially disseminated through the NWS or the
FAA communications networks to the NWS Telecommunications Gateway (NWSTG) and the System Monitoring
and Coordination Center (SMCC) where they are made
available for redistribution to NWS offices and compilation on the daily Service Records Retention System (SRRS)
tape for the National Climatic Data Center (NCDC). These

Daily and Monthly cumulative precipitation totals:
These precipitation totals are summed each minute and included along with other data in the Daily Summary and
Monthly Summary Products/messages.

The information in the Daily Summary is valid for the
24-hour period beginning at 00:00 LST and ending at 23:59
LST. This message is issued on the following calendar day
at programmable transmission times (see Section 5.6 for
further details).

The tendency to under-report accumulation during
freezing rain or snow events is moderated by using a less
intense heat source to melt the frozen precipitation from
the tipping bucket at a slower rate. The slower heating rate
however, can sometimes allow unmelted snow to bridge
over the heated funnel surface and form a snow cap over
the orifice opening which prevents any further accretion
into the gauge.

The Monthly Summary message is valid for the calendar month. This message is issued on the first day of the
following month at a time specified by the system manager. The Monthly Summary message contains the
amount(s) and date(s) of the maximum 24-hour precipitation accumulation during the calendar month; the number
of calendar days with precipitation equal to or greater than
0.01, 0.10, 0.50 and 1.00 inches respectively; and departure of monthly cumulative precipitation totals from normal. Other parameters are also included in the Daily and
Monthly Summary messages. See Sections 5.6.1 and 5.6.2
for further details.

3.4.3

At some time after the precipitation event ends, under
bright sunshine and/or warming ambient temperature, the
snow cap melts and falls into the gauge. This causes a delayed accumulation to be registered and falsely reported at
a time when no precipitation is occurring. “False tips” may
also be caused by dew, frost, or heavy fog.

Precipitation Accumulation
Strengths and Limitations

There are well known problems (referenced in Section 3.4) associated with a HTB precipitation gauge. A
major problem occurs during high rainfall rate events when
the tipping bucket cannot keep up with the water flow and
under-reports the accumulation. The ASOS software corrects for the HTB bias to under report precipitation accumulation during most heavy rainfall events (greater than
1.80 inch per hour); however, during extremely heavy rainfall events (greater than 10 inches per hour), the HTB may
still under-report the total rainfall accumulation.
During freezing conditions, the application of heat to
melt snow and prevent gauge icing also induces evaporation or sublimation, especially during light freezing rain or
snow events at temperatures near 32°F. This results in a
tendency to under-report freezing or frozen precipitation
accumulation.

CHAPTER FOUR
4.0

Automating the Subjective
Weather Elements

While automating most objective weather elements is
fairly straightforward, there are numerous complexities in
automating subjective visual elements such as sky condition, visibility, and present weather. The major problem is
how to objectively quantify subjective human judgement.
With subjective elements, observers traditionally read
instruments and watch weather conditions in their area at a
fixed time to produce a “snap-shot” observation—a method
called spatial averaging. To create a similar observation,
automated systems repetitively sample conditions in a relatively small volume of air and then average these data over
a set time period—a technique called time averaging. Technological advances have made it possible for modern observations to progress from the periodic, subjective, spatial
averaging methodology of the observer to an objective,
nearly continuous, temporal averaging method of automated observing systems.

Figure 9.

ASOS ceilometer is located near the touchdown zone of
the primary instrument runway at most airports. At large
airports, a secondary Cloud Height Indicator (CHI) may
be located elsewhere on the airport to provide additional
information when there is a meteorological discontinuity.
At small airports the ceilometer may be collocated with
other sensors near a center-field location or touchdown
zone, depending on local siting requirements.

The rules for observing sky condition (clouds), visibility, present weather, and obscurations were designed
for observers and are still defined for subjective use; however, the FMH-1 now includes expanded rules for automated techniques. Because of the complexity of the
subjective weather elements, separate sections are devoted
to sky condition, visibility, and present weather. Each section discusses the differences between manual and automated rules and techniques.

4.1

Laser Beam Ceilometer

4.1.1

Cloud Height Indicator Sensor

The ASOS uses a laser beam ceilometer with a vertical measuring range of 12,600 feet and reporting range of
12,000 feet (Figure 9). The ASOS cloud sensor, or CHI, is
a vertically pointed laser transmitter and receiver. Its operation is similar to radar in that the time interval between
pulse transmission and reflected reception from a cloud
base is used to determine the cloud height. Sophisticated
time-averaging algorithms in the ACU are also used to interpret “cloud hit” information from the CHI and determine cloud height and amount.

Automating Sky Condition

Observers have used rotating beam ceilometers
(RBC) and the newer laser beam ceilometers (LBC) for
years to measure the height of clouds. Visual estimates were
still needed to determine the amount of clouds. The challenge of automating the data from such sensors was not
only to process the height accurately, but also to provide a
representative description of the amount of cloud coverage. Because the atmosphere is normally in motion, it was
found that processing the ceilometer signal through a sophisticated algorithm over a 30-minute time period provided an optimally representative and responsive
observation similar to that depicted by an observer. To be
sensitive to the latest changes in sky conditions, the most
recent 10 minutes of the data are processed twice (doubleweighted). To be most responsive to operational needs, the

The CHI consists of a gallium arsenide laser beam
ceilometer operating in the near Infrared (IR) portion of
the electro-magnetic spectrum at a wavelength of about a
0.9 microns. The instrument employs Light Detection and
Ranging (LIDAR) principles and computer algorithms to
provide cloud coverage and height information. The noise
inherent in the return signal varies with ambient tempera-

ture. To optimize the laser’s signal-to-noise performance,
the CHI’s pulse frequency varies with the ambient temperature. The frequency range is 620 Hertz (Hz) to 1,120
Hz with a nominal pulse frequency of 770 Hz at room temperature (68 ºF). The frequency is also modulated by the
age of the equipment to maintain a constant power output.
The width of the beam is confined to a divergence of ± 2.5
milliradians (mrad) so that at 12,000 feet the beam’s sample
area is a circle with a diameter of 60 feet.
The CHI reports will contain only opaque clouds.
Moisture layers, or thin clouds detected by the CHI and
considered too thin to be a cloud, will be reported as a restriction to vertical visibility or simply not reported. The
reporting of vertical visibility is dependent on the thickness and density of the moisture layer. To correctly classify these signals received by the ceilometer, sensor
software processes the data into three categories: “no hit,”
“cloud hit,” and “unknown hit.”

Figure 10. Example Of Cloud “Hit” At 4,500 Feet

4.1.2

The signal signature of a cloud return or “cloud hit” is
characterized by a rapid increase in backscatter when the
beam passes from the clear air beneath the cloud into the
moist conditions within the cloud. Figure 10 is an example
of a “cloud hit” at 4,500 feet. At the end of each 12-second
sampling, the CHI produces a detailed, high-resolution
back-scatter profile from which a unique determination of
the cloud base can be made. The cloud base “hits” (or returns) from each pulse are assigned to one of the 252 50foot vertical data bins within the 12,600 foot measurement
range. This results in a vertical resolution of 50 feet!

Sky Condition Algorithm

Computer algorithms (organized processing steps) are
used to process the signal data as described above into sky
condition reports that are encoded into METAR and SPECI
reports. Describing exactly how the algorithms work is quite
complicated, so what follows is a simplified explanation.
Every 30 seconds a sample is compiled from the CHI’s
back scatter returns taken from the most recent two or three
12-second processing intervals completed within the 30second period. Each 12-second interval processes more than
9,000 signals for back scatter returns. These data are processed to determine the height of the returns and whether

Not all signal returns exhibit the sharp signature pattern of a “cloud hit,” (as shown in Figure 10). Those signals without sharp returns are classified as “unknown hits.”
A return from deep fog where the scatter return pattern
extends from the surface to around 600 feet is shown in
Figure 11. These “unknown hits” are primarily caused by
precipitation and fog that mask the base of the clouds. This
broadened moisture field returns laser back scatter signals
from various heights within the moisture field. Because
these signals cannot be processed as a definite cloud return, they are processed as a vertical visibility (VV). VV is
defined as the distance in feet a person can see vertically
from the surface of the earth into an obscuring phenomena
or indefinite ceiling.
Figure 11. Example Of “Unknown Hit”

All the values of “unknown hits” are processed and
stored in height bins reserved for the VV values (separate
from the “cloud hit” bins). Finally, all the returns in the
VV bins are processed into a single mean height and assigned to a single bin. The VV returns are then processed
by the sky condition algorithms to determine cloud cover,
cloud height, or VV.

the sample compiled from these returns is a “cloud hit” or
an “unknown hit.” Every minute, ASOS processes the most
recent 30 minutes of 30-second sample data; the last 10
minutes of data are processed twice (double weighted) to
be more responsive to the latest changes in sky condition.
This technique provides a total of 80 samples; 40 in the

first 20 minutes and 40 in the last 10 minutes. The cloud
signal hits for the latest 30 minutes are then rounded or
“binned” to the nearest 100 feet for cloud heights between
the surface and 5,000 feet; to the nearest 200 feet for heights
between 5,000 and 10,000 feet; and to the nearest 500 feet
for heights above 10,000 feet.

human observation. All cloud layer heights are reported
Above Ground Level (AGL) with respect to field elevation. The cloud amounts reported by ASOS are Clear (CLR)
below 12,000 feet, Few (FEW), Scattered (SCT), Broken
(BKN), or Overcast (OVC). Table 3 shows the relationship between the ASOS measured cloud amount, the human equivalent, and the reported category.

Each minute, if more than fives bin height values have
been recorded (during the last 30 minutes), the cloud heights
are clustered into layers using a least-square statistical procedure until there are only five bins remaining (each bin
can have many “hits’ in it). These bins, or clusters, are
then ordered from lowest to highest height.

The sky condition algorithm also tests for total
obscurations. Necessary conditions for reporting totally obscured sky include a surface visibility of one mile or less
and a high percentage of “unknown hits” at or below 2,000
feet AGL. When these conditions are met, ASOS processes
and formats cloud return values classified as “unknown
hits” into the sky condition report as “VVaaa.” VV is an
obscuration; “aaa” represents the visible height in hundreds
of feet an observer can see vertically into the obscuring
phenomena.

Following this clustering, ASOS determines whether
the clusters can be combined into “meteorologically significant” height groups. This second clustering is done so
that very close layers are not reported (e.g., BKN030
OVC032). At the end of this combining process, all cluster
heights between the surface and 5,000 feet are rounded to

Table 3.

Finally, when a ceiling layer (BKN or OVC) is reported
below 3,000 feet AGL, an algorithm tests for conditions
requiring a variable ceiling remark. If these conditions exist, ASOS places the entry of CIG minVmax (where min,
max is the height in hundreds of feet of the upper and lower
limits of the variability) in the remarks section.

Cover ASOS Cloud Amount
Report —Percent of Sky

ASOS Measured
Amount in %
of sky cover

Human
Equivalent
in oktas

ASOS
Report

00 to < 05
> 05 to < 25
> 25 to < 50
> 50 to < 87
> 87 to 100

0
> 0 to 2/8
> 2/8 to < 4/8
> 4/8 to < 8/8
8/8

CLR
FEW
SCT
BKN
OVC

4.1.2a Meteorological Discontinuity
Sensors
A sky condition algorithm has been developed for use
where cloud formation (or advection) typically occurs in
(or from) a known location and results in significant concurrent differences in sky conditions over the airport. The
meteorological discontinuity algorithm uses output from
two CHI sensors. The primary sensor is sited near the touchdown zone of the primary instrument runway. The second
CHI is typically sited 2 to 4 miles away from the primary
sensor, upwind in the most likely direction of the advection, or closer to the fixed source of the unique sky condition. The second CHI serves to detect operationally
significant differences in sky conditions.

the nearest 100 feet. Above 5,000 feet, the algorithm rounds
the cluster height values to the nearest reportable value
(i.e., nearest 500 ft. up to 10,000 ft. and nearest 1,000 ft.
above 10,000 ft.). These bins now are called “layers” and
the algorithm will select up to three of these layers to be
reported in the METAR/SPECI in accordance with cloud
layer reporting priority as specified in FMH-1.

Information from the meteorological discontinuity sensor is included in the ASOS METAR under appropriate
conditions described below. Data from the primary and
meteorological discontinuity sensors are independently
processed through the single sensor algorithm and then
compared. Only data from the primary sensor is used in
the body of the METAR.

The amount of sky cover is determined by adding the
total number of hits in each layer and computing the ratio
of those hits to the total possible. If there is more than one
layer, the “hits” in the first layer are added to the second
(and third) to obtain overall coverage. For reporting purposes, the ASOS measured cloud amount for each layer is
then converted to a statistical functional equivalent of a

Table 4.

Criteria For Reporting A Meteorological Discontinuity Ceiling Remark

CEILING REPORTED
BY PRIMARY SENSOR

CEILING DIFFERENCE CRITERIA
(Primary - Met Discontinuity)

UP TO 1,000 FEET
> 1,000 FEET UP TO 3,000 FEET
> 3,000 FEET UP TO 5,000 FEET
> 5,000 FEET UP TO 8,000 FEET
> 8,000 FEET

> 300 FEET
> 400 FEET
> 600 FEET
> 1,000 FEET
> 1,600 FEET

4.1.3

If the primary sensor is not operational, the sky condition is not reported, a maintenance check indicator ($) is
appended to the METAR, and no further comparisons are
made. If the meteorological discontinuity sensor is not operational, the remark “CHINO LOC” is added to the
METAR, where LOC is the nominal location of the meteorological discontinuity sensor (e.g., CF, RWY26L), a maintenance check indicator ($) is appended to the METAR,
and no further comparisons are made.

Sky Condition Strengths
and Limitations

The ASOS ceilometer is a vertically pointing laser that
measures the height and infers the amount of cloud elements that pass over the sensor. It operates continually and
once every minute it determines the sky condition for the
most recent 30 minutes. The sensor does not measure or
know what is happening along the horizon, nor does it report on clouds above 12,000 feet. The horizontal limitation can be partially overcome by using a second
meteorological discontinuity CHI sensor at selected major
airports where studies have shown that more than one sensor is needed. Due to the inherent motion of the atmosphere
however, a single CHI is usually sufficient to report accurate cloud conditions.

If both sensors are operational and report a ceiling,
and the ceiling reported by the meteorological discontinuity sensor is lower than the ceiling reported by the primary
sensor by the difference criteria listed in Table 4, then a
remark “CIG VALUE LOC” is included in the METAR
where VALUE is the ceiling height (in hundreds of feet)
reported by the meteorological discontinuity sensor. For
example, if the primary sensor reported a ceiling at 8,000
feet and the meteorological discontinuity sensor reported a
ceiling at 6,500 feet on runway 26L then the remark “CIG
065 RWY26L” is reported.

A common concern is that the algorithm does not respond fast enough to changes in the sky condition. If a
solid cloud deck suddenly appears, the algorithm will report a “FEW” layer within 2 minutes and a “BKN” ceiling
layer within 10 minutes. One limitation however is that if
the CHI becomes inoperative for three consecutive 30-second readings (1.5 minutes) or misses five or more readings
in the most recent 30 minutes, it does not report sky condition. The algorithm requires a full 30-minutes of data from
the CHI after restart before a valid sky condition report is
once again generated.

4.1.2b Back-Up Sensors
At some locations, mainly major airports, a back-up
CHI may be available. The back-up is generally collocated
with the primary sensor suite while the meteorological discontinuity CHI is not. The back-up sensor operates continuously in stand-by mode. When a METAR/SPECI report
is created, the ASOS checks for data from the primary CHI
sensor. If the primary sensor is inoperative, then ASOS
will interrogate the back-up sensor, process that data
through single-sensor algorithms, and enter the value into
the report. When the primary sensor is back in service, the
back-up sensor is once again placed in stand-by mode.

Precipitation poses a double challenge. Because the
laser can only look vertically, precipitation particles directly
overhead will scatter the laser light, often leading to an
increased number of “unknown” signal returns. The algorithms described above help distinguish a vertical obscuration caused by falling rain drops from the base of the
cloud layer above it. Precipitation can also fall onto or collect on top of the instrument. To limit that problem, a blower
is used to move air over the slanted glass cover housing of
the laser lens.

Occasionally, the laser CHI can see “too many”
clouds. For example, a very sensitive laser will sometimes detect invisible moist layers before they coalesce
to become visible clouds. Also, occasional sharp
inversions, especially in very cold winter conditions,
may trigger the report of clouds. Algorithm developers
will continue to work on eliminating these small but
annoying traits.
There are times when the CHI may report fewer
clouds than the observer. Studies have shown that up to
20 percent of the reports of “FEW” cloud events will be
missed by using only one ceilometer. These events are
usually widely scattered fair weather cumulus, which the
ASOS may report as “CLR.”
What are some of the advantages of ASOS? It will
report the onset of low stratus moving over the ceilometer
within 2 minutes, and the formation/dissipation of a low
ceiling within 10 minutes. If a low cloud layer forms or
falls below 1,000 feet AGL, or if the ceiling rises to equal
or exceed 1,000 feet AGL, the ASOS will transmit a SPECI
report. ASOS will continually monitor the sky condition
and automatically create and issue a report with a speed
and responsiveness comparable to an observer. The
ceilometer does not suffer from night adaptation. Observers must wait for their eyes to adapt to the dark before
being able to accurately distinguish night time sky conditions. It is difficult, even for the experienced observer, to
distinguish gray on gray when lower stratus cloud layers
move in under higher stratus clouds.

Figure 12. Example of Packing Effect

The value reported as the visibility may reflect only
the distance an observer can see due to obstructions such
as nearby buildings and trees, not how far the eye can actually see in the absence of such obstructions. It is not unusual to see differing reports of visibility observations under
the same conditions because of perspective. The challenge
facing observing system developers was to automate visibility reporting to accurately reflect both the meteorological and non-meteorological elements that enter into the human
evaluation of visibility.
The NWS and FAA jointly developed the concept of
Runway Visual Range (RVR). This concept combined visibility theory with extensive empirical testing of
transmissometer measurements to represent the distance a
pilot could see runway lights or objects during take off or
landing. Thus, it was natural to apply this knowledge in
automating the measurement of surface visibility.

Finally, ASOS reports more broken cloud layers and
fewer overcast layers than an observer because the ceilometer does not suffer from the “packing effect” (Figure
12). The “packing effect” is a condition where the observer
tends to overestimate the cloud coverage because clouds
near the horizon appear to blend together or overlap. This
effect is due to the curvature of the earth and parallax view
of distant objects.

4.2

The task of automating surface visibility began earnestly in the early 1970s when the FAA and NWS jointly
conducted the AV-AWOS experiments. These test demonstrate the feasibility of automated surface observations.
By the 1980s, the agencies were able to show that it was
practical to automate surface visibility and report a value
that was suitable and responsive to the needs of the aviation community. From these tests, a working group in 1981,
under the auspices of the Office of the Federal Coordinator
for Meteorology (OFCM), produced a set of definitions
and guidance for automating sensor derived visibility.

Automating Surface
Visibility

Visibility remains one of the most difficult elements
to automate. Sensors can successfully measure atmospheric
elements relating to visibility, but it is very difficult to
relate these measured elements to the characteristics of
human vision. The physical limitations of the human eye
and human subjectivity are greatly impacted by precipitation, day and night vision adaptation, contrast, physical
obstructions, and perspective.

4.2.1

Principles in Visibility
Automation

In the early 1970s, NWS scientists developed the concept of Sensor Equivalent Visibility (SEV). SEV was defined as any equivalent of human visibility derived from
instrument measurements. This means that visibility is not
measured directly but is inferred by measuring other physical characteristics and properties of the air. These properties include the transmittance (or conversely attenuation)
of light due to absorption and scattering by atmospheric
contaminants such as aerosols, course particulates, and
hydrometeors. Studies have shown that human visibility is
closely correlated with transmittance. A sensor can measure this atmospheric capacity and quantify it through sophisticated algorithms to represent visibility. This is SEV.
Before automating visibility and applying the SEV
concept, developers had to create a standard value that could
be consistently reproduced in electronic sensors. The fundamental measure of atmospheric clarity, the extinction
coefficient, was defined by using the Optec Transmissometer, which measures the attenuation of light by scattering
and absorption in the mid-visible light wavelength of 550
nanometers (550 billionths of a meter). It was found that,
in general, scattering was the primary cause of attenuation
(and therefore visibility reduction). Furthermore, scatter
meters were less costly to build and easier to install and
maintain than transmissometers. Therefore, qualification
testing focused on scatter meters with the goal that they
could be used as an alternative to the Transmissometer.

Figure 13. Forward Scatter Visibility Sensor

A forward scatter meter was found to correlate better
with Transmissometer extinction coefficients and human
visibility than a back scatter meter, particularly during snow
events. Later testing showed that with slight adjustments
in derived visibility based on the type of weather (i.e., rain
or snow) a forward scatter meter was fully acceptable as a
visibility sensor. A visible light source was found to work
best with the forward scatter meter because of better agreement with observers during small particle (e.g., haze)
events.

A photocell on the visibility sensor turns on at dawn
or off at dusk at a light level between 0.5 and 3 foot candles
(deep twilight). This function determines whether the sensor uses the day or night equation to calculate visibility.
For a given extinction coefficient, the day calculation will
provide a visibility from 1/2 to 1/3 of that derived by the
night equation. Therefore, an abrupt change in visibility
may be reported after sunrise or sunset if there is a significant obstruction.
SEV and the prevailing visibility (PV) of the observer
are based on different concepts. The SEV in ASOS is derived by frequently measuring the forward scatter characteristics of a small atmospheric sample (0.75 cubic feet)
near the sensor and processing these data for 10 minutes.
PV, as determined by the observer, relies on visual interpretation of current conditions in a full 360 degree circle
around the observation point. This is not done continuously, but only during hourly observation time and when
significant weather changes are occurring and noted. SEV
and the human-derived prevailing visibility agree most con-

The next step was to develop algorithms to process
the sensor data into a representative visibility. This step
required identifying those key elements the human eye responds to in determining visibility. During the day, the
human eye relies on variations in contrast between the visibility target and its background when viewing through fog,
mist, haze, rain, or snow. At night, the human eye uses a
distant unfocused light to determine visibility. The illuminance threshold, like contrast threshold, assumes that the
average observer can see a light source of two mile-candle
illuminance.

sistently when the weather is homogeneous over a large
area; however, testing has shown that there is also strong
agreement between human and automated visibilities, even
during periods when weather conditions are quite variable.
The AV-AWOS experiments in 1979 showed that the
values obtained from three visibility sensors (in a 3-mile
triangular arrangement) were, in most cases, in close agreement. Further testing supported the premise that only one
sensor, located in a representative area, could be used to
describe the visibility for an airport area. It was still recognized that sites plagued by advection fog or other visibility
discontinuities would require additional sensors. Every airport required a site survey to properly locate the visibility
sensor and identify locations that might need additional
sensors.
The location of the visibility sensor(s) was critical. The
small sampling volume of the sensor dictated that the sensor be located as near to the area of concern as possible. As
a result, most primary visibility sensors were placed near
the touchdown zone (TDZ) of the primary instrument runway. The actual siting of the sensors had to meet, as closely
as possible, the requirements established by the Federal
Standard for Siting Meteorological Sensors at Airports, and
the clearance requirements of the FAA. This included consideration of local sources of visibility reduction (e.g.,
plowed fields, snow blowing operations, and smokestacks),
the proper sensor height (at least 10 feet), and adequate
obstacle clearances.

4.2.2

Figure 14. Visibility Sensor—Top View
impinge on the detector lens. Only that portion of the beam
that is scattered forward by the intervening medium in the
sampling volume is received by the detector (see Figure
14). The sensor sampling volume is 0.75 cubic feet and the
sensor response time is 20 seconds. A measurement sample
is taken every 30 seconds. Visibility sensor measurement
accuracy is specified in reference to comparison with two
NWS visibility standards and is summarized in Table 5. In
this regard, the forward scatter sensor has shown excellent
performance when compared with the “Optec” Transmissometer standards.

Forward Scatter Sensor

The ASOS visibility sensor (Figure 13) operates on a
forward scatter principle in which light from a pulsed Xenon flash lamp in the blue portion of the visible spectrum
is transmitted twice a second in a cone-shaped beam over
a range of angles. The projector and detector are protected
with a lens hood and canted down at 15 degrees from the
horizon to prevent snow blockage.

Visibility in METAR is reported in statute miles (SM).
The reportable increments are: M1/4SM, (less than
1/4SM), 1/4SM, 1/2SM, 3/4SM, 1SM, 1 1/4SM, 1 1/2SM,
1 3/4SM, 2SM, 2 1/2SM, 3SM, 4SM, 5SM, 6SM, 7SM,
8SM, 9SM and 10SM. Note that visibilities between zero
and less than 1/4 mile are reported as M1/4SM8. Measured
visibilities exactly half way between reportable values are
rounded down. Visibilities of 10 miles or greater are reported as “10SM.”

The detector “looks” north to minimize sun glare, particularly from low sun angles near sunrise or sunset. To
optimally balance the detection efficiency and differentiation ability of the sensor under varying conditions, a nominal 45 degree horizontal incident angle is set between the
projector beam and the detector field-of-view within the
sampling volume. This is achieved by offsetting the projector about 45 degrees to the left (i.e., northwest) of the
detector. As a result, the projector beam does not directly

4.2.3

Visibility Algorithm

The ASOS visibility algorithm samples sensor data
every minute, obtaining a 1-minute average extinction coefficient and day/night indication. The algorithm calculates
a 1-minute average visibility every minute and stores the
value for 10 minutes. Finally, ASOS computes a running
10-minute harmonic mean once per minute from the stored
data to provide the latest visibility.

Manual observations of 0, 1/16, and 1/8SM may be
augmented.

Table 5.

Visibility Sensor—Accuracy in Statute Miles
(As compared to NWS Standard Transmissometer)

ASOS
Visibility
Measurements

at least 80%
of data within
these limits.

at least 98%
of data within
these limits

all data
within these
limits

0 thru 1 ¼
1 ½ thru 1 ¾
2 thru 2 ½
3 thru 3 ½
4 thru 10

±¼
+ ¼, - ½
±½
+ ½, - 1
± 1 RI*

+½
+ ½, - ¾
±1
+ 2 RI*/ - 1
± 2 RI*

±1
±1
±1
+ 2 RI*/ -1
± 2 RI*

*RI + Reportable Increment

A harmonic mean is used in the final computation
rather than an arithmetic mean because it is more responsive to rapidly decreasing visibility conditions and will
generally yield a lower value than the arithmetic mean. This
result is preferable because it provides an earlier warning
of deteriorating conditions. Conversely, when visibilities
are rising rapidly, the harmonic mean will be slower than
the arithmetic mean in reporting the improving visibility.
This conservative bias is intended to provide an additional
margin of aviation safety. A 10-minute computation and a
harmonic averaging technique was chosen to strike an optimal balance. A longer averaging period tends to dampen
short-term non-homogeneous fluctuations while harmonic
averaging may respond conservatively to rapidly changing conditions. The harmonic mean is computed from the
formula:

The value obtained from this computation is then
rounded down to the nearest reportable visibility value. This
visibility is stored in memory for 31 days as part of the
METAR/SPECI reports and made available for distribution. The raw extinction coefficient and day/ night settings
are stored for 12 hours in the engineering data for maintenance technician monitoring purposes. Eight one-minute
data samples in the last 10 minutes are required to form a
report. If less than 8 of the current 10, 1-minute visibility
values are available in memory, the 1-minute visibility is
not reported by ASOS.
The newest visibility value is checked against the visibility reported in the last METAR or SPECI report to determine if it has passed any specified visibility thresholds.
If so, ASOS creates a SPECI report. The algorithm also
checks for variable visibility. If requirements for variable
visibility are met, ASOS generates an appropriate automated remark.

V = n / (1/V1 + 1/V2 + ...+ 1/Vn)
Where V is the harmonic mean, n = 10, and V1, V2,... Vn,
are the individual 1-minute values.

At airport locations with control towers, the ASOS provides the ability to enter tower visibility. If tower visibility
is less than 4 statute miles and is also less than the reported
surface visibility, then tower visibility is entered in the body
of the report and surface visibility is entered in remarks. If,
on the other hand, the surface visibility is less than the tower
visibility, then surface visibility is entered in the body of
the report and tower visibility is entered in remarks.

The difference between the harmonic mean and the
arithmetic mean can be seen when a fog bank moves in
and suddenly drops the visibility from 10 miles to 1/4 mile.
Initially there are 10, 1-minute values of visibility equal to
10 miles and both the arithmetic and harmonic mean are
reporting 10 mile visibility. In the first minute, there are
nine values of 10 miles and one value of 1/4 mile used to
compute the current visibility. The arithmetic mean yields
9.025 miles while the harmonic mean yields 2.041 miles.
In the second minute, there are now eight values of 10 miles
and two values of 1/4 mile used to compute current visibility. In this case, the arithmetic mean is 8.050 miles and the
harmonic mean is 1.136 miles.

4.2.3a Meteorological Discontinuity
Visibility Sensor
At some airports a second visibility sensor is placed
where unique weather, not necessarily representative of the
entire airport, may impact flight operations for a portion
of the airport or a particular runway. The secondary

4.2.4

meteorological discontinuity visibility sensor may be used
to provide an early alert of deteriorating conditions, such
as fog rolling in off a nearby bay or river. Whereas the
primary visibility sensor is sited near the touch-down zone
of the primary instrument runway, the second (meteorological discontinuity) visibility sensor is sited about
2-4 miles away in the most likely location for a meteorological discontinuity.

One of the main advantages of the visibility sensor is
its location at the touchdown zone of the primary instrument runway where it provides a precise visibility value
appropriate for that location. Another strength of the sensor is consistency of observations. Under the same weather
conditions, an ASOS visibility reported at one site will be
identical to the visibility reported at another site because
both ASOSs use identical sensors and algorithms. Variations introduced by human observers from such limitations
as perspective, sun angle, day and night differences, and
poor locations are eliminated.

The data from the primary and meteorological discontinuity sensors are independently processed through the
single sensor algorithm and then compared. Only data from
the primary sensor is used in the body of the METAR.
If the primary visibility sensor is not operational, the
visibility is not reported, a maintenance check indicator
($) is appended to the METAR, and no further comparisons are made. If the meteorological discontinuity visibility sensor is not operational, the remark “VISNO LOC” is
added to the METAR, where LOC is the nominal location
of the meteorological discontinuity visibility sensor (e.g.,
CF, RWY26L), a maintenance check indicator ($) is appended to the METAR, and no further comparisons made.

During certain daylight hours the human eye can
readily be affected by light back-scattered toward the observer. This may occur around sunrise or sunset when sunshine is pouring into a low cloud deck or a local weather
obstruction, (such as mist, snow, rain, or haze) from behind the observer. In this situation, the bright sky and the
back-scattered light can overwhelm the human eye causing the observer to report a lower visibility than ASOS.
This back-scattering of light from the sun and sky into the
observer’s vision by particulates in the air is called
“airlight.”

If both sensors are operational, a meteorological discontinuity visibility remark is reported when the visibility
measured by the meteorological discontinuity sensor is less
than 3 miles and is also less than the visibility measured
by the primary visibility sensor by one-half mile or more.
When these conditions are met, a remark in the form “VIS
VALUE LOC” is added to the METAR, where VALUE is
the visibility reported by the meteorological discontinuity
sensor and LOC is the nominal location of the meteorological discontinuity sensor. For example, a meteorological discontinuity visibility value of 1SM on runway 26L is
reported as: VIS 1 RWY26L.

4.2.3b

Visibility Strengths and
Limitations

Visibility restrictions under back-scatter conditions are
considerably less common than under forward-scatter conditions. Under airlight conditions, a direct 180 degree backscatter reflection is required for reduced human visibility.
Visibilities in other directions are not as restricted. Consequently, airlight visibilities are not representative
of the entire viewing area. Under forward-scatter conditions, however, ASOS uses a wider 0-45 degree scattering
angle range to obtain a more representative measurement
of conditions.

Back-Up Visibility Sensor

The major disadvantage of an automated forward scatter sensor is the small sampling volume (0.75 cubic feet)
which can give undue weight to small scale differences in
the atmosphere. Even broadening the sample by including
all the sensor measurements over a 10 minute period (20
samples) may not provide enough data to create a visibility report that represents the entire airport area. For instance,
if patches of fog (BCFG) or shallow fog (MIFG) form, or a
fog bank moves over one end of the runway, ASOS may not
“see” it if the sensor is not in the right location. Initial site
surveys were conducted at each airport to determine proper
placement of the primary visibility sensor. Follow-on studies
are conducted to decide if additional sensors are needed.

A back-up visibility sensor is available at selected major airports where a requirement has been established. It is
located within 2 statute miles of the primary visibility sensor and operates continually in “sleep mode,” i.e., measurements are taken, stored internally in ASOS, but not
included in the METAR report until activated by failure of
the primary sensor. The ASOS algorithms process the primary and back-up sensor data separately and determine
when data from the back-up sensor are substituted in the
observation. When the primary sensor is restored the backup sensor returns to “back-up” status.

4.3

Automating Present
Weather and Obscurations

As noted earlier, the algorithms developed for automated reporting of visibility and sky condition required a
new perspective. By contrast, the automated detection and
reporting of present weather and obstructions required
whole new observing technologies and techniques. Present
weather and obstructions arguably are the most complex
and difficult elements to automate due to their variety of
composition and appearance. Once the first generation of
present weather sensors was developed, the reporting algorithms were refined to give a present weather report similar to the human report. Improvements in sensor capability
and algorithm sophistication have resulted in the current
generation of highly-capable, automated, present weather
and obstruction reporting technology.
The ASOS employs a variety of sensors to correctly
report present weather and obscurations. A new lightning
sensor has recently been added to the ASOS sensor suite at
a limited number of sites, which provides information on
thunderstorms. The precipitation identification (PI) sensor
discriminates between rain and snow; a freezing rain sensor detects ice accretion caused by freezing precipitation;
and the visibility sensor and hygrothermometer provide data
for algorithms that further refine the reports of present
weather and infer the existence and type of obscuration.

4.3.1

Figure 15: Lightning Sensor

bins: 0 to 5 miles, 5 to 10 miles, and 10 to 30 miles. Because the cloud-to-cloud detection is less efficient than
cloud-to-ground detection, the ASOS considers cloud-tocloud strikes to be within 5 miles.
A cloud-to-ground strike is made up of one or more
individual flashes. Within one flash, numerous discharges
can occur; these individual discharges are called strokes.
The sensor groups all strokes occurring within 1 second
of each other into a single flash. The range of a cloud-toground strike is determined by the range of the closest
stroke within a flash.

Single Site Lightning Sensor

Where required, the ASOS uses the Global Atmospherics Inc. (GAI) Model 924 single site lightning sensor as a
source for reporting a thunderstorm (see Figure 15.Lightning Sensor). The ASOS Lightning Sensor (ALS) is installed at selected Service-Level “D” ASOS sites that do
not have the FAA Automated Lightning Detection And Reporting System (ALDARS). The ALDARS is another
source of lightning information provided through the National Lightning Detection Network described in Section
4.3.3. The ALS sensor is a single-point omnidirectional
system that requires two criteria before reporting a thunderstorm: an optical flash and an electrical field change
(radio signal), which occur within milliseconds of each
other. The requirement for simultaneous optical and radio
signals virtually eliminates the possibility of a false alarm
from errant light sources.

The sensor automatically “ages” each lightning strike
for 15 minutes. Because a thunderstorm is defined to be
in progress for 15 minutes after the last lightning or thunder occurs, the sensor continues to report each strike in
the appropriate bin for 15 minutes after it is first detected.
A thunderstorm is determined to end when no strikes are
detected within the last 15 minutes.

4.3.2

Single Site Lightning
Sensor Algorithm

ASOS polls and processes data from the single site
lightning sensor once a minute. The raw sensor data can
be viewed through Direct Command Mode using a
“THUNDER” command. To determine the starting and
stopping times of a thunderstorm, ASOS examines data

The sensor can detect cloud-to-ground and cloud-tocloud strikes. All strikes are counted, but only the cloudto-ground strikes are used to generate an estimate of the
range. Cloud-to-ground strikes are grouped into three range

in the 0-5 and 5-10 mile cloud-to-ground range bins and
in the cloud-to-cloud bin. For the single site lightning sensor, ASOS does not use data in the 10-to-30 mile range bin.

The FAA has constructed a processing architecture
called Automated Lightning Detection and Reporting Systems (ALDARS) at each of the 20 Air Route Traffic Control Centers (ARTCCs) in the CONUS. There is no NLDN
lightning coverage in Alaska or Hawaii. The AWOS Data
Acquisition System (ADAS) at each ARTCC filters NLDN
data for its particular ARTCC coverage area and identifies
strikes within 30NM of any Airport Reference Point (ARP)
within the ARTCC coverage area. A strike is placed in one
of three range bins: within 5NM of ASOS (at the airport),
5-10NM of ASOS (vicinity of the airport), and 10-30NM
(distant from ASOS). These data are sent to the ASOS by
the ADAS where they are processed and treated by ASOS
as any other sensor derived data, and are included in the
transmitted ASOS reports when appropriate. At sites that
may concurrently receive ALS and NLDN-ALDARS information, the ALS data will take precedence for processing and reporting by ASOS. Current plans, however, call
for no sites to have both ALS and ALDARS data.

A thunderstorm is declared to start when the sum of
strikes in the three bins (0-5, 5-10, and cloud-to-cloud) is
equal to, or greater than, two. To minimize the possibility of
a false alarm, the ASOS algorithm requires two strikes to
start a thunderstorm; false alarms are more likely with a singlestroke starting algorithm. ASOS declares a thunderstorm to
end when the last strike is “aged” out of the three bins and the
sum of the strikes in the three bins falls to zero.
There are two possible thunderstorm reports: “TS”
when strikes are occurring within 5 miles, and “VCTS”
when strikes are occurring outside 5 miles but within 10
miles. ASOS transmits a SPECI report at the start and end
of any thunderstorm condition.
To begin a “TS” report, the two-or-more strike condition must occur in one of the following formats:
n
n
n
n
n
n

4.3.4

0-5 bin only
cloud-to-cloud bin only
0-5 and 5-10 bins
0-5 and cloud-to-cloud bins
cloud-to-cloud and 5-10 bins
0-5, 5-10, and cloud-to-cloud bins

A “VCTS” report is transmitted when the strikes occur only in the 5-10 mile bin. If a thunderstorm begins as a
“VCTS” report, and a single strike is detected in either the
0-5 or the cloud-to-cloud bin, the report is changed to “TS.”
Similarly, a “TS” report will be changed to “VCTS” when
the only strikes detected are occurring in the 5-10 mile bin.

4.3.3

The ALS occasionally reports valid lightning within
10 miles when thunder cannot be heard by an observer.
An extensive field evaluation in the summer of 1997
indicated that the sensor can report 15-20% more valid
thunderstorm minutes than an observer, primarily because an observer’s ability to hear thunder at most airports is restricted.

Due to the two-part criterion (optical and radio) for
identifying lightning strikes, the sensor has an inherently low false alarm rate. The 1997 field evaluation
indicated a false alarm rate of 0.0039 (i.e., about 0.4
percent of ASOS thunderstorm minutes may be false).

The ranging estimates provided by the sensor have
significant uncertainty and can contribute to reporting
thunderstorms when cells are beyond 10 miles. A very
strong lightning strike at 15-20 miles can be erroneously placed in the 5-10 mile bin by the ASOS sensor.

The 1997 field evaluation identified cloud-to-cloud
strikes detected at ranges near 30 miles. The cloud-tocloud bin was retained in the ASOS algorithm because
of the importance of cloud-to-cloud lightning in identifying thunderstorms. A significant number of thunderstorms contain primarily cloud-to-cloud lightning
and if it were eliminated from the reporting algorithm,
there would be a risk of failing to report thunderstorms.

Automated Lightning Detection
And Reporting System

A potential alternative source of lightning information
is the National Lightning Detection Network (NLDN) operated by GAI. It consists of approximately 100 lightning
direction finder sensors located throughout the Continental U.S. (CONUS). The sensors record the parameters of
each cloud-to-ground lightning strike with precise time and
location. All strike reports are sent by satellite to the GAI
processing center at Tucson, AZ, where a computer processing algorithm compares strike reports from multiple
sensors, computes each strike location, and provides this
information to the FAA communications network within
seconds of the strike.

ASOS Lightning Sensor
Strengths and Limitations

1 millimeter horizontal slit aperture through which the transmitter light beam passes before it is focused by the lens
and impinges on the photo diode. The narrow aperture
makes the receiver more sensitive to beam fluctuations
caused by particles down to the size of a small raindrop
(0.04 inch diameter).
Because the slit is much wider than its height, the receiver is more sensitive to beam fluctuations induced by
the vertical velocity component of particles passing through
the beam than the horizontal component. Built-in sensor
algorithms minimize the possibility of any false identification caused by greater sensitivity.
As a particle of rain or snow passes through the coherent light beam, the particle creates a shadow that modulates the light, which then passes through the receiver’s
horizontal slit aperture as a partially coherent (intermittently
disrupted), colliminated (parallel to the slit) beam. The
shadow varies depending on the size and speed of descent
of the particle as it falls across the receiver.
When many particles fall through the beam, a scintillation pattern is created. The fluctuating beam pattern is
sensed by a photo diode and amplified, creating a jumble
of frequencies containing information on the size and speed
of the falling particles. A spectral analysis reveals how much
energy or power is contained in the various frequency
bands. For example, a predominance of power in low frequencies from 75 to 250 Hz indicates snow. When energy
is predominantly in a band from 1000 to 4000 Hz, the precipitant is almost certainly rain. The LEDWI registers rain
and snow mixed as a “smearing” of the spectral power,
which is usually reported by ASOS as unknown precipitation (UP). This analysis is the basis of the discrimination
algorithm, which differentiates rain from snow.

Figure 16. The Precipitation
Identification Sensor

4.3.5

Precipitation Identification
Sensor

When the precipitation is not mixed, (i.e., pure rain or
pure snow) the LEDWI can determine the intensity of the
precipitation. The intensity is determined by the power of
the signal return in the rain (1-4 kHz) or snow (75-250 Hz)
portion of the power spectrum. The power return of rain is
derived from the size and fall velocity of the particles whose
size distribution, correlates well with liquid water content.
It is possible to accurately determine the rain intensity
through an empirical relationship (the Marshall-Palmer distribution), which can distinguish light (up to 0.10 inch per
hour), moderate (0.11-0.30 inch per hour), or heavy (greater
than 0.30 inch per hour) intensities for rain.

The Precipitation Identification sensor (PI), better
known as a Light Emitting Diode Weather Identifier
(LEDWI), differentiates rain from snow and determines
the intensity of the precipitation.
The LEDWI contains a coherent light transmitter (i.e.,
there is a continuous relationship among the various phases
of the light waves within the beam) and a photo diode receiver. The transmitter and receiver are mounted on a crossarm 10 feet above the ground or base of the platform. They
are equipped with heated lens hoods, face directly at each
other, are separated by a distance of 2 feet and are oriented
in a north-south direction with the receiver looking north.
(See Figure 16 for a view of the LEDWI.)

In the case of snow, it is again the size and fall velocity of the snowflakes that determines their size distribution. This correlates well with rate of snow accumulation.
Unlike rain however, the density of snow can vary significantly depending on whether the snow is “wet” or “dry,”
and so the liquid content cannot be accurately determined.

The transmitter generates a coherent Infrared (IRED)
light beam, 50 millimeters in diameter, aimed directly at
the receiver. The receiver lens is masked with a narrow
34

The LEDWI report of intensity is well correlated with the
rate of snow accumulation but is not directly related to the
visibility reduction due to snow, which observers use to
differentiate between light, moderate, and heavy snow.
ASOS processing algorithms use the visibility sensor data
to modify the LEDWI snow intensity report so that moderate snow is not reported when the visibility is greater than
1/2 mile and heavy snow is not reported when the visibility is greater than 1/4 mile.

tion is based on the hierarchal scheme for reporting present
weather: liquid (-RA, RA, +RA), freezing (-FZRA, FZRA),
and frozen (-SN, SN, +SN) in ascending order.
ASOS reports only one precipitation type at a time.
For instance, if both freezing rain and snow are detected,
snow is reported. Additional precipitation elements may
be added to the report by observers. The PI algorithm performs a temperature check on the PI sensor output. If -SN,
SN, or +SN is reported, and the temperature is > 38 F, then
the PI sensor output is set to “No Precipitation” (NP). The
PI algorithm also formats and reports precipitation beginning and ending remarks just as the observer does.

The LEDWI has an “adaptive baseline,” which adjusts
the power spectrum threshold to reduce reports of false
precipitation. For instance, wind turbulence and thermals
(e.g., the shimmer seen across an open field on a sunny
day) can induce scintillations that are near the frequencies
characteristic of snow. Therefore, ASOS sets the threshold
for snow detection above the spectral power induced by
turbulence.

Once the precipitation type is determined from the last
10 minutes of data, then the 1-minute samples from the
past 5 minutes are used to compute intensity. Precipitation
intensity is determined from the highest common intensity
derived from three or more samples. Common intensities
for heavy precipitation are light, moderate and heavy; for
moderate precipitation, common intensities are light and
moderate. For example, if rain is the determined precipitation type, and there are three moderate rain and one light
rain detected in the past 5 minutes, then ASOS reports
moderate rain in the METAR/ SPECI report. Likewise, if
snow is the determined type and there are one light, two
moderate, and one heavy snow in the past 5 minutes, then
moderate snow is reported.

This “adaptive baseline” can pose a problem when
snow increases so slowly that the baseline rises without
snow being detected. When this occurs, only a sudden increase in snow may trigger the sensor to report snow. A
similar condition may occur when the LEDWI is turned
on (say after a power failure) and precipitation is falling.
The initial adaptive baseline may be set much too high to
detect precipitation correctly and will not be reset to a representative threshold until the precipitation ends.

As a third example, if the most recent 5 minutes of
sensor data contains one report of light rain, and two reports of moderate rain, then light rain is reported by ASOS.
If, on the other hand, there are less than three common
intensities of the reported precipitation type, then ASOS
reports the lightest intensity. For example, if rain is the
determined type, and there are only one moderate rain and
one heavy rain reported by the sensor, then the precipitation intensity is set to moderate rain (RA) in the ASOS
METAR/SPECI report. If snow is the determined precipitation type and there are only one moderate and one heavy
snow, then ASOS reports moderate snow.

Rain detection is generally not a problem. Occasionally, if the rain is preceded by a gradually increasing drizzle,
the rain channel adaptive baseline threshold may rise to a
point where light rain is not sensed. As a general rule, rain
lighter than 0.01 inches per hour will not be detected.

4.3.6

PI Algorithm

Every minute, the PI algorithm requests the PI sensor
data, stores the data in memory for 12 hours, and examines
the latest 10 minutes of data stored in memory. If three or
more samples are missing, the algorithm sets the sensor status to “inoperative” and reports “missing” for that minute. If
the 10-minute memory buffer contains less than two sensor
samples of precipitation, the precipitation report terminates.

The ASOS PI algorithm formats and reports precipitation beginning and ending remarks just as the observer
does. The PI sensor output is further compared with the
freezing rain sensor output to ultimately determine the precipitation reports and remarks (see description of the freezing rain algorithm for further details).

If, however, two or more samples in the latest 10 minutes indicate precipitation, the algorithm then determines
the type and intensity to report. In general, to report anything other than “Unknown Precipitation” (UP), two of the
samples are required to be the same type. If there is a tie
between two types of precipitation (e.g., two rain samples
and two snow samples) snow is reported. This determina-

Under blowing snow conditions, particularly where
snow is blown to a height of 10 feet or more, the PI sensor
(LEDWI) can mistakenly interpret the scintillations from
blowing snow rising up and/or settling through the sensor’s
IRED beam as rain, and occasionally as snow, depending
on the vertical velocity of the snow particles.

At higher wind speeds (10 knots or greater), the upward vertical velocity of the snow particles usually is sufficient to be incorrectly interpreted as rain. The ASOS
blowing snow algorithm interrogates the LEDWI and other
ASOS sensors for concurrent blowing snow conditions and
rain reported by the LEDWI. When this “error” is detected,
ASOS replaces the false LEDWI rain report with a correct
blowing snow report in the ASOS METAR/SPECI present
weather field. This change is made by evaluating sky condition and 15-minute average data for ambient temperature and wind. A 15-minute average is used to reduce the
risk of the output oscillating from one present weather report “solution” to another. ASOS evaluates all LEDWI reports of rain with an ambient temperature of 32 oF or less.
Under these conditions, either blowing snow (BLSN) or
unknown precipitation (UP), is reported in the present
weather field. When all data are available, ASOS reports
blowing snow when:
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on intensity of the ice pellets). The PI sensor will more
likely interpret this as rain and output -RA, RA, or +RA
rather than UP. The ASOS processing algorithm will, in
turn, also report either -RA, RA, or +RA, as output by the
PI sensor.
ASOS may report other types of frozen precipitation
as rain or snow, depending on the scintillation pattern
caused by their size and fall velocity. For example, snow
grains are larger than drizzle and fall slower so they are
sometimes reported as snow. Snow pellets also fall slower
than raindrops and will therefore be reported as snow. Hailstones are larger than raindrops and fall faster, and therefore will be reported as rain.
In mixed precipitation conditions, such as snow and
ice pellets, the PI sensor likely interprets the ice pellets as
rain (see above). The snow, however, has a higher reporting precedence and thus snow will be reported by the processing algorithm. A mixture of snow grains and drizzle
may also be reported as snow. In cases where snow grains
are mixed with freezing drizzle, ASOS will either report
snow or no precipitation.

Visibility is less than 7 statute miles
Ambient temperature is 14 oF or less
Sky cover is less than overcast or the ceiling height is
greater than 10,000 feet
Wind speed is greater than 22 knots,

Sometimes the LEDWI may sense non-atmospheric
phenomena. For example, insects flying in the receiver
field of view can cause it to report precipitation. Spiders
that run threads between the cross arms can trigger a false
precipitation report when the wind moves the threads up
and down. Even sun glint, particularly on a bright day when
there are snow crystals (diamond dust) floating in the air,
can trigger false precipitation indications.

If these conditions are not met, ASOS reports UP.

4.3.7

PI Strengths and Limitations

Because of its continuous monitoring capability, the
ASOS PI sensor often detects and reports the beginning
and end of rain or snow before an observer. In addition,
because of its sensitivity, the PI sensor can detect light precipitation even at times when it cannot distinguish between
rain and snow (as the observer can). In these situations, the
ASOS may report too many “UPs.” Earlier updates to the
sensor and the algorithm minimized generation of excessive remarks (e.g., UPB04E07R AB07E14UPB14, etc).

Strong winds during snow and blowing snow occasionally pack the receiver or projector heads of the LEDWI
with snow. A total blockage of the lens will cause a loss of
any detected signal creating an error condition. A partial
blockage may contaminate the measurement and lead to
inaccurate reports of precipitation type or intensity. An
intensive ongoing effort is underway to eliminate the problem and prevent lens blockage.

Because drizzle particles are small and fall slower than
raindrops, their power spectrum is weak and smeared; therefore, drizzle is often not detected. On occasion, however, it
may elevate either the high or low frequency channel of
the PI sensor sufficiently to be reported as rain or snow.
When light drizzle is falling, the PI sensor may, on occasion, interpret the scintillation pattern created by these suspended water droplets as light snow when the temperature
is < 38 F. Product improvement efforts are aimed at reliably detecting and correctly reporting drizzle (see Section
7.2.5).

4.3.8

Freezing Rain Sensor

The ASOS Freezing Rain (FZRA) Sensor is based on
technology initially developed to detect icing on aircraft
in flight. The sensing device consists of a small cylindrical probe that is electrically stimulated to vibrate at its resonant frequency. A feedback coil is used to measure the
vibration frequency, which is proportional to the mass of
the probe. Figure 17 shows the design of this
“magnetostrictive oscillator” (i.e., Freezing Rain Sensor).
Magnetostriction is a property of certain metals in which

Ice pellets (PL) and rain have a similar size and fall
velocity. Consequently, ice pellets will force an output from
the PI sensor of either UP, -RA, RA, or +RA (depending

stores this data in memory for 12 hours. Data from the last
15 minutes are used to compute the current 1 minute freezing rain report. If three or more freezing rain sensor outputs in the last 15 minutes are missing, the sensor status is
set to “inoperative,” and the freezing rain report is set to
“missing.” The current minute’s frequency output is converted
to an ice accretion thickness using the following equation:
Zt = (40,000 - Fn) * 0.000152
Where Zt is the current ice accretion thickness in inches
and Fn is the current sensor frequency.
Each minute the Freezing Rain sensor outputs a report. The Freezing Rain report is set to “FRZA” under the
following combined conditions:
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Figure 17. Freezing Rain Sensor
n

a change in the (axial) dimension of a body caused a change
in magnetization. It is used in the ASOS sensor to drive the
probe at a natural resonant frequency of 40kHz. The axial
vibration is of such low amplitude that it cannot be seen or
felt. The probe is orientated vertically to provide optimal
uniform exposure to freezing precipitation regardless of
wind direction. This position also prevents birds from
alighting.

Otherwise, the freezing rain report is set to indicate a
lack of freezing precipitation for the current minute. The
freezing rain report for each minute is saved for 15 minutes. Finally, the freezing rain report is checked against the
present weather report from the previous minute and special alert criteria for the beginning and ending of freezing
precipitation, or changes in intensity, and if necessary a
SPECI will be generated.

When ice freezes on the probe, the combined mass
increases and the resonant vibration frequency decreases.
There is a well defined relationship between the measured
frequency and the ice accretion on the probe. The freezing
rain instrument is sensitive enough to measure accumulation rates as low as 0.01 of an inch per hour. The freezing
rain sensor continuously monitors the resonant frequency
of the vibrating probe, obtains a sample once a second, and
once each minute averages the results to update the probe’s
current resonant frequency. When excessive freezing rain
accumulates, (i.e., equal to or greater than 0.08 inch) the
sensor goes into a heating cycle to melt the freezing rain
from the probe and return it to the base resonating frequency. This process normally takes two to three minutes.
During this time, the sensor status is set to “deice” and the
output is not updated.

4.3.9

If the current ice accretion exceeds 0.005 inch,
If the current ice accretion exceeds the minimum accretion since the sensor was last declared operational
or during the past 15 minutes, whichever is less, by
0.002 inch
If the current five-minute ambient temperature is less
than 37oF.

Table 6 shows the reporting scheme for the ASOS
Present Weather Report. The reporting scheme follows the
familiar hierarchy of LIQUID-FREEZING-FROZEN, in
ascending order of priority. Only the highest priority precipitation phenomena observed will be reported at any one
time (i.e., ASOS does not report mixed or multiple precipitation types). For example, if the freezing rain sensor
indicates “-FZRA” and the precipitation identifier reports
“UP”, then the ASOS will report “-FZRA.”
The previous minutes’ present weather reports are further examined to determine which present weather remarks
need to be generated and appended to the METAR report.
This includes precipitation beginning and ending times in
minutes past the current hour (e.g., FZRAB05E21). Once
freezing rain has been sensed and the ambient air temperature is 36oF or below, it will be encoded in subsequent
METAR reports for 15-minutes after it is no longer sensed.

FZRA Algorithm

Once each minute, the freezing rain algorithm accesses
the current 5-minute average ambient temperature and the
current frequency output from the freezing rain sensor and

4.3.10 FZRA Strengths and Limitations

freezing fog (FZFG) and haze (HZ). Other obscurations,
such as dust, smoke, and blowing sand are not automatically reported by ASOS, but may be augmented by the
observer in accordance with agency policy.

The major strength of the freezing rain sensor is its sensitivity and continuous monitoring, usually allowing it to detect freezing precipitation conditions before an observer,
especially at night. Working in conjunction with other sensors, mixed precipitation events can be better defined. For
example, under conditions of mixed precipitation, such as
ice pellets (PL) and freezing rain (FZRA), the PI sensor will
output UP, -RA, RA, or +RA (for the PL), and the FZRA
Table 6.

The obscuration algorithm checks the reported visibility once each minute. When the surface visibility drops
below 7 statute miles, the algorithm obtains the current dew
point depression (DD) to distinguish between FG, BR, and
HZ. If the DD is less than or equal to 40F (~20 C), then FG
or BR will be reported (Table 8). Visibility will then be
used to further differentiate between FG and BR. If the
visibility is less than 7 miles and down to 5/8ths of a mile,
BR is reported. If the visibility is less than 5/8ths of a mile,
FG is reported. If the ambient temperature is also below
freezing, freezing fog (FZFG) is reported. When precipitation is reported, FG or BR may also be reported when the
preceeding conditions are met. When the DD is greater than
40F (~20 C) and no precipitation is reported by the PI and
freezing rain sensors, then HZ is reported as the obscuration; however, when precipitation is reported HZ is not reported.

Present Weather Reporting
Hierarchy

Freezing
Rain
Sensor
Report

Precipitation
Identification
Sensor
Report

ASOS
Present
Weather
Report

FZRA
FZRA
FZRA
FZRA
FZRA
FZRA
FZRA
FZRA
FZRA

No Precip.
Missing
UP
- RA
RA
+RA
- SN
SN
+SN

NP
NP
- FZRA
- FZRA
FZRA
FZRA
- SN
SN
+SN

Table 7.

sensor will output FZRA. In any event, the ASOS processing
algorithms will finally interpret and correctly report this condition as -FZRA or FZRA (see Table 7).
At temperatures near freezing, snow can become
attached to the probe firmly enough to cause a frequency
shift that could be misinterpreted as freezing rain. This problem of differentiating snow from freezing precipitation is
overcome by checking the output of the PI sensor to determine if it is raining or snowing when the freezing rain
sensor has a frequency shift. If the PI sensor indicates rain,
then the report will be freezing rain. If the PI sensor
indicates snow, then snow will be reported in the body of
the METAR message.

ASOS Report of Freezing/Frozen
Phenomena

Type of Weather
Occurrence

ASOS Present
Weather Report

Ice Pellets (PL)
Ice Pellets and Snow (SN)
Ice Pellets and Freezing Rain
Ice Crystals
Hail (GR)
Snow Grains (SG)
Snow Pellets (GS)

-RA, RA, or +RA
-SN, SN, or +SN
-FZRA, or FZRA
No Precipitation
+RA
-SN, SN, or +SN
-SN, SN, or +SN

In the event the DD is missing (i.e., temperature and/
or dew point is missing) the obscuration algorithm relies
solely on visibility to discriminate between FG, BR and
HZ. If the reported visibility is less than 7 miles and equal
to or greater than 4 miles, HZ is reported. If the visibility is
less than 4 miles and down to 5/8ths of a mile, then BR is
reported as the obscuration. If visibility is less than 5/8ths
of a mile, FG is reported. When present weather is also
reported, FG or BR is appended to the present weather re-

4.3.11 Obscuration Algorithm
Obstructions are not directly measured by ASOS, but
rather inferred from the measurement of visibility, temperature, dew point, and present weather (precipitation).
ASOS reports only four obstructions: fog (FG), mist (BR),

Table 8.

ASOS Report of Present Weather

PRESENT WEATHER

VIS
T

PRECIPITATION

NONE

1/4

SN FZFG

NONE

1/2

NONE

5/8

NONE

port. If reported visibility is equal to or greater than 4 miles
but less than 7 miles, and no present weather is reported,
then HZ is reported as the obscuration.

4.3.12 Obscuration Algorithm
Strengths and Limitations
One obvious characteristic of the obscuration algorithm
is that FG or BR cannot be reported, even if it is the only
obstruction to vision, when the DD is greater than 40F or
when DD is missing and visibility is 4 miles or more. In
these situations, if FG or BR is actually occurring, then HZ
is incorrectly reported when the visibility is less than 7
miles. The ASOS, of course, cannot report obscuration remarks for distant phenomena such as “FG BANK NE-SE.”
The ASOS, on the other hand, will provide more timely
reporting of the formation and dissipation of obscuring phenomena due to its continuous monitoring capability.

OBSCURATION

CHAPTER FIVE
5.0

Explanations/Examples
of ASOS Observations

Computer-generated voice messages of the current
OMO (or METAR / SPECI). These messages are sent
to pilots (generally within 50 miles of an airport)
through the ASOS Ground-To-Air VHF radio or navigation aid. They are also made available to the aviation community through telephone number provided
for dial-in.

Long-Line Dissemination: OMO data will not be sent
over long-line until both the modernization of the FAA
and the NWS communications networks is complete
(see Figures 18 and 19). At that time, FAA ADAS and
NWS AWIPS communications nodes will collect
OMO data from local ASOS locations. These data will
be made available to FAA and NWS users in METAR
format. Future communication links will allow each
NWS WFO to access the OMO data from selected
ASOS locations every 5-minutes when operating in
the “warning” mode, or every 15-minutes when operating in the “alert” mode.

ASOS provides six basic types of data:
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5-Second Wind Update
The OMO Data
The METAR / SPECI Messages
Auxiliary Data
The SHEF Messages
The Daily and Monthly Summary
Products/Messages

5.1

5-Second Wind Updates

The current 2-minute average wind is computed once
every 5 seconds and automatically updated on FAA provided ASOS Controller Equipment (ACE) video displays
at selected sites. A list of the 24 most recent 5-second average wind values used to compute the current 2-minute average wind is also available on the OID screen. (See
Appendix A for example of 5-second average wind).

5.2

5.3

All ASOS locations prepare and disseminate Aviation
Routine Weather Reports in METAR / SPECI code format. Although ASOS is designed to operate completely
unattended, it can also function in an attended mode with
an observer augmenting data and providing backup support if needed. In either case, the ASOS report is identified
by the “AO2” designator near the beginning of the remarks
(RMK) section of the report. This indicates that the ASOS
report is an Automated Observation–Type 2, with the added
capability to report present weather phenomena (e.g., RA,
SN, FZRA). When operating in the unattended mode, an
“AUTO” report modifier is inserted near the beginning of
the report after the Date-Time group.

One-Minute Observations

Once each minute, ASOS performs internal diagnostic and QC checks on sensor data and then updates the One
Minute Observation (OMO). The OMO is encoded in
METAR format and includes all basic weather parameters
found in the body of the METAR plus selected automated
remarks. The OMO also accepts manually entered remarks
(via OID keyboard entry), such as variable visibility and
tower visibility. The basic difference between the OMO
and the METAR/SPECI is that the OMO is generally not
transmitted long-line beyond the local FAA or NWS communications network node (see bullet 3 below). OMO
information is collected during the 60-second period ending at M+00 and made available to users each minute at
M+23 (23 seconds past the current minute) through the
following media:
n

ASOS Aviation Routine
Weather Reports

When operating in the attended mode, the AUTO designator does not appear. The absence of the AUTO designator indicates that an observer and/or an air traffic
controller is signed-on to the OID and is capable of providing backup or augmentation support if necessary. Only
one observer is permitted to sign-on the ASOS at a time.
Remote (off-site) observer sign-on is not permitted. If the
observer corrects the ASOS METAR/SPECI report, the
“COR” indicator appears as the report modifier after the
Date-Time group. When the observer and the air traffic

OID, VDU, and authorized dial-in user computer
display screens. Various screen displays of OMO
data are available. See Appendix A for examples,
and the ASOS Software User’s Manual (SUM) for
details.

controller are both signed-off the ASOS, the AUTO report
modifier is restored to the METAR/SPECI report.

matically be used to compute the daily and monthly summary temperature information for the site. The extent of
backup and augmentation provided at each location is dependent on the national and regional policies of the host
agency. Each agency provides observer support in coordination with interagency NWS/FAA and aviation industry
agreement on the service level assigned to each location.

When an aircraft mishap occurs, the on-site observer
can (and should) initiate a local special observation. In this
event, the remark “(ACFT MSHP)” is appended to the local SPECI report, which is sent to the ASOS printer and
stored in memory for 31 days. The long-line version of the
SPECI report is sent without the (ACFT MSHP) indicator.

5.3.2
All ASOS reports that are transmitted through the NWS
long-line communications network are captured at the NWS
Systems Monitoring and Coordination Center (SMCC) and
compiled with other transmitted data into a daily Selective
Records Retention System (SRRS) tape. This tape is sent
to the National Climatic Data Center (NCDC) once a day
for archiving and inclusion in various climatic summaries.
NCDC also directly downloads data from selected sites for
special climatological processing.

5.3.1

Missing vs. Non-Event Data

Generally, if less than 75 percent of the data required
for the computation of any weather element for the
METAR/SPECI report are available, then ASOS simply
does not report that element. For example, if three 1-minute
visibility values are missing in the past 10 minutes, then
the current 1-minute visibility will be missing. One notable exception to the 75 percent rule is that precipitation
accumulation requires nearly 100 percent of the 1-minute
data samples during the accumulation period. For example,
if just one 1-minute precipitation accumulation value is
missing in the past hour, then the current hourly precipitation accumulation report will be missing.

Backup and Augmentation

“Backup” is observer editing of the ASOS report for
missing , clearly erroneous, or temporarily unrepresentative elements that ASOS should report correctly, or transmission of an entire report when total ASOS equipment
failure has occurred. This activity may include use of alternative data sources and transmission medium. Only data
elements normally generated by ASOS are added to the
ASOS observation through backup. Backup may be applied to the ASOS-generated METAR/SPECI reports or to
daily and monthly summary products. For example, backup could occur if the ASOS Hygrothermometer becomes
inoperative and the temperature/dew point data are omitted in the METAR. To backup the ASOS observation, an
observer using alternate source(s) of information manually edits temperature/dew point data into the ASOS observation in place of the missing data (See Example 1 in
Section 5.3.4). This complete observation is then transmitted as the METAR report.

When a schedule-driven or event-driven data element
is missing in the body of the METAR/SPECI report, the
corresponding group and the preceding space for that data
element are omitted from the report. Trailing elements in
the report shift to fill in the blank space left by the omitted
data.
Schedule-driven elements should always appear in the
METAR/SPECI report: Wind, Visibility, Sky Condition,
Ambient and Dew Point Temperature, and Altimeter. It is
readily apparent from the compressed form of the report
when these elements are missing. For event-driven (i.e.,
criteria triggered) elements such as runway visual range,
present weather and obstructions, it is not always clear from
the report that their omission is due to missing data or simply because the event has not occurred.
To clearly show when event-driven elements are missing, a sensor status indicator is added to the remarks section when ASOS cannot report the element. A sensor status
indicator is also added to the remarks section when eventdriven meteorological discontinuity sensor data for secondary visibility and/or sky condition are not available.
Additionally, a sensor status indicator is included in the
remarks section when the event-driven hourly precipitation accumulation remark is not available. Finally, although
Sea-Level Pressure (SLP) information is included in both
the METAR and SPECI reports from ASOS, a sensor status indicator for missing SLP is only added to the METAR
report when SLP information is not available. The sensor
status indicators are:

Augmentation is the process of manually adding information to an ASOS METAR/SPECI report or Daily
Summary Message / Monthly Summary Message (DSM /
MSM) product that is beyond the capability of ASOS to
provide on its own. This task is performed before transmission. FAA/NWS policy permits augmentation at designated locations where qualified staff are available.
Edited and augmented data are processed by the ASOS
when computing the daily and monthly summary products.
For example, if the operator edits a temperature in a
METAR report, that edited temperature value would auto-

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RVRNO—Runway Visual Range Information Not
Available
FZNO—Freezing Rain Sensor Information Not Available
PWINO—Present Weather Identifier Information Not
Available
TSNO—Thunderstorm Information Not Available
PNO—Precipitation Accumulation Gauge Information Not Available
SLPNO—Sea Level Pressure Information Not Available
CHINO LOC—Cloud Height Indicator Information
at Meteorological Discontinuity Location Not Available (e.g., CHINO E)
VISNO LOC—Visibility Sensor Information at Meteorological Discontinuity Location Not Available
(e.g., VISNO RWY06)

At this automated site where observer oversight is not
available, the word “AUTO” is encoded after the date/time
group. In the remarks section, the “AO2” indicates that this
is an automated system with a precipitation discriminator.
In other words, this site can report the occurrence of precipitation in the present weather field. In this METAR report, the only element reported is the altimeter (A3029). If
an element is missing, METAR code does not permit an
“M” to be encoded. Missing elements are simply not encoded. The only exceptions to this rule are found in the
remarks section. When sea-level pressure is missing,
“SLPNO” (sea-level pressure not available) is encoded.
When precipitation accumulation amounts are not available, the 3- or 6-hourly and 24-hourly precipitation additive data groups, i.e., 6RRRR and 7RRRR, will appear as
“6////” and “7////.” METAR code does allow for sensor status indicators to be encoded at the end of the report. Since
this site is in need of maintenance, a maintenance indicator
($) is placed at the end of the report.

In general, when data for an automated remark are not
available the remark is omitted from the METAR/SPECI
report. A notable exception is when more than a trace of
precipitation has occurred but accumulation data for the
period is incomplete (i.e., some data are missing and therefore the amount is undetermined). In this case, the 3/6
hourly and 24 hourly precipitation accumulations are coded
as 6//// and 7////, respectively.

5.3.3

Example 2: Full Report
METAR KMTJ 031153Z AUTO 36003KT 3SM -SN BR
BKN009 BKN016 OVC070 00/M01 A3008 RMK AO2
SNB1057 CIG 006V010 SLP166 P0000 60000 70002
T00001006 10017 20000 51005 FZRANO TSNO
This 1200 UTC report is from a fully operational automated site without observer oversight. Because the site
does not have a freezing rain sensor installed and does not
have the capability to automatically report thunderstorms,
FZRANO and TSNO are encoded in the remarks section.
The obscuration mist (BR) has been encoded since the surface visibility is less than 7SM, but greater than or equal to
5/8SM, and the dew point depression is less than or equal
to 4o F. ASOS automatically provided all of the encoded
data including the begin time for the snow (SNB1057), the
variable ceiling height remark (CIG 006V010), and the sealevel pressure value (SLP166).

Estimated Data

The Federal Standard Algorithms for Automated
Weather Observing Systems Used for Aviation Purposes
(November 1988), does not allow an automated system to
use algorithms to generate estimated data for a METAR/
SPECI report. Because ASOS conforms to the federal standard, estimated automated data is not included in any ASOS
generated METAR/SPECI report. Estimated data however,
may be manually included in the ASOS METAR/SPECI
report through backup when the automated data are missing or deemed inaccurate. The METAR/SPECI report does
not indicate whether a data element has been estimated.

5.3.4

Example 3: Surface Visibility vs. Tower Visibility
SPECI KDEN 060409Z 03008KT 2 1/2SM BR OVC003
M06/M07 A3012 RMK AO2 TWR VIS 3 VIS 1 1/4V5

Examples of METAR/SPECI Reports

This SPECI report is from a site with observer oversight; note that the AUTO indicator after the date-time group
is omitted. In this report both the surface, sensor derived
visibility and the tower, human derived visibility are included. When the tower visibility is less than 4SM, the lower
of the two visibility values is encoded in the body of the
report and the other is included in the remarks section. In
this case, the surface visibility is lower and therefore encoded in the body, while the tower visibility is found in the

The examples in this section conform to the FMH-1,
Federal Meteorological Handbook No. 1, Surface Weather
Observations and Reports (December 1995). See Appendix C for a quick reference guide for the ASOS-generated
METAR/SPECI reports.
Example 1: Missing Data
METAR KSP1 021156Z AUTO A3029 RMK AO2
SLPNO 6//// 7//// 51018 PWINO FZRANO TSNO RVRNO
PNO $
43

remarks section of the report. The tower visibility is manually entered into the ASOS report through the OID. The
variable visibility remark (VIS 1 1/4V5) is automatically provided by ASOS and refers to the surface visibility.

ASOS can automatically report the obscurations of fog
(FG), freezing fog (FZFG), mist (BR), haze (HZ), and blowing snow (BLSN). Reporting these phenomena is based on
surface (sensor) visibility, temperature, dew point depression, and the occurrence of precipitation. If there is precipitation, ASOS does not automatically report HZ or
BLSN; however, it may report FG, FZFG, or BR during
precipitation.

Example 4: Meteorological Discontinuity Sensor Data
a:

METAR KBOS 291856Z 03006KT 1 3/4SM BR
FEW001 SCT019 BKN023 12/10 A2961 RMK AO2
SFC VIS 3 CIG 018 E SLP028 T01170100 $

Looking at the three reports above, the obscuration
changes from FZFG to HZ and then to BR. In the first
report, the surface visibility is less than 5/8SM, the temperature is below 0oC, and the dew point depression is less
than 4oF. Therefore FZFG is encoded. FZFG would also
have been encoded if precipitation were occurring. In the
second report, the dew point depression changes to 5oF
(ambient temperature = -23.3oC and dew point temperature = -26.1oC) and the obscuration changes to HZ. If precipitation were occurring, HZ would not have been
encoded. In the third report, the surface visibility increases
to 2 1/2SM and dew point depression changes to 4oF (ambient temperature = -23.3oC and dew point temperature =
-25.6oC). Therefore, BR is encoded as the obscuration. Note
that no distinction is made between BR and freezing mist,
as is done with FG and FZFG. Mist is reported as BR regardless of the temperature.

The tower visibility is 1 3/4SM and the surface (sensor) visibility is 3SM. In this case, the tower visibility is
encoded in the body of the report and surface visibility is
placed in remarks (SFC VIS 3). A meteorological discontinuity sensor grouping (ceilometer and visibility sensor)
is located east of the primary ASOS sensor suite. A ceiling
height of 1,800 feet is being reported east of the primary
sensors (CIG 018 E). If the meteorological discontinuity
ceilometer was not operational, then the remark “CHINO
E” would have been encoded.
b:

SPECI KBOS 291710Z 06004KT 1 1/2SM BR
OVC001 09/08 A2970 RMK AO2 SFC VIS 1 3/4 VIS
1/2 E $

A visibility remark (VIS ½ E) has been encoded for
the meteorological discontinuity sensor installed east of the
primary sensors. If this visibility sensor were not operational, then the remark “VISNO E” would have been encoded.
c:

Example 6: Obstructions vs Visibility

METAR KBOS 292256Z 04004KT 1 1/2SM RA BR
BKN011 BKN050 OVC060 11/11 A2956 RMK AO2
SFC VIS 3 TSB29E50 SLP008 VIS W-N 1 3/4 TS
DSPTD P0016 T01110106 $

Example 5: Obstructions
SPECI KATY 080515Z AUTO 00000KT 1/4SM
FZFG VV001 M23/M25 A3036 RMK AO2 FZRANO
TSNO PNO

METAR KATY 080553Z AUTO 00000KT 1/4SM
HZ VV001 M23/M26 A3036 RMK AO2 SLP326 6//
// T12331261 11139 21239 410671239 58007
FZRANO TSNO PNO

SPECI KSP1 311245Z 31007KT 1/2SM BR 26/25
A3003 RMK AO2 SFC VIS 3 $

SPECI KSP1 311247Z 31007KT 1/2SM FG 26/25
A3004 RMK AO2 TWR VIS 3 $

In the first report of this example, the tower visibility
is 1/2SM and the surface (sensor) visibility is 3SM. Determining surface obscuration is based on surface visibility,
not tower visibility. In this case, note that BR is encoded
as an obscuration. In the second report, we changed the
surface visibility to 1/2SM and the tower visibility to 3SM.
The obscuration correctly changed from BR to FG.

In the remarks section, the observer has augmented a
sector visibility (VIS W-N 1 3/4). ASOS does not now automatically report a sector visibility.

Since surface observations are taken at the surface,
obcuration entries should accurately report conditions at
the surface, not at the height of the tower cab. Both reports
are correct as encoded.
Example 7: Obstructions vs. Surface Visibility

METAR KATY 080653Z AUTO 00000KT 2 1/2SM
BR OVC001 M23/M26 A3037 RMK AO2 SLP329
T12331256 FZRANO TSNO PNO

SPECI KSP1 041606Z 03013G22KT 2 1/2SM BR
CLR 23/21 A3005 RMK AO2 TWR VIS 10 $

SPECI KSP1 041608Z 36011G18KT 2 1/2SM CLR
23/21 A3004 RMK AO2 SFC VIS 10 $

Because ASOS provides reports of surface conditions,
algorithms that automatically encode obscurations, as well
as the intensity of snow, use the surface (sensor) visibility
as input. In the first report, the surface visibility is
2 1/2SM and the tower visibility is 10SM. In this case,
ASOS automatically encoded mist (BR) as the obscuration. In the second report, the visibilities were changed to
10SM at the surface and 2 1/2SM from the tower cab. Since
the surface visibility is now 10SM, ASOS does not encode
an obscuration since it is not occurring at the surface. Both
reports are correct as encoded.

This example also illustrates the order of remarks in
an ASOS report. After the “RMK” (remarks) contraction,
the following order is used:
1. Tornadic remarks: e.g., FUNNEL CLOUD B06 FUNNEL CLOUD E10
2. “AO2” indicator
3. Automated remarks: e.g., PK WND 04036/1135 SFC
VIS 1/4 SNB07 PRESFR SLP096
4. Manual remarks: e.g., SNINCR 2/12 4/012
5. Additive data: 6//// 7//// T10201020 10167 21022
58199
6. Sensor status indicators: e.g., PNO
7. Maintenance check indicator: $

Example 8: Variable Wind
a.

METAR KSP1 301756Z AUTO 33007KT 290V360
10SM FEW049 19/M01 A2981 RMK AO2 SLP229
T01941006 10200 20122 58017 PWINO TSNO $

Example 10: Surface Visibility and RVR

METAR KSP1 301856Z AUTO VRB06G15KT
10SM SCT050 19/M01 A2978 RMK AO2 SLP220
T01941011 PWINO TSNO $
SPECI KORE 291712Z AUTO 33003KT 6SM HZ
SCT014 BKN038 BKN049 11/07 A2965 RMK AO2
TSNO

SPECI KSP1 041202Z 35008KT M1/4SM BR CLR
18/18 RMK A02 SFC VIS 2 SNE02 PNO $

SPECI KSPI 041205Z 35008KT 340V070 1/4SM FG
CLR 18/18 RMK A02 TWR VIS 2 SNE02 RVRNO
PNO $

If for any reason the RVR should not be available from
a site designated to report RVR, the remark RVRNO is
encoded when the surface visibility is 1 statute mile or less.
In the first report above, the surface visibility is 2SM. In
the second report, the surface visibility reduces to 1/4SM
and the RVRNO remark is encoded.

Variable wind direction encoding is based on the observed sustained wind speed. When the wind has a sustained speed of greater than 6 knots, the variability of the
wind’s direction is encoded in the body of the report. (See
the first report in this example.) When the sustained wind
speed is 6 knots or less, a variable wind has an encoded
direction of “VRB.” As is shown in the second report of
this example, the sustained wind speed is 6 knots but the
wind is gusting to 15 knots. (Gusts are reported for a minimum of 10 minutes after they are first observed.) The direction is still encoded as “VRB.” In the third report the
winds are not variable and the actual direction is encoded.

Example 11: Corrected Report

Example 9: Surface Visibility and RVR, Plus Remarks
METAR KSP1 041156Z 05024G33KT 020V080 M1/4SM
R04/2800V5000FT +SN FZFG VV002 M02/M02 A2953
RMK FUNNEL CLOUD B06 FUNNEL CLOUD E10 AO2
PK WND 04036/1135 SFC VIS 1/4 SNB07 PRESFR
SLP096 SNINCR 2/12 4/012 6//// 7//// T10201020 10167
21022 58199 PNO $

METAR KSP1 081153Z 22006KT 9SM FEW250
M10/M13 A3015 RMK AO2 SLP259 T11001128
11094 21133 58006 4/002

METAR KSP1 081153Z COR 22006KT 9SM
FEW250 M10/M13 A3015 RMK AO2 SLP259 4/002
T11001128 11094 21133 58006

This example illustrates the ASOS correction feature.
Anyone signed-on as an observer can generate a corrected
report. This feature allows the observer to enter data any
place in the report. The observer is completely responsible
for data quality and proper encoding practices. ASOS does
not do any error checking in the correction feature. In the
first report, the observer added the “4/sss” (snow depth on
ground) group, however, it was not encoded in the proper
order of entry. Using the ASOS correction feature allows
the observer to correct data. In the second example a corrected report was issued with the “4/sss” group encoded in
the proper location, between the “SLP259” and

At selected sites, Runway Visual Range (RVR) is encoded when conditions warrant. As shown above, RVR is
encoded immediately after the visibility in the body of the
report. RVR is encoded whenever surface visibility is one
statute mile (1SM) or less, and/or if the minimum RVR
value is less than or equal to 6,000 feet. In this case, surface visibility is 1/4SM (see remarks section).

5.5

“T11001128” group. Future enhancement to the ASOS QC
algorithms will include format error checking when manual
entries are made.

Standard Hydrometeorological
Exchange Format (SHEF)
Messages

Example 12: Augmentation For Severe Present Weather
In addition to METAR messages (which may contain
precipitation accumulation remarks such as Prrrr, 6RRR/,
and 7RRRR), ASOS also generates two distinct SHEF message types.

SPECI KSP1 241321Z 04008KT 7SM +FC -TSRA
FEW035 BKN070 OVC090 31/20 A2964 RMK TORNADO B14 NE AO2 TSB21RAB21 P0000 $
This SPECI was generated to report the beginning of
a thunderstorm (TS), which began at 1321Z. Note that a
tornado (+FC) also is occurring and began at 1314Z. The
observer added the +FC indicator in the body of the report
and the direction (northeast - NE) from the station where
the tornado was located when it began.

One type of message is in “.E” SHEF message format.
This message is generated when precipitation accumulation onset criteria are exceeded and is discontinued when
precipitation accumulation falls below the termination criteria. This type of message is referred to as the “SHEF 15minute Precipitation Criteria Message.” This message
contains a chronological listing of precipitation accumulation during the four most recent 15-minute reporting time
periods. The 15-minute Precipitation Criteria Message is
issued at either H+00, H+15, H+30, or H+45.

Example 13: Augmentation For Volcanic Eruption:
SPECI KSP1 241258Z 01007KT 300V040 3/4SM VA
FEW050 BKN070 OVC095 10/M03 A2963 RMK AO2 VIS
1/4V5 MT AUGUSTINE VOLCANO 70 MILES SW
ERUPTED 241255 LARGE ASH CLOUD EXTENDING
TO APRX 30000 FEET MOVING NE RVRNO $

The second type of SHEF message is in “.A” SHEF
format and is routinely generated at a fixed time each hour
(H+00). This message is referred to as the “SHEF Hourly
Routine Precipitation Message” and contains precipitation
accumulation information for a 60-minute time interval ending at a fixed time each hour.

When a volcanic eruption is first noted, a SPECI report
is generated. Because there was volcanic ash (VA) at the site,
the observer augmented the SPECI to report this obscuration. More information about the eruption was entered by the
observer in the remarks section (shown in bold print).

5.4

The onset and termination criteria for the SHEF 15minute Precipitation Criteria Message and the end time for
the SHEF Hourly Routine Precipitation Message are set
during system acceptance. They may be subsequently
changed only by an authorized ASOS system manager.
SHEF messages are stored in ASOS memory for a minimum of 3 days plus so far for the current day. SHEF messages are available from all NWS and FAA ASOS locations.

Auxiliary Data

Auxiliary data consists of:
n
n
n
n
n
n

Relative Humidity
Sea-Level Pressure
Station Pressure
Density Altitude
Pressure Altitude
Magnetic Wind

All SHEF messages originating from NWS ASOS locations and transmitted on the NWS AFOS network are
identified by the generic message header:
“CCCNNNXXX,” where NNN = RR6 (for the SHEF 15minute Precipitation Criteria Message), or RR7 (for the
SHEF Hourly Routine Precipitation Message). For NWS
ASOS locations, CCC is the Station Identifier (SID) for
the primary responsible WSFO/WFO, and XXX is the
ASOS location’s SID. SHEF messages originating from
NWS ASOS locations are transmitted individually.

These data are available on the VDU, user-provided
video monitor screens and selected OID screens; they are
updated every minute. The auxiliary data are generally derived from other processed sensor data by the weather reporting algorithms, and, therefore, cannot be edited directly,
but may be altered indirectly by editing of the component
parameters through the OID. Examples of auxiliary data
displays are shown in Appendix A.

Those SHEF messages originating from FAA ASOS
locations are captured at the FAA WMSC (WMSCR in the
future) and passed across Gateway to the NWS as collectives. These collectives are identified by the AFOS PIL
header: “NMCRR7NKA.” Collectives contain both types
of SHEF messages (RR6, RR7) from commissioned and
pre-commissioned FAA ASOS locations.

Example I.

SHEF 15-Minute Precipitation Criteria Message

MESSAGE
NUMBER

RR6 SHEF MSG #1
RR6 SHEF MSG #2

TIME

ONSET
CRITERIA

TERMINATION
CRITERIA

PCPN
VALUE

1200 UTC
1215 UTC
1230 UTC
1245 UTC
1300 UTC
1315 UTC

0.50 inch
0.50 inch
0.50 inch
0.50 inch
0.50 inch
0.50 inch

0.25 inch
0.25 inch
0.25 inch
0.25 inch
0.25 inch
0.25 inch

0.00 inch
0.25 inch
M
0.75 inch
0.25 inch
0.20 inch

SHEF MSG #1:
TOPRR6TOP
TTAA00 KTOP 201245
.E TOP 1020 DH1200/PPC/DIN15/0.00/0.25/M/0.75: C = 0.50/.25
SHEF MSG #2:
TOPRR6TOP
TTAA00 KTOP 201300
.E TOP 1020 DH1215/PPC/DIN15/0.25/M/0.75/0.25: C = 0.50/.25

Legend for Example I. (See SHEF Message #1)
.E TOP:

.E format message from Topeka, KS (TOP).

1020:

Month (October) and day (20) for first observation in time series (current year assumed).

DH1200:

Hour and minute UTC time stamp of first data value in time series. Example given here is 12:00 UTC.

PPC:

Precipitation, Actual Increment (PP); Fifteen minute increment (C).

DIN15:

DI = Data Interval; N15 = Fifteen Minutes.

/0.00/
/0.25/
/M/
/0.75:

Incremental precipitation (in inches) for first, second, third and fourth 15-minute periods
of time series. Missing data are replaced with “M.” The first value (0.00 inches) is valid
for 12:00 UTC. The last value (0.75) is valid for 12:45 UTC.

C = n.nn/.tt SHEF 15-minute onset threshold criteria/termination threshold criteria. In this example: n.nn = 0.50
inches, and .tt = .25 inches.

Distinctions between different SHEF message formats
are also made in the body of the individual messages. See
SHEF Version 1.1 issued by the NWS Office of Hydrology, 31 January 1985 for further details.

UTC. Note that the detection threshold specified for the
ASOS HTB is 0.01 inch per hour, and the precipitation rate
accuracy is the larger of 10 percent or 0.01 inches per hour.
SHEF MSG #4 shows a large 60-minute accumulation of 1.35 inches ending at 1500 UTC.

5.5.1

SHEF 15-Minute Precipitation
Criteria Message

SHEF MSG #5 valid at 1600 UTC is a typical example
of a 60-minute reporting period with no measurable precipitation.

The messages in Example I are identified by the “RR6”
in the AFOS header, the “.E” message type designator and
the “DIN15” 15-minute time interval indicator in the body
of the message.

SHEF MSG #6 valid at 1700 UTC is a typical example
of a 60-minute reporting period where some of the 1-minute
data were missing. If precipitation accumulation is missing during any minute within the discrete 60-minute period, then the value reported in the SHEF Hourly Routine
Precipitation message after “PPH” is “M.”

In Example I, MSG #1 was issued at 12:45 UTC because during the current 15-minute period (12:31 - 12:45
UTC) the precipitation accumulation first exceeded the 15minute precipitation accumulation onset criteria of 0.50
inch. The three previous 15-minute time periods are also
included in MSG#1 and listed in chronological order (even
though precipitation accumulation during those time periods did not exceed the onset threshold criteria).

5.6

All ASOS locations are capable of generating and
transmitting daily and monthly weather summaries, however, only selected NWS ASOS locations transmit these
data long-line. These summaries are valid for the calendar
day, or the calendar month, ending at 23:59 LST. The daily
summaries are normally transmitted early on the following day at a programmable transmission time specified by
the system manager. Backup transmission times are generally 1 and 2 hours after the initial scheduled transmission time. The monthly summaries are normally transmitted
early on the first day of the new month at programmable
transmission times. Backup transmission times are generally 1 and 2 hours after the initial scheduled transmission
time. The ASOS generated daily and monthly summaries
replace the “F-6” data previously provided from staffed
locations.

MSG #2 was issued at 13:00 UTC because precipitation accumulation during the immediately preceding 15minute period (ending at 12:45 UTC) exceeded the onset
threshold criteria and the precipitation accumulation during the current 15-minute period (12:46 - 13:00 UTC) is
still equal to or greater than the termination criteria of 0.25
inch. 15-Minute Precipitation Criteria Messages cease to
be issued when the discrete 15-minute precipitation
acccumulation falls below the termination threshold criteria. If precipitation accumulation is missing during any
minute within a discrete 15-period, then the value reported
for that 15-minute period is “M.”

5.5.2

Daily and Monthly
Summaries

SHEF Hourly Routine
Precipitation Message

Daily and monthly summaries are stored in ASOS
memory for 10 days after transmission. The content and
format of the daily and monthly summary messages is
only briefly summarized here.

The messages in Example II are identified by the
“RR7” in the AFOS header, and the “.A” SHEF format
designator in the body of the message. In example II the
end time of the SHEF Hourly Routine Precipitation Message is set at H+00. Thus, all precipitation accumulation
reports in example II are for the 60-minute period from
H+01 to H+00. A SHEF Hourly Routine Precipitation Message is issued routinely every hour, regardless of whether
or not there was precipitation during the hour.

5.6.1

Daily Summary Message

The Daily Summary Message (DSM) contains
data to complete the Preliminary Local Climatic Data
(PLCD) record for each specified location. These data are
valid for the previous day and include:

SHEF MSG #3 is valid at 1400 UTC. This 60-minute
period includes the 0.20 inch precipitation accumulation
which occurred during the 15-minute period ending at 1315

Example II.

SHEF Hourly Routine Precipitation Message

MESSAGE NUMBER

TIME

PCPN
VALUE

RR7 SHEF MSG #3
RR7 SHEF MSG #4
RR7 SHEF MSG #5
RR7 SHEF MSG #6

1400 UTC
1500 UTC
1600 UTC
1700 UTC

0.20 inch
1.35 inch
0.00 inch
Missing

SHEF MSG #3:

TOPRR7TOP
TTAA00 KTOP 201400
.A TOP 1020 DH1400/PPH 0.20

SHEF MSG #4:

TOPRR7TOP
TTAA00 KTOP 201500
.A TOP 1020 DH1500/PPH 1.35

SHEF MSG #5:

TOPRR7TOP
TTAA00 KTOP 201600
.A TOP 1020 DH1600/PPH 0.00

SHEF MSG #6:

TOPRR7TOP
TTAA00 KTOP 201700
A TOP 1020 DH1700/PPH M

Legend for Example II. (See SHEF MSG #3)

.A TOP:

“.A” format message from Topeka, KS (TOP).

1020:

Month (October), and day (20) of observation (current year assumed).

DH1400:

Observation time in UTC. Example given here is for MSG

PPH:
0.20 (a.aa):

Precipitation, Actual Increment (PP); Hourly increment (H).
Incremental precipitation is in hundredths of an inch. In the example given here, precipitation
accumulation for the current hour in MSG # 3 (ending at 14:00 UTC) is 0.20 inch

# 3 (14:00 UTC).

n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n

Daily (00:00 to 23:59 LST) Maximum/ Minimum
Temperatures and Times of Occurrence
Daytime (0700 to 1900 LST) Maximum Temperature
and Time of Occurrence
Nighttime (1900 to 0800 LST) Minimum Temperature and Time of Occurrence
Daily Minimum Sea-Level Pressure
Daily Total Precipitation Accumulation (Liquid
Equivalent)
Hourly Precipitation Amounts (H+00 to H+59)
Daily Average 2-Minute Wind Speed
Fastest 2-Minute Wind Direction, Speed (MPH) and
Time of Occurrence
Daily Peak Wind Direction and Speed (MPH) and
Time of Occurrence
Weather Occurrence Symbols
Minutes of Sunshine and Percentage of Sunshine
(When Available)
Daily Total Snowfall, Ice Pellets Accumulation (When
Available)
Depth of Snow, Ice and/or Ice Pellets on the Ground
(In Whole Inches) at a Designated Observation Time
(When Available)
Average Daylight Sky Cover (Sunrise to Sunset)
(When Available)
Average Daily Sky Cover (When Available)
Remarks for Estimated Data

n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n

5.6.2

Monthly Summary Message

The Monthly Summary Message (MSM) contains data
necessary to complete the monthly Preliminary Local Climatic Data (PLCD) record for each specified location.
These data are valid for the previous calendar month and
include:
n
n
n
n
n
n
n

Number of Days with Minimum Temperature
of 32o F and Below.
Number of Days with Minimum Temperature of 0o F
and Below.
Monthly Heating Degree Days
Monthly Cooling Degree Days
Monthly Mean Station Pressure
Monthly Mean Sea-Level Pressure
Monthly Maximum Sea-Level Pressure and Date and
Time of Occurrence
Monthly Minimum Sea-Level Pressure (Nearest 0.01
Inch Hg) and Date and Time of Occurrence
Monthly Total Precipitation (Water Equivalent)
Number of Days with Precipitation Greater Than or
equal to 0.01, 0.10, 0.50, and 1.00 inch
Greatest 24-Hour Precipitation (Water Equivalent) and
Date(s) of Occurrence
Short Duration Precipitation (5, 10, 15, 20, 30, 45,
60, 80, 100, 120, 150, 180 min) including Date of Occurrence and Time when period ended
Monthly Total Hours of Sunshine Observed (When
Available)
Monthly Percentage of Total Sunshine Observed
(When Available)
Greatest 24-Hour Snowfall and Date(s) of Occurrence
(When Available)
Greatest Depth Of Snow on Ground and Date(s) of
Occurrence (When Available)
Number of Clear, Partly Cloudy, and Cloudy Days
during Month (When Available)
Remarks For Estimated Data

5.7

ASOS High-Resolution
Sensor Data

High-resolution sensor data from the ASOS 12-hour
archive file are available for review at the ASOS OID or to
authorized remote users. These data have not undergone
final quality control checks. They are primarily intended
for maintenance troubleshooting purposes and should not
be used as valid meteorological data without extensive
evaluation. The high-resolution data which are available
for review are summarized in Table 9. Examples of ASOS
1-Minute Sensor Data are shown in Appendix A.

Monthly Maximum Temperature and Date of Occurrence
Monthly Minimum Temperature and Date of Occurrence
Average Daily Maximum Temperature
Average Daily Minimum Temperature
Average Monthly Temperature
Number of Days with Maximum Temperature
of 32o F and Below
Number of Days with Maximum Temperature of 90oF
and Above (Use 70o F in NWS Alaska Region)

Table 9.

ASOS High-Resolution Data

n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n

30-Second Cloud Height Reading (Ceilometer Data)
1-Minute Average Visibility Extinction Coefficient
1-Minute Photometer Reading
1-Minute Average Ambient Temperature
1-Minute Average Dew Point Temperature
2-Minute Average Wind Speed and Direction (Every Minute)
Maximum 5-Second Average Wind Speed and Direction For Each Minute
1-Minute Precipitation Identification Sensor Data
1-Minute Lightning Data *
1-Minute Precipitation Amount
15-Minute Incremental Precipitation Amount Stored Every 15 Minutes
1-Minute Sunshine Data *
1-Minute Frozen Precipitation Water Equivalent *
1-Minute Snow Depth *
1-Minute Freezing Rain Occurrence
1-Minute Average Pressure For Each Pressure Cell

When Sensors Become Available

CHAPTER SIX
6.0

ASOS Data Outlets

prevents remote editing or augmenting of the ASOS data.
The various types of data available in these sign-on levels
are listed in Figure 4, and shown in Appendix A, ASOS
Video Displays.

ASOS data are provided through multiple data outlets. There are five basic types of ASOS data outlets. The
basic ASOS data outlets are described below along with a
brief description of the types of data available through each
of these outlets. The basic ASOS data outlets are:
n
n
n
n
n

6.2

On-Site and Remote OID Video Screen Displays, and
Remote User Interactive Video Screen Displays
On-Site, Non-Interactive Screen Displays
On-Site Printer Hard copy
Long-Line Coded Messages
Computer-Generated Voice Messages

6.1

On-site displays, other than the OID, include the FAA
ACE Video Display (VD) and the Video Display Unit
(VDU). The VD is intended for Air Traffic Controller use.
The VDU (up to 4 per airport) is intended for observer use
at the primary observation location. Other users, such as
fixed base operators, may interface with the ASOS by providing their own off-the-shelf, non-interactive video monitors. Up to 50 such monitors can be interfaced with ASOS
at a single airport. The data which may be displayed on
these monitors is identical to the data displayed on the VDU.
See Appendix I for examples of the FAA VD and VDU
screens.

Interactive Video Screen
Displays

At staffed ASOS locations, up to two Operator Interface Devices (OID) can be provided. The OID consists of
a monitor and keyboard and provides interactive access to
ASOS data through the ACU. A primary OID is located at
the designated primary observer location. At airports with
air traffic control towers, an OID is also provided which
may be designated either the primary or secondary OID,
depending upon primary observer location. On-site users
may “sign-on” to the ASOS OID in any one of four access
levels with the proper password. Those four levels include:
n
n
n
n

The FAA VD includes the 5-second wind update of
the current 2-minute average wind (magnetic), the current
1-minute update of the altimeter setting, the last transmitted METAR, and the density altitude at the time of the last
transmitted METAR. This data is automatically updated
by the ASOS.
The VDU displays the most current OMO data, the
last transmitted METAR, and auxiliary data such as Relative Humidity, Sea-Level Pressure, Density Altitude, Pressure Altitude, and wind (direction given in both magnetic
and true degrees). Most auxiliary data are refreshed automatically once every minute (see Section 5.4 for details).
The data available on the user provided monitor is identical with that provided on the VDU. No user interaction
with ASOS is permitted with the FAA VD, VDU, or user
provided monitor.

Observer Level
Air Traffic Control (Tower) Level
Technician Level
System Manager Level

OID users in both the Observer Level and the Air Traffic Control Level can be signed on to the ASOS simultaneously, however no two users can be signed on in the
Observer Level at the same time. Remote OID users and
remote interactive computer users may only sign on as:
n
n
n

Non-Interactive Screen
Displays

6.3

Unsigned User
Technician
System Manager

ASOS Printer

One printer may be provided at the primary OID
equipped location. The following data are automatically
printed:

Up to 8 on-site OID/remote OID users may be signedon at one time, however, only one on-site OID user at a
time may be signed-on in the (primary) Observer Level or
Air Traffic Control (ATC) Level. It is not possible to remotely sign-on in the Observer level or ATC level. This

All transmitted METARs

n
n
n

The OMO generated just before and after the OID edit
function has been accessed
Equipment failures
Daily and monthly summaries (after computed)

tent of the computer-generated voice includes the following:
n
n

In addition, any data displayed on the primary OID
screen may be printed upon command.

6.4

n
n

Long-Line Dissemination

Location identification
Type of observing station (automated), but not type
of observation (i.e., METAR or SPECI)
The valid time of the observation
The individual reported weather elements normally included in the METAR (except sea-level pressure). One
major difference from the METAR however, is that wind
direction in the voice message is given in reference to
magnetic direction instead of true direction
Selected automated remarks and manual OID keyboard entered remarks

The ASOS coded messages are transmitted nationwide
through NWS and FAA communication networks (see Figures 18 and 19).

Initially, the FAA will provide OMO data for local
use only; no networking of these data is possible. Additionally, all ASOS METAR messages are available to users on both the FAA and NWS networks. Exchange of
ASOS METAR messages between the FAA and NWS communications networks occurs through the NWS Telecommunication Gateway (NWSTG). ASOS METAR messages,
SHEF data, and Daily Summary and Monthly Summary
Messages from both FAA and NWS locations are additionally available to other users through the NWSTG.

After the altimeter setting information is given, the
word “remarks” is spoken to indicate that additional information follows. This additional information may be ASOS
generated and/or manually entered. The ASOS is capable
of selecting appropriate words from the approved ASOS
voice vocabulary (see Appendix D) and producing computer-generated voice remarks for:
n
n

In the final network configuration, OMO data from all
ASOS locations will be made available to users upon request. OMO data from specified ASOS locations will be
provided to the NWS Weather Forecast Offices (WFO),
upon request, once every 5 minutes when the WFO is operating in the “warning” mode, or once every 15 minutes
when the WFO is operating in the “alert” mode.

6.5

Variable visibility (e.g., “visibility two miles variable
three miles”)
Pressure (e.g., when density altitude is > 1,000 feet an
automated remark is generated; “Density altitude two
thousand five hundred”)
Lightning (e.g., “Lightning Distant Northeast”)

Manually generated remarks are appended to the computer-generated voice remarks through the OID keyboard.
For example, manual entry remarks through the OID keyboard may include tower-visibility.
At towered airports, the air traffic controller has the
option of selecting for the broadcast weather message either METARs (scheduled or unscheduled) or OMOs. At
non-towered locations, the broadcast weather message defaults to the OMO.

Computer-Generated Voice
Messages

ASOS computer-generated voice messages include
those which are broadcast directly to pilots through the
ASOS ground-to-air radio, and those which are made available to the general aviation public through FAA sponsored
telephone dial-in access. The information contained in the
ground-to-air radio message and the telephone dial-in message are identical. ASOS computer-generated voice observations are spoken at the rate of 100 words per-minute.

Each computer-generated voice message begins with
an identification of the location, the phrase “AUTOMATED WEATHER OBSERVATION” and the UTC
(ZULU) time of the observation.
To clarify and distinguish the different data elements
in the computer-generated voice message a verbal identifier (prefix) accompanies each weather element (e.g., “temperature four dew point three”). Weather elements missing
in the METAR or OMO alpha-numeric message are enunciated as “missing” in the corresponding computer-gener-

The content of the computer-generated voice message
is either the current OMO or the current METAR, depending on location. At unstaffed locations, the OMO is used;
at FAA towered locations the OMO or METAR may be
used at the discretion of the air traffic controller. The con-

Figure 18. ASOS Network Data Flow–—Initial Configuration (1997)

Figure 19. ASOS Network Data Flow––Final Configuration

ated voice message (e.g., “sky condition missing,”; “visibility missing”).

“BREMERTON NATIONAL AIRPORT, AUTOMATED
WEATHER OBSERVATION, TEMPORARILY INACTIVE, ONE FOUR FIVE SIX ZULU, ...”

Example 1
Example 5
If the airport’s specific location is included in the
airport’s name, the airport’s name is announced.

In this example, a “clear below 12,000 feet” observation is given. Notice that the number “9” is pronounced
NINER. The magnetic declination at MHK is 7oE. The true
wind direction reported in the METAR (280 degrees) is
reported as 270 degrees magnetic (rounded to the nearest
10 degrees) in the computer-generated voice message.

“BREMERTON NATIONAL AIRPORT, AUTOMATED
WEATHER OBSERVATION, ONE FOUR FIVE SIX
ZULU, ...”
“RAVENSWORTH JACKSON COUNTY AIRPORT,
AUTOMATED WEATHER OBSERVATION, ONE
FOUR FIVE SIX ZULU, ...”

METAR KMHK 010355Z AUTO 28008KT 10SM CLR
20/13 A2988 RMK AO2 SLP929 T02000130
“MANHATTAN MUNICIPAL AIRPORT, AUTOMATED WEATHER OBSERVATION, ZERO THREE FIVE
FIVE ZULU, WIND TWO SEVEN ZERO AT ZERO
EIGHT, VISIBILITY ONE ZERO, SKY CONDITION
CLEAR, TEMPERATURE TWO ZERO CELSIUS, DEW
POINT ONE THREE CELSIUS, ALTIMETER TWO
NINER EIGHT EIGHT”

Example 2
If the airport’s specific location is not included in the
airport’s name, the location name is first announced, and
then the airport name is given.
“SAULT STE MARIE, CHIPPEWA COUNTY INTERNATIONAL AIRPORT, AUTOMATED WEATHER OBSERVATION, ONE FOUR FIVE SIX ZULU, ...”

Example 6
In this example, a manual lower tower visibility is
given. The measured true wind direction is 164 degrees
(rounded to 160 for reporting in the METAR). The measured magnetic wind direction is 157 degrees (rounded to
160 degrees for reporting in the computer-generated voice
message). Although magnetic declination at MHK is 7oE,
note that both the true and magnetic wind directions are
reported as 160 degrees in this example.

“SANDUSKY, COWLEY FIELD, AUTOMATED
WEATHER OBSERVATION, ONE FOUR FIVE SIX
ZULU, ...”
Example 3
During the nationwide implementation of the ASOS,
each location will operate in test mode for approximately
30-60 days before commissioning. A longer period of test
broadcasts may occur at selected locations. During this test
period, computer-generated voice messages will have the
word “TEST” in the message following the word
“OBSERVATION” to indicate that the observation is not
official.

SPECI KMHK AUTO 090735Z 16005KT 1SM -RA BR
OVC050 20/20 A2992 RMK AO2 SFC VIS 3 P0001
“MANHATTAN MUNICIPAL AIRPORT, AUTOMATED WEATHER OBSERVATION, ZERO SEVEN
THREE FIVE ZULU, WIND ONE SIX ZERO AT ZERO
FIVE, VISIBILITY ONE, LIGHT RAIN MIST, SKY
CONDITION OVERCAST FIVE THOUSAND, TEMPERATURE TWO ZERO CELSIUS, DEW POINT TWO
ZERO CELSIUS, ALTIMETER TWO NINER NINER
TWO, REMARKS SURFACE VISIBILITY THREE”

“BREMERTON NATIONAL AIRPORT AUTOMATED
WEATHER OBSERVATION TEST ONE FOUR FIVE
SIX ZULU, ...”
Example 4

Example 7

The phrase “TEMPORARILY INACTIVE” is added
to the message after the word “OBSERVATION” to indicate that the system is inoperative.

ASOS does not report a cloud height of zero, but is
capable of reporting a vertical visibility with an obscura-

tion. Observed visibilities of zero, 1/16 or 1/8 mile are all
reported as “less than one quarter” mile by ASOS but may
be manually augmented. The pressure sensors are inoperative and therefore the altimeter is missing (note the “$”
maintenance flag in the METAR message). The magnetic
declination at ABI is 8oE. With a calm wind however, both
the true wind direction in the METAR (00000) and the
magnetic wind direction in the computer-generated message are reported as “calm.”
SPECI KABI 121255Z AUTO 0000KT M1/4SM FG
VV001 16/16 RMK AO2 $
“ABILENE MUNICIPAL AIRPORT, AUTOMATED
WEATHER OBSERVATION, ONE TWO FIVE FIVE
ZULU, WIND CALM, VISIBILITY LESS THAN ONE
QUARTER, FOG, VERTICAL VISIBILITY ONE HUNDRED FEET, TEMPERATURE ONE SIX CELSIUS,
DEW POINT ONE SIX CELSIUS, ALTIMETER MISSING”
Example 8
In this example, variable visibility is automatically included as a remark. The magnetic declination at BUF is
8oW. The computed true wind direction (rounded to the
nearest degree) is 237 degrees, and the computed magnetic wind direction (also rounded to the nearest degree) is
245 degrees. For reporting purposes, wind directions are
rounded down to the nearest 10 degrees. Thus the true direction reported in the METAR is given as 240 degrees
while the magnetic direction reported in the computer-generated voice message is also given as 240 degrees.
SPECI KBUF 111541 AUTO 24010 11/2SM -SN SCT010
OVC018 M02/M03 A2955 RMK AO2 VIS 1V2
“GREATER BUFFALO AIRPORT, AUTOMATED
WEATHER OBSERVATION, ONE FIVE FOUR ONE
ZULU, WIND TWO FOUR ZERO AT ONE ZERO, VISIBILITY ONE AND ONE HALF, LIGHT SNOW, SKY
CONDITION SCATTERED AT ONE THOUSAND
OVERCAST AT ONE THOUSAND EIGHT HUNDRED,
TEMPERATURE MINUS ZERO TWO, DEW POINT
MINUS ZERO THREE, ALTIMETER TWO NINER FIVE
FIVE, REMARKS VISIBILITY ONE VARIABLE TWO”

CHAPTER SEVEN
7.0

Table 10.

Introduction

This chapter lists elements ASOS does not now provide and describes plans for product improvement.

n
n
n

7.1

n
n
n
n
n

Data Not Provided by ASOS

The elements not currently sensed or reported by ASOS
are listed in Table 10. Other complementary technology or
supplementary observing networks will report these weather
conditions. Some additional data may be added to the ASOS
report through augmentation. Data from separate observing networks or from remote sensing technologies such as
WSR-88D, GOES, and LDS can be used to complement
the sensor observation provided by ASOS.

n
n
n
n
n

The operational implementation of ASOS is clearly a
major technological achievement in automation of surface
observations. As with any new sophisticated technology,
continuous development and product improvement are essential to keep pace with an ever changing world. The ASOS
product improvement program is an on-going effort to enhance existing capabilities and develop new sensors and/
or algorithms for reporting hail, snowfall, liquid equivalent of frozen precipitation, and snow accumulation. Work
continues on incorporating independent ALS and NLDNALDARS data into the ASOS to provide real-time information on thunderstorm location.

7.2

n
n
n
n

Weather Elements Not Reported
by ASOS

Tornado, funnel cloud, waterspout b
Hail abc
Ice crystals a
[Snow grains, Ice Pellets, Snow pellets] abc
Drizzle, freezing drizzle abc
Volcanic ash b
Blowing obstructions (sand, dust, spray) abc
Smoke b
Snow fall (accumulation rate) and snow depth
(6-hourly) abc
Hourly snow increase (SNINCR) remarks abc
Liquid Equivalent of Frozen Precipitation abc
Water equivalent of snow on the ground ac
Clouds above 12,000 feet abcd
Operationally significant clouds above 12,000 feet
in mountainous areas b
Virga b
Distant precipitation in mountainous areas
and distant clouds obscuring mountains b
Operationally significant local variations
in visibility b
Minutes of sunshine c

a = Provided by supplementary observing networks
b = May be provided by manual augmentation
c = May be provided by future ASOS sensors/
algorithms
d = Provided by complementary technologies
(Satellite Cloud Product, ALDARS, etc.)

Planned Product
Improvement
baseline capabilities. The current ASOS baseline includes
reports for the following elements: Wind, Visibility, Present
Weather/Obscurations, Ambient Temperature/Dew Point
Temperature, Altimeter Setting (Pressure), and Liquid Precipitation Accumulation.

The development and fielding of the ASOS network
represents a large initial commitment of time and tax dollars. The goal of ASOS planned product improvements is
to ensure the most cost-effective benefits to the nation well
into the next century, to prevent premature system obsolescence and the need for a costly full system replacement,
and to more adequately meet changing user requirements
and decrease maintenance demands. To meet this goal, the
NWS has initiated a long-term product improvement
program.

Typically, several years of test and evaluation activities are required before a new sensor is ready for implementation. The process begins by first defining the
operational and performance requirements that a potential
sensor must meet, defining the specification, and then selecting a number of vendors from which to procure suitable prototypes. These prototypes are generally tested and
evaluated over at least a two year period in selected environments to ensure that the sensors are subjected to a full
spectrum of weather conditions.

The objectives of this multi-year program are to refine
and improve the ASOS baseline suite, while taking advantage of future scientific and technical breakthroughs in sensors and algorithms which will enhance and expand the

An iterative process of refining the hardware and software capabilities occurs during this period. Evaluation results and requests for specific changes are continuously
sent to the vendors. As potential technology improvements
become available, they are incorporated into the prototype
sensors for further testing.

ment of the silver coated mirror with a gold coated mirror.
This increases the mirror’s reflectivity and the sensor’s
ability to compensate for mirror contamination over a
longer period of time before maintenance is needed. An
effort is now underway to completely replace the chilled
mirror dew point sensor with an alternative technology.

The most promising prototype sensors, usually from
one vendor, are then integrated into the operational ASOS
sensor suite and tested at approximately 20 sites for another year or two. This procedure enables test and evaluation to occur in a more realistic setting and under more
varied weather conditions. It also helps to ensure that the
hard ware/software components of the new sensor are compatible with those of the existing ASOS sensor suite.

An alternative vendor/technology search was conducted in Spring 1996 and revealed five potential alternative sensor technologies:
n
n
n
n
n

The iterative process of sensor refinement continues
as the vendor receives evaluation data and requests for
changes. At this stage, the evaluation process includes feedback from potential users, made possible by the more operational setting. After evaluation and filtering of user
feedback comments, recommendations for improvement
are forwarded to the vendor. Again, as sensor improvements become available, they are incorporated into the prototype sensors for further testing. Upon successful
completion of this process, a new sensor/capability is added
to the ASOS nationwide network. The following sections
describe specific product improvement activities.

7.2.1

Based on analysis of the competing technologies, the
thin-film capacitive technology was selected for testing.
Initial testing began at four test locations in 1997.

7.2.3

Ceilometer/Sky Condition

The ASOS specification requires the ceilometer to report cloud heights up to 12,000 feet. Under most circumstances, the sky condition algorithm processes the cloud
height information properly. The resultant sky condition
report is fairly representative of the actual sky condition
near the airport. However, the algorithm can be improved
to enhance reporting of cloud bases and vertical visibility
during fog, virga, and precipitation events. Algorithm improvements are also needed to reduce the false reporting
of lower mid-level moisture layers as clouds.

Ice-Free Wind Sensor

The current ASOS wind sensor uses the traditional cup
and vane design that occasionally experiences freeze-up in
icing conditions. To minimize these occurrences, the ASOS
Program Office is testing a variety of replacement wind
sensors with respect to the ability to continue operation
under adverse winter conditions such as freezing rain, freezing drizzle, and snow. First-year testing started during the
fall of 1995 with ice-free wind sensors from four vendors.
Second-year testing (field, chamber, and wind tunnel)
started in December 1996 and is ongoing as of this writing.

7.2.2

Chilled surface-capacitive
Infrared hygrometer
Hygro-mechanical arch
Lithium chloride
Thin-film capacitive

The ASOS Program can and will make some of the
proposed algorithm enhancements using the current ceilometer. Laser beam ceilometer technology employed in the
current model has advanced in recent years to provide improved cloud base detection. In addition, new generation
cloud height sensors (e.g., 25,000 foot ceilometers) may
provide more reliable information in the lower atmosphere
during adverse weather conditions. Initial testing of the
25,000 foot ceilometers from three vendors started in August 1996.

Dew Point Sensor

The current ASOS dew point sensor exhibits several
problems, including corrosion and calibration problems,
resulting in inaccurate and unreliable performance of the
sensor. Consequently, ASOS technicians make more frequent periodic maintenance trips than the ASOS specified
90-day preventive maintenance interval. Several modifications have been made, and continue to be made, to the
existing dew point sensor. One improvement is the replace-

7.2.4

All-Weather Precipitation
Accumulation Gauge

The current ASOS Heated Tipping Bucket gauge is
strictly a liquid precipitation accumulation sensor. A Frozen Precipitation Water Equivalent Sensor was a planned
addition to ASOS. Numerous attempts were made to design a suitable frozen precipitation water equivalent sen60

sor. These sensors typically consisted of a collection bucket
mounted on a load cell. The basic problem was that the
load cell exhibited too much hysteresis with temperature.
In June 1996, the NWS Offices of Hydrology and Meteorology finalized a requirement for an “all-weather” gauge
that will measure both liquid and frozen precipitation water equivalent. Initial testing of all-weather precipitation
accumulation gauges (AWPAG) from two vendors started
in January 1997. The AWPAG requirement was modified
in Spring 1997 based on initial test results. Follow-on testing is planned for 1998.

7.2.5

be fielded as early as FY 1999. Limited production and
field implementation of other sensors/algorithms is estimated to start in the year 2000 and be completed by 2007.

7.3

The characteristics of the ASOS include:
n
n
n
n
n
n
n
n
n

Enhanced Precipitation
Identification

The current precipitation identification sensor does not
consistently detect and report light precipitation (< 0.01
inch/hour), ice pellets, or hail. The current precipitation
identification sensor was designed to meet a requirement
to detect precipitation that falls at a rate >0.01 inch/hour.
ASOS’s existing precipitation identification sensor can
detect rain or snow; however, the sensor identifies ice pellets and hail as rain, or undetermined precipitation (UP).
Field reports have identified a resurgent need for the detection and identification of drizzle or very light precipitation (< 0.01 inch/hour), ice pellets, and hail. Finally,
although the first generation algorithms for blowing snow
are now part of ASOS, further development and refinement is being pursued. Initial sensor testing from two vendors started during 1995-1996 winter. A single vendor’s
technology was tested during 1997.

7.2.6

Continuous Operation; Reports Updated Every Minute
Fast Response to Changing Conditions
Consistency of Observations
Designed for Aviation Operational Use
Critical Sensors Normally Located Near TDZ
Visibility Determinations to 10 Miles
Cloud Height Determination Up to 12,000 Feet
Present Weather Sensors Included
Multiple Sensors Where Needed

Among the basic strengths of the ASOS observation
is the ability to measure critical weather parameters (such
as sky condition and visibility) where they are needed most
—at the touchdown zone(s). Other parameters may be
measured at a representative location, usually near the center of the airport. ASOS data are updated once each minute
and transmitted directly to the air traffic controller. ASOS
generated METAR messages, SHEF messages, and daily
and monthly summary products are routinely made available for nationwide dissemination. The ASOS is capable
of performing all the basic observing functions and operating fully unattended, thus freeing observing personnel
for other demanding duties.
ASOS data may be accessed through a variety of media never before available from a surface observing site.
Various local on-site video screen displays are available
on the OID (proper access code/password may be required), and VDU. Remote monitor hook-up can be made
available to airlines and other external users on the airport. Authorized remote users (with modem-equipped
computers and the proper access code/password) may also
acquire a wide variety of OID screen displays through the
ASOS remote user dial-in port. Computer-generated voice
messages are made available to local aviation users through
ground-to-air broadcast and a dial-in telephone number
provided at each ASOS location. Long-line users are provided with routine access to ASOS generated METAR and
SHEF messages, and daily and monthly summary data.
The wealth of high-quality ASOS data now available to
users provides new resources and exciting opportunities
in meeting future needs for environmental data.

Sunshine

The ASOS specification has a requirement for a sunshine sensor. Test and evaluation of candidate sensors began in 1992. The sunshine sensors from test vendors were
tested and evaluated. All candidates’ sensors were compared with two reference sensors, the Foster-Foskett and
the Eppley pyrheliometer. Test results revealed that one
vendor was clearly superior in meeting the ASOS specification. In 1995, the Office of Meteorology redefined the
ASOS specification for the sunshine sensor. Sunshine sensors from the leading vendor were installed at four test sites
in early 1996. A one-two year demonstration began at eight
other sites in Summer 1997.

7.2.7

Summary

Schedule

The earliest product improvement sensor forecasted
to reach production is the sunshine sensor. It is expected to

ASOS DATA OUTLET
OID/REMOTE USER
INTERACTIVE SCREENS

ASOS
DATA

ON-SITE
SCREEN

TYPE
OBSVR
LEVEL
(1)

ATC
LEVEL
(1)

MAINT
LEVEL

SYS
MGR
LEVEL

UNSIGNED
USER
LEVEL

VDU

(STAFFED
NWS
SITES)
AFOS

METAR DATA

AUXILIARY
DATA

SHEF DATA

DAILY
SUMMARY
DATA

MONTHLY
SUMMARY
DATA

NWS
AWIPS

FAA
NADIN

GROUNDTO-AIR
RADIO

MAINTENANCE

DATA

u Data type available through data outlet

Figure 4.

Availability of ASOS Data

1
2
3
4

TELEPHO
DIAL-IN

OMO DATA

COMPUTER GENERA
VOICE

LONG-LINE

ON-SITE
PRINTER

This level not available remotely
Selected OMO data provided once every 5 or 15 minutes upon request
Either OMO or last transmitted METAR/SPECI - Not both
Available through FAA ADAS interface

Level 1
Site Specific Area

Level 2
County Warning
Area

NCEP QC
MESSAGES

“Near Real Time”
“Real Time”
On-Site Observer

FAA
Reg HQ

FAA
AFSS

Level 3
National Area
FAA
WMSCR

FAA
HQ

FAA Sites
NWS Sites

NWS
Reg HQ
WFO

NWS
HQ

AOMC

On-Site ASOS
Continuous
Self Checking

Auto
QC
Programs

NWSTG

• System Oversight
• Meteorological

Maintenance

Figure 1.

ASOS Quality Control Concept

Expertise
• WFO Monitoring
(human &
automated)
• Regional Oversight

Based on METAR
Observations
• Synoptic Overview
• Interactive Technology
• Automated QC Systems

FAA
ASOS

METAR/SPECI
SHEF DS/MS

GS200/ADAS

WMSCR
MET
SPE

SHEF
DS/MS

NWSTG
55

NWS
ASOS

METAR/SPECI
SHEF DS/MS

AFOS
OR
PACE*

SHEF
DS/MS

M
S

METAR/SPECI
SHEF DS/MS

SMCC
- Interagency Interface
METAR/SPECI - Aviation Routine Weather Report
SHEF - Standard Hydrom eteorological Exchange Format Reports
NWSTG - National Weather Service Telecommunications Gateway
ADAS - AWOS/ASOS Data Acquisition System
WMSC - Weather Message Switching Center
Figure 18. ASOS Network Data Flow–—Initial Configuration (1997)

* - PC-Based Asynchrono
Communications Exten
(for ASOS)

FAA
NETWORK
ASOS

ADAS

OMOs

Os
OM

NADIN-II

WMSCR

METAR/SPECI
SHEF DS/MS

METAR/SPECI
SHEF DS/MS
5/15-Minute Data*

SHEF
DS/MS

Proposed

5/15-Minute
Data

o
Pr

d
se
o
p

RS
A
D
AL

ta
Da

5/15-Minute Data*

METAR/SPECI
SHEF DS/MS

NWS
NETWORK
ASOS

METAR/SPECI

Proposed ALDARS
Data

NWSTG

AWIPS
5/15-Minute Data*

- Interagency Interface
METAR/SPECI - Aviation Routine Weather Report
SHEF - Standard Hydrometeorological Exchange Format Reports
OMOs - One Minute Observations
DS/MS - Daily Summary/Monthly Summary Messages
WMSCR - Weather Message Switching Center Replacement
ALDARS - Automated Lightning Detection and Reporting System
Figure 19. ASOS Network Data Flow––Final Configuration

* - Alternate proposal for delivery
the 5/15-minute data from FAA to

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