S 190_sw Preface 190 Student Workbook

User Manual: S 190

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PREFACE
Introduction to Wildland Fire Behavior, S-190, is identified training in the National
Wildfire Coordination Group’s (NWCG), Wildland and Prescribed Fire
Curriculum. This course has been developed by an interagency development
group with guidance from the National Interagency Fire Center (NIFC), Fire
Training Group under authority of the NWCG, with coordination and assistance of
personnel from the following agencies:
Bureau of Land Management
Timothy Mathewson
Bureau of Land Management
Christine Keavy
National Park Service
Patrick Morgan
USDA Forest Service
Michelle Ellis
Fire Behavior Committee Liaison
Risa Lange-Navaro
NWCG Fire Training
Noble Dunn, Deana Parrish, Sue Hickman
The NWCG appreciates the efforts of these personnel, and all those who have
contributed to the development of this training product.

CONTENTS

PREFACE ...................................................................................................................i

UNITS OF INSTRUCTION
Unit 0 – Introduction.....................................................................................0.1
Unit 1 – Basic Concepts of Wildland Fire....................................................1.1
Unit 2 – Principles of Wildland Fire Behavior
Lesson 2A – Topographic Influences ..............................................2A.1
Lesson 2B – Fuels............................................................................ 2B.1
Lesson 2C – Weather ....................................................................... 2C.1
Unit 3 – Wildland Fire Behavior and Safety ................................................3.1

iii

Introduction to Wildland Fire Behavior, S-190
Unit 0 – Introduction

OBJECTIVES:
During this unit the instructor will:
1.

Introduce instructors and students.

Discuss administrative concerns.

Explain the purpose of the course.

Explain the course objectives.

Discuss expectations.

Explain course evaluation methods.

Explain where the course fits in the wildland fire behavior curriculum.

0.1

0.2

INTRODUCTIONS

II.

ADMINISTRATIVE CONCERNS

III.

PURPOSE OF COURSE
To provide the student with wildland fire behavior knowledge applicable for
safe and effective fire management activities (wildfires, prescribed fire, and
fire use).
This course introduces students to characteristics and interactions of the
wildland fire environment (fuels, weather, and topography) that affect
wildland fire behavior for safety purposes.
The materials in this course are elements of the wildland fire behavior
curriculum.
This is the first formal wildland fire behavior training course the students
will receive.

IV.

COURSE OBJECTIVES
•

Identify and discuss the three sides of the fire triangle.

•

Identify the environmental factors of fuels, weather, and topography
that affect the start and spread of wildland fire.

•

Describe the contributing factors that indicate the potential for
increased fire behavior that may compromise safety.

0.3

EXPECTATIONS
A.

Student Expectations
What are your expectations for the course?

VI.

Instructor Expectations
•

Attendance at all sessions

•

Be prepared to start on time

•

Participate and share ideas

EVALUATIONS
A.

Student Evaluations
Students must obtain 70% or higher on the final exam to receive a
certificate of completion for this course.

Course Evaluations
This is an opportunity for students to comment on the course and
instructors for the purpose of improving future courses.

0.4

VII. WHERE DOES THIS COURSE FIT IN THE WILDLAND FIRE
BEHAVIOR CURRICULUM?
A.

Introduction to Wildland Fire Behavior, S-190
Entry-level course designed around the basics of fuel, weather, and
topography.

Intermediate Wildland Fire Behavior, S-290
Provides a better basis for analyzing variables and understanding how
they interact and affect wildland fire behavior. Introduces the Fireline
Assessment Method (FLAME); a practical fireline tool used for
predicting significant short term changes in wildland fire behavior.

Introduction to Wildland Fire Behavior Calculations, S-390
Introduces wildland fire behavior calculations by manual methods
such as tables and nomograms.

Advanced Wildland Fire Behavior Calculations, S-490
Teaches students to use state of the art computer models to project fire
perimeter growth based on weather predictions and knowledge of
fuels and topography.

0.5

0.6

Introduction to Wildland Fire Behavior, S-190
Unit 1 – Basic Concepts of Wildland Fire

OBJECTIVES:
Upon completion of this unit, students will be able to:
1.

Define basic terminology used in wildland fire.

Identify the elements of the fire triangle.

Describe three methods of heat transfer.

1.1

1.2

BASIC TERMINOLOGY USED IN WILDLAND FIRE
A.

Parts of the Fire
1.

Point of origin
The precise location where a competent ignition source came
into contact with the material first ignited and sustained
combustion occurred.

Head of a fire
The side of the fire having the fastest rate of spread.

Flank of a fire
The part of a fire’s perimeter that is roughly parallel to the main
direction of spread.

Rear of a fire
•

That portion of a fire spreading directly into the wind or
down slope.

•

That portion of a fire edge opposite the head.

•

Slowest spreading portion of a fire edge. Also called
heel of a fire.

1.3

Fire perimeter
The entire outer edge or boundary of a fire.

Fingers of a fire
The long narrow extensions of a fire projecting from the main
body

Pockets of a fire
Unburned indentations in the fire edge formed by fingers or
slow burning areas.

Island
Area of unburned fuel inside the fire perimeter.

Spot fire
Fire ignited outside the perimeter of the main fire by a
firebrand.

1.4

1.5

Fire Behavior Terms
1.

Smoldering
Fire burning without flame and barely spreading.

Creeping fire
Fire burning with a low flame and spreading slowly.

Running fire
Behavior of a fire spreading rapidly with a well defined head.

Spotting
Behavior of a fire producing sparks or embers that are carried
by the wind and which start new fires beyond the zone of direct
ignition by the main fire.

Torching
The burning of the foliage of a single tree or a small group of
trees, from the bottom up.

Crown fire
A fire that advances from top to top of trees or shrubs more or
less independent of a surface fire. Crown fires are sometimes
classed as running or dependent to distinguish the degree of
independence from the surface fire.

1.6

Flare up
Any sudden acceleration in the rate of spread or intensification
of the fire. Unlike blowup, a flare-up is of relativity short
duration and does not change existing control plans.

Firewhirl
Spinning vortex column of ascending hot air and gases rising
from a fire and carrying aloft smoke, debris, and flame. Fire
whirls range in size from less than one foot to over 500 feet in
diameter. Large fire whirls have the intensity of a small
tornado.

Backing fire
That portion of the fire with slower rates of fire spread and
lower intensity, normally moving into the wind and/or down
slope. Also called heel fire.

10.

Flaming front
That zone of a moving fire where the combustion is primarily
flaming.
Behind this flaming zone combustion is primarily glowing or
involves the burning out of larger fuels (greater than about 3
inches in diameter).
Light fuels typically have a shallow flaming front, whereas
heavy fuels have a deeper front.

1.7

Other Useful Firefighting Terms
1.

Anchor point
An advantageous location, usually a barrier to fire spread, from
which to start constructing a fireline. The anchor point is used
to minimize the chance of being flanked by the fire while the
line is being constructed.

Control line
An inclusive term for all constructed or natural barriers and
treated fire edges used to contain a fire.

Fireline
The part of a containment or control line that is scraped or dug
to mineral soil.

Mop-up
Extinguishing or removing burning material near control lines,
felling snags, and trenching logs to prevent rolling after an area
has burned, to make a fire safe, or to reduce residual smoke.

Contained
The status of a wildfire suppression action signifying that a
control line has been completed around the fire, and any
associated spot fires, which can reasonably be expected to stop
the fire's spread.

1.8

Controlled
The completion of control line around a fire, any spot fires, and
any interior islands to be saved.
Burn out any unburned area adjacent to the fire side of the
control lines.
Cool down all hot spots that are immediate threats to the control
line, until the lines can reasonably be expected to hold under the
foreseeable conditions.

Chain
Unit of measure in land survey, equal to 66 feet (20 M)
(80 chains equal 1 mile).
Commonly used to report fire perimeters and other fireline
distances.
Popular in fire management because of its convenience in
calculating acreage (example: 10 square chains equal one acre).

1.9

II.

ELEMENTS OF THE FIRE TRIANGLE
Three elements must be present and combined before combustion can occur
and continue. There must be:
•

Fuel to burn

•

Air to supply oxygen for the flame

•

Heat to start and continue the combustion process

These three elements or sides compose what we call the “fire triangle.”
Remove any single one, and there can be no fire.

III.

THREE METHODS OF HEAT TRANSFER
We have learned that heat is a necessary condition for combustion, and part
of the fire triangle. There are many methods by which heat can be supplied
to a fuel to start a fire. Examples:
•
•
•

Matches
Lightning
Cigarettes

More importantly, we must know how the fire spreads once it has started.
Heat must be able to move from one burning piece to another, or the fire
triangle will be broken. This movement is called heat transfer. Heat is
transferred by three processes:
•
•
•

Radiation
Convection
Conduction

1.10

Radiation
Think of radiant heat as a ray or wave. Radiant heat warms you as
you stand close to a campfire, or stand in the sunlight. Radiant heat
can dry surrounding fuels and sometimes ignite them.

Convection
Think of convection as a smoke column above the fire. Convection
occurs when lighter warm air moves upward. The hot gases and
embers which compose the smoke column can dry and ignite other
fuels.

Conduction
Think of conduction as a spoon in a hot drink. Heat is conducted
from one fuel particle to another in the same way, through direct
contact.
Since wood is a poor conductor (meaning heat will not travel through
it easily), this process is the least important of the three to fire
behavior.

1.11

OPEN BOOK EXERCISE.
1.

What are the three methods of heat transfer?

The fire triangle consists of oxygen, heat, and ____________________?

When is a fire controlled?

Should you fight fire without an anchor point?

Why?

1.12

Introduction to Wildland Fire Behavior, S-190
Unit 2 – Principles of Wildland Fire Behavior
Lesson A – Topographic Influences

OBJECTIVE:
Upon completion of this lesson, students will be able to:
•

List the basic characteristics of topography and describe how they affect
wildland fire behavior.

2A.1

2A.2

TOPOGRAPHY
Topography is the configuration of the earth’s surface including its relief and
the position of its natural and man-made features.
It is much easier to predict the influences which topography will have on a
wildfire than the influences of fuel and weather.
Following are topographic terms and how they affect wildland fire behavior.
A.

Aspect
Aspect is the direction a slope is facing (its exposure in relation of the
sun).
The aspect of a slope generally determines the amount of heating it
gets from the sun; therefore, determines the amount, condition, and
type of fuels present.
1.

South and southwest slopes are normally more exposed to
sunlight and generally have:
•
•
•
•

lighter and sparser fuels
higher temperatures
lower humidity
lower fuel moisture

They are the most critical in terms of start and spread of
wildland fires.

2A.3

North facing slopes have more shade which causes:
•
•
•
•

heavier fuels
lower temperatures
higher humidity
higher fuel moistures

A north facing aspect will have less fire activity than a south
facing slope.

Slope
The amount or degree of incline of a hillside (a steep slope).
Fires burn more rapidly uphill than downhill. The steeper the slope,
the faster the fire burns.
This is because the fuels above the fire are brought into closer contact
with the upward moving flames.
Convection and radiant heat help the fuel catch fire more easily.
Another concern about steep slopes is the possibility of burning
material rolling down the hill and igniting fuel below the main fire.
The position of the fire in relation to the topography is a major factor
in the resulting fire behavior.
A fire on level ground is primarily influenced by fuels and wind.
A fire which starts near the bottom of a slope during normal upslope
daytime wind conditions will normally spread faster and has more
area to spread upslope than a fire that starts near the top of the slope.

2A.4

Shape of the Country – Terrain
Certain topographic features can influence the wind speed and
direction for small areas, independent of general weather conditions
for an area.
The shape of the country can also influence the direction of fire
spread, rate of spread, and the intensity.

Box Canyons
Fires starting near the base of box canyons and narrow canyons may
react similar to a fire in a wood burning stove or fireplace.
Air will be drawn in from the canyon bottom creating very strong
upslope drafts. These upslope drafts create rapid fire spread up the
canyon, also referred to as the chimney affect. This affect can result
in extreme fire behavior and can be very dangerous.

Narrow Canyons
Fire in a steep narrow canyon can easily spread to fuels on the
opposite side by radiation and spotting. Wind eddies and strong
upslope air movement may be expected at sharp bends in canyon.

Wide Canyons
Prevailing wind direction can be altered by the direction of the
canyon. Cross-canyon spotting of fires is not common except in high
winds. Strong differences in fire behavior will occur on north and
south aspects.

2A.5

Ridges
Fire burning along lateral ridges may change direction when they
reach a point where the ridge drops off into a canyon. This change of
direction is caused by the flow of air in the canyon.

Saddle
Wind blowing through a saddle or pass in a mountain range can
increase in speed as it passes through the constricted area and spreads
out on the downwind side with possible eddy action.

Elevation
The height of the terrain above mean sea level, usually expressed in
feet (ASL - Above Sea Level).
Elevation plays a large role in determining the conditions and amount
of fuel.
Because of higher temperatures, fuels at lower elevations dry out
earlier in the year than those at higher elevations.
In extremely high elevations there may be no fuel.
Elevation affects fire behavior in several other ways like the amount
of precipitation received, wind exposure, and its relationship to the
surrounding terrain.

2A.6

Barriers
Any obstruction to the spread of fire, typically an area or strip lacking
any flammable fuel.
Barriers to fire include many things, both natural and man-made.
1.

Natural barriers:
•
•
•
•

rivers
lakes
rock
slides

Fuels which have a high moisture content do not burn as well as
others in the same area.
2.

Man-made barriers:
•
•
•
•

roads
highways
reservoirs
fireline constructed by fire resources

2A.7

OPEN BOOK EXERCISE.
1.

A barrier is:

Under normal conditions, a north facing aspect will have more fire activity
than a south facing aspect.
a.
b.

True
False

A box canyon is dangerous because:

2A.8

Introduction to Wildland Fire Behavior, S-190
Unit 2 – The Principles of Wildland Fire Behavior
Lesson B – Fuels

OBJECTIVES:
Upon completion of this lesson, students will be able to:
1.

Identify the basic fuel types.

Identify the fuel characteristics that influence the behavior of the fire.

2B.1

2B.2

THE SIX BASIC FUEL TYPES
A.

Definition of Fuel
A simple definition of fuel is any burnable material.
•

Wildland fuels are basically live and/or dead plant material.

•

Houses, sheds, etc., can also be fuels

Fuels are the source of energy that drives the fire.
Regardless of the area of the country, fire behavior is dependent on
certain fuel characteristics:
•
•
•

Fuel type
Fuel loading
Fuel availability

Fuel Types
Wildland fuels are grouped into fuel types based on the primary fuel
that carries the fire. There are six major fuel types:
•
•
•
•
•
•

Grass
Grass – Shrub
Shrub
Timber – Understory
Timber litter
Slash - Blowdown

Fuels vary in type from one area of the country to another and within
the same area.
Differences in the amount of water in the soil is one reason that types
of fuels vary and elevation changes is another.

2B.3

Grass
•

Found in most areas.

•

More dominant as a fuel in desert and range areas.

•

Can become prevalent after a fire in timber areas.

•

Burns hottest and fastest.

Grass – Shrub
•

Found in the plains regions and high deserts.

•

A significant contributor to fire spread due to the fine
fuels mixed with the aerial/shrub fuel.

Shrub
•

Found throughout most areas.

•

Some highly flammable shrub fuels are:
–
–
–

Palmetto/gallberry in the southeast
Sagebrush in the Great Basin
Chaparral in the southwest and California

Timber - Understory
•

Found throughout most areas

•

Provides ladder to aerial crown fuels

2B.4

Timber litter
•

Most dominant in mountainous topography, especially in
the Northwest.

•

Provides fuel for ground fire.

Slash - Blowdown
•

Debris left after natural events or human activities:
–
–
–
–
–
–
–
–

•

Debris may include:
–
–
–
–
–
–
–

•

Logging
Road building
Pruning
Thinning
Shrub cutting
Wind
Fire
Snow

Logs
Chunks of wood
Bark
Branches
Stumps
Broken understory trees
Shrubs

Provides fuel for fire spread

2B.5

EXERCISE 1.
For your assigned fuel type, answer the following questions on a flip chart:
1.

List examples, near your area, where this fuel type occurs.

Why is this fuel type a possible concern to firefighters?

II.

FUEL CHARACTERISTICS THAT INFLUENCE THE BEHAVIOR OF
THE FIRE
A.

Fuel Type

Fuel Loading
The amount of fuel present expressed quantitatively in terms of
weight of fuel per unit area (tons per acre).
•

This may be available fuel (consumable fuel) or total fuel and is
usually dry weight.

•

The loading of the fuels in any given area does not necessarily
mean the fire will burn with great intensity.

•

What is more important is the quantity of fuels available for
combustion.

2B.6

Fuel Availability (for combustion)
Many factors are involved when talking about the availability of a fuel
for combustion.
1.

Fuel size classes and shape
The physical characteristics of fuels, divided into four
categories on the basis of their size:

1-hour fuels: 0 – ¼ inch in diameter

10-hour fuels: ¼ – 1 inch in diameter

100-hour fuels: 1 – 3 inches in diameter

1000-hour fuels: 3 – 8 inches in diameter

Surface area to volume ratio
•

Relates to the amount of the outer surface of the fuel that
is exposed to the air.

•

The more surface exposed, the more easily the fuel will
dry and burn.

•

Smaller (fine) fuels have a higher surface area to volume
ratio than larger (heavy) fuels.

2B.7

An example to illustrate surface-area-to-volume ratio concepts
is the process of building a campfire:
•

Start with small fuels (such as dry grass, pine needles,
and small twigs), then add larger fuels (such as larger
twigs and sticks), and finally add the largest fuel – the
logs.
The smaller fuels (grass, needles, etc.) have a larger
surface area to volume ratio than the logs, and therefore
ignite more readily than the logs.

Fuel Arrangement
The manner in which fuels are spread over a certain area.
1.

Horizontal continuity
Horizontal continuity affects fire’s rate of spread.
a.

Uniform fuels
Include all fuels distributed continuously over the area.
•

Areas containing a network of fuels which connect
with each other to provide a continuous path for a
fire to spread are included in this category.

2B.8

Patchy fuels
Include all fuels distributed unevenly over the area, or
areas of fuel with definite breaks or barriers present.
Examples:

•

Patches of rock outcroppings.

•

Bare ground.

•

Areas where another dominant type of fuel is much
less flammable.

Vertical arrangement
a.

Ground fuels
All combustible materials lying beneath the surface:
•
•
•
•

Deep duff
Tree roots
Rotten buried logs
Other organic material

Ground fire consumes the organic and combustible
materials beneath the surface, such as a smoldering duff
or peat fire.

2B.9

Surface fuels
All combustible materials lying on or immediately above
the ground:
•
•
•
•
•
•
•
•

Needles or leaves
Duff
Grass
Small dead wood
Downed logs
Stumps
Large limbs
Low shrubs

Surface fire burns surface litter, debris, small shrubs, and
other vegetation.
c.

Ladder fuels
Combustible materials that aid the spread of fire from the
surface to the upper canopy.
•

Ladder fuels can include surface litter, shrubs, and
other moderate height vegetation that provides a
pathway from the surface to the canopy.

Aerial fuels
All green and dead materials located in the upper canopy:
•
•
•
•

Tree branches and crowns
Snags
Hanging moss
Tall shrubs

Crown fire burns through the tops of trees or shrubs and
can advance in conjunction with or be independent of the
surface fire.

2B.10

Fuel moisture
The amount of water in a fuel, expressed as a percentage of the
oven-dry weight of that fuel.
•

Fuel moisture is expressed as a percent of total weight.

•

How well a fuel will ignite and burn is dependent, to a
large extent, on its moisture content.

•

Dry fuels will ignite and burn much more easily than the
same fuels when they are wet.
–

•

You don’t use wet wood to make a campfire!

Before a wet fuel can burn, the moisture it contains must
evaporate.
–

This process requires more heat. As fuel moisture
increases, the amount of heat required to ignite and
burn that fuel also increases.

•

Because of their various sizes and characteristics, different
fuels in the same area will have various moisture levels.

•

Likewise a similar type of fuel, across a broad area, will
have different moisture levels based on the amount of
precipitation received and period of warm, dry weather.

Remember, light (small) fuels take on and lose moisture faster
than heavier (larger) fuels.

Wet fuels
Fuels that have a high moisture content because of exposure to
precipitation or high relative humidity.

2B.11

Dry fuels
Fuels that have low moisture content because of prolonged
exposure to sunshine, dry winds, drought, or low relative
humidity.

Timelag
The rate at which dead fuel gains or loses moisture.
Time needed under specified conditions for a fuel particle to
lose about 63 percent of the difference between its initial
moisture content and its equilibrium moisture content.
If conditions remain unchanged, a fuel will reach 95 percent of
its equilibrium moisture content after four timelag periods.
Firefighters use a concept of “Timelag” to identify the different
sizes of dead fuels as they relate to increasing moisture or
drying-out over time.
Smaller fuels can dry out in an hour larger fuels may take ten to
a thousand hours to dry.
One-hour timelag fuels react to changes in relative humidity
much faster than 100-hour timelag fuels.
The timelag categories are:
1-hour
10-hour
100-hour
1000-hour

0 - ¼ inch in diameter
¼ - 1 inch in diameter
1 - 3 inches in diameter
3 - 8 inches in diameter

2B.12

EXERCISE 2.
Part 1
View each photo and match it to the correct description.
Photo #1: ____

a. These are patchy fuels.

Photo #2: ____

b. This is a large volume of fuel.

Photo #3: ____

c. These are 1-hr timelag fuels.

Photo #4: ____

d. This is a shrub fuel type.

Part 2
Match the definitions to the correct descriptions.
Uniform Fuels

____

A. The amount of water in a fuel, expressed as a
percentage of the oven-dry weight of that fuel.

Ladder Fuels

____

B. Grasses, leaves, pine needles

Fuel Timelag

____

C. Fuels distributed continuously over the area.

Light Fuels

____

D. All combustible materials lying on or
immediately above the ground.

Fuel Moisture

____

E. The rate at which dead fuel gains or loses
moisture.

Surface Fuels

____

Combustible materials that aid the spread of fire
from the surface to the upper canopy.

2B.13

2B.14

Introduction to Wildland Fire Behavior, S-190
Unit 2 – Principles of Wildland Fire Behavior
Lesson C – Weather

OBJECTIVES:
Upon completion of this lesson, students will be able to:
1.

Describe the effect temperature and relative humidity have on wildland fire
behavior.

Describe the effect of precipitation on wildland fire behavior.

Describe the differences between a stable and unstable atmosphere.

Describe general and local winds.

Describe critical fire weather conditions.

List the different types of fire weather forecasts and outlooks available.

2C.1

2C.2

WEATHER
Short-term variations in the atmosphere are what we call weather. Weather
is one of three components of the fire environment.
Weather conditions can result in the ignition of fire by lightning from
thunderstorms and the rapid spread of fires as a result of strong winds. On
the other hand, an increase in humidity or precipitation can slow or
extinguish fires.
Of the three fire environment components, weather is the most variable over
time, and at times, difficult to predict.
Firefighters conducting fire suppression must monitor the weather at all
times to make safe and effective firefighting decisions.
The importance of monitoring weather and predicting the resultant fire
behavior cannot be overstressed. It is one of the 10 Fire Orders and three of
18 Watchout Situations that all firefighters must obey.
The risk involved in fire suppression can be reduced if firefighters and fire
managers pay attention and understand weather conditions that impact fire
behavior.
The basic principles and concepts of fire weather as they relate to wildland
fire behavior include:
•

Air Temperature and Relative Humidity (RH)

•

Precipitation

•

Atmospheric Stability

•

Wind

2C.3

II.

THE EFFECT TEMPERATURE AND RELATIVE HUMIDITY HAVE
ON WILDLAND FIRE BEHAVIOR
A.

Air Temperature
Air temperature is the degree of hotness or coldness of the air.
1.

Air temperature varies with:
•
•
•

Changes in air temperature near the surface of the earth are
caused by:
•
•
•

Time
Location
Height above the earth’s surface

Changing seasons
Alternations of night and day
Migrating weather systems

Seasonal and diurnal temperature changes can be large or small,
depending on:
•
•
•
•

Latitude
Elevation
Topography
Proximity to the moderating influences of nearby
oceans or lakes

Abrupt changes in temperatures can occur when migrating weather
systems transport colder or warmer air into a region.
Heating of the earth’s surface and the atmosphere is primarily a result
of solar radiation from the sun; however, on a smaller scale, heat may
be caused by a large fire.

2C.4

In the wildland fire environment, direct sunlight and hot temperatures
can preheat fuels and bring them closer to their ignition point, whereas
cooler temperatures have the opposite affect.
Above average temperatures are common on large fires. Many
firefighter fatalities have occurred on fires where record high
temperatures were set.
Temperature is measured with a thermometer calibrated either to the
Fahrenheit scale or the Celsius scale.

Relative Humidity
Relative humidity is the amount of moisture in the air divided by the
amount the air could hold when saturated at the same air temperature;
usually expressed in percent.
Relative humidity can range from 1% (very dry) to 100% (very
moist). Low relative humidity is an indicator of high fire danger.
Moisture in the atmosphere, whether in the form of water vapor, cloud
droplets, or precipitation, is the primary weather element that affects
fuel moisture content and the resulting flammability of wildland fuels.
The amount of moisture that fuels can absorb from or release to the air
depends largely on relative humidity. Light fuels, such as grass, gain
and lose moisture quickly with changes in relative humidity. Heavy
fuels respond to humidity changes much more slowly.
Firefighters can usually see or feel most of the elements of weather
such as:
•
•
•

Wind
Rain
Increasing temperatures

2C.5

Small changes in relative humidity that cannot be felt or seen can have
a significant impact on wildland fire behavior.
Relative humidity values for extreme wildland fire behavior vary over
time and location, and are different for different fuels types.
Fuels in the southeast part of the United States and Alaska typically
burn with considerably higher relative humidities than fuels in the
western U.S.

Temperature and Relative Humidity Relationships
Temperature and relative humidity have an inverse relationship.
•

When temperature increases, relative humidity decreases.

•

When temperature decreases, relative humidity increases.

In the early morning hours, temperature typically reaches its lowest
point and relative humidity reaches its highest point. As the sun rises
and the temperature increases, relative humidity decreases.
When the temperature reaches its maximum for the day (usually mid
to late afternoon) relative humidity decreases to a minimum. This is
the time when fine fuel moisture reaches its minimum. As the sun
sets the temperature drops and the relative humidity increases.
There can be a large fluctuation of temperature and relative humidity
in time and location. However, the majority of large fire outbreaks
occur when air temperature is high and relative humidity is low.
It is very important for firefighters to routinely monitor temperature
and relative humidity trends. The most common field instrument used
to measure and determine these two important weather elements is a
sling psychrometer, which is part of the belt weather kit.

2C.6

Exercise 1. Temperature and Relative Humidity
1.

Temperature is:
A.

The degree of hotness or coldness of a substance.

The amount of moisture in the air.

The amount of moisture in the air divided by the amount the air could
hold when saturated at the same air temperature.

Relative humidity is:
A.

The degree of hotness or coldness of a substance.

The amount of moisture in the air divided by the amount the air could
hold when saturated at the same air temperature.

Expressed in degrees Fahrenheit.

Expressed as a percentage.

B and D

As temperature increases, relative humidity:
A.

Increases

Decreases

2C.7

III.

THE EFFECT OF PRECIPITATION ON WILDLAND FIRE BEHAVIOR
A.

Precipitation
Precipitation is liquid or solid water particles that originate in the
atmosphere, and become large enough to fall to the earth’s surface.

Precipitation Amount vs. Duration
Fuel moisture is affected by the amount and also the duration of the
precipitation.
Fine fuels react quite rapidly by precipitation since they gain or lose
moisture usually within one hour.
Heavy fuels are not affected as drastically since they gain or lose
moisture more slowly.
A large amount of precipitation in a short time will not raise the fuel
moisture as much as less rainfall over a longer period of time where
the fuels can absorb more moisture before it runs off.

2C.8

IV.

THE DIFFERENCES BETWEEN A STABLE AND UNSTABLE
ATMOSPHERE
A.

Atmospheric Stability
Wildfires are greatly affected by atmospheric motion and the
properties of the atmosphere that affect its motion.
Surface winds, temperature, and relative humidity are most commonly
considered and easy to measure in the fire environment.
Less obvious, but equally important, is atmospheric stability and
related vertical air movements that influence wildfire.
Atmospheric stability is the degree to which vertical motion in the
atmosphere is enhanced or suppressed. Stability is directly related to
the temperature distribution of the atmosphere.
The temperature and stability of the atmosphere is constantly
changing with variations over time (day-to-day or season-to-season),
location, and from one layer of the atmosphere to the next.

Stable Atmosphere
A stable atmosphere is defined as an atmosphere that resists upward
motion.
In a stable atmosphere, the extensive heat of the fire generates vertical
motion near the surface, but the vertical motion above the surface is
weakened, thus limiting indrafts into the fire at low levels and fire
intensity.

2C.9

Visual Indicators of a Stable Atmosphere
In the fire environment, visual indicators can give clues about the
stability of the atmosphere. Keeping in mind that stable air resists
upward vertical motion, the following are visual indicators of a stable
atmosphere:

•

Clouds in layers

•

Stratus type clouds

•

Smoke column drifts apart after limited rise

•

Poor visibility due to smoke or haze

•

Fog layers

•

Steady winds

Inversions
The usual temperature structure of the lower atmosphere is
characterized by a decrease in temperature with altitude. However, a
layer where temperature increases with altitude (warm air over cold
air) may exist. This layer is referred to as an inversion.
Warm air
------------- = Inversion (stable condition)
Cold air
Under an inversion, fuel moisture content is usually higher, thus
decreasing fire spread rates and intensities.
Updrafts containing smoke and warm gases generated by a fire are
typically weak and will only rise until their temperature equals that of
the surrounding air. Once this occurs, the smoke flattens out and
spreads horizontally.

2C.10

When inversions break or lift, as a result of heating the lower
atmosphere by the sun or a fire, increased wildland fire behavior is
almost certain.
1.

Watch for the following indicators when an inversion breaks:
•
•
•

Increase in temperature
Decrease in relative humidity
Increase and/or shift in wind

There are four types of inversions that may be encountered in
the wildland fire environment:
•
•
•
•

Nighttime (radiation)
Subsidence
Frontal
Marine

Though all inversions are important, nighttime and subsidence
inversions are most common in the wildland fire environment.

Nighttime (Radiation) Inversions
Air cooled at night, primarily by contact with cold surfaces, gradually
deepens as the night progresses and forms a surface inversion.
Inversions forming at night near the earth’s surface are commonly
referred to as a radiation or nighttime inversion.
Nighttime inversions develop on calm, clear nights when radiational
cooling of the earth’s surface is greatest, and can differ in strength
depending on time of year. Inversions in the winter are typically
stronger than inversions that develop in the summer.
Nighttime inversions are easy to identify because they trap smoke and
gases resulting in poor visibilities in valleys or drainages.

2C.11

Thermal Belts
Nighttime inversions in mountainous regions increase in depth during
the night. They form early in the evening at the canyon bottom or
valley floor and at first are quite shallow. The cold layer gradually
deepens, with the nighttime inversion coming in contact with and
reaching farther up the slope below the main ridges.
The warmest nighttime air temperatures in valleys are often found at
the inversion top. The height of the warmest air temperature, at the
top of the inversion, can be found by measuring temperature along the
slope. From the top of the inversion, temperature decreases as one
goes farther up or down the slope. This region of warmer air,
typically found on the middle third of the slope, is called the
THERMAL BELT.
The thermal belt is characterized by the highest minimum temperature
and the lowest nighttime relative humidity. Within the thermal belt,
wildland fires can remain rather active throughout the night. Below
the thermal belt, fires are in cool, humid, and stable air.

Subsidence Inversion
Subsidence is the large-scale sinking of air associated with high
pressure systems. As air from higher elevations in high pressure
systems descends to lower elevations, it warms and dries.
The warming and drying of air sinking is so pronounced that saturated
air (air with 100% RH), can produce relative humidity less than 5
percent in a very short period of time. If a high pressure system
persists for a period of days, the subsidence inversion may reach the
surface with only very little external modification or addition of
moisture.
Skies are typically clear or cloudless under these high pressure
systems, and extended periods of above average temperatures and
below average relative humidities can dry out fuels to the point that
burning conditions become severe. Subsidence is usually a
contributor in the development of foehn winds.

2C.12

Unstable Atmosphere
An unstable atmosphere is defined as an atmosphere that encourages
upward motion.
When the atmosphere is unstable, vertical motions increase,
contributing to increased fire activity by:
•

Allowing convection columns to reach greater heights,
producing stronger indrafts and convective updrafts.

•

Increasing the lofting of firebrands by updrafts.

•

Increasing the occurrence of dust devils and fire whirls.

•

Increasing the potential for gusty surface winds.

Wildland fires burn hotter and with more intensity when the air is
unstable. Cold air over warm air represents an unstable condition.
Cold air
------------- = Unstable condition
Warm air

Visual Indicators of an Unstable Atmosphere
•

Clouds grow vertically and smoke rises to great heights

•

Cumulus clouds

•

Good visibility

•

Gusty winds

•

Dust devils and firewhirls

2C.13

Exercise 2. Atmospheric Stability
1.

A stable atmosphere:
A.

Encourages upward vertical motion

Resists upward motion

An unstable atmosphere:
A.

Encourages upward vertical motion

Resists upward motion

An inversion is:
A.

A layer of air where temperature increases with altitude.

A layer of air where temperature decreases with altitude.

A layer of air where there is no temperature change with altitude.

2C.14

GENERAL AND LOCAL WINDS
A.

Wind and Wind Direction
Wind is the horizontal movement of air relative to the surface of the
earth. Wind direction is the direction from which the wind is blowing
(a north wind means the wind is blowing from the north).
Wind is the most critical weather element affecting wildland fire
behavior, the most difficult to predict, and the most variable in both
time and location.
This variability (especially in rough terrain) can pose safety and fire
control problems, which can result in firefighter fatalities. Wind
direction and wind speed must be constantly monitored by all
firefighters.

Characteristics of Wind and its Effect on Wildland Fire
Wind impacts the fire environment by:
•

Increasing the supply of oxygen to the fire.

•

Determining the direction of fire spread.

•

Increasing the drying of the fuels.

•

Carrying sparks and firebrands ahead of the main fire causing
new spot fires.

•

Bending flames results in the preheating of fuels ahead of the
fire.

•

Influencing the amount of fuel consumed by affecting the
residence time of the flaming front of the fire. The stronger the
wind, the shorter the residence time and the less fuel is
consumed.

2C.15

Wind Systems
1.

General wind
General winds are large scale upper level winds caused by high
and low pressure systems.
If strong enough, these winds can influence wildland fire
behavior, but are generally modified in the lower atmosphere by
terrain.

Local wind
Local winds are found at lower levels of the atmosphere.
Local winds are induced by small-scale (local) differences in air
temperature and pressure, and are best developed when skies
are clear and general winds are weak.
Terrain also has a very strong influence on local winds; the
more varied the terrain, the greater the influence.
Local winds can be as important to wildland fire behavior as the
winds produced by the large-scale pressure patterns. In many
areas, especially in rough terrain or near large bodies of water,
local winds can be the prevailing daily winds.
The different types of local winds include:
a.

Slope winds
Slope winds are local winds that develop in mountainous
terrain where the differences in heating and cooling
occur.
During the day, the typical local wind pattern is upslope
and downslope during the night. There will be cases
where this rule does not apply.

2C.16

Local personnel are usually aware of prevailing wind
conditions.
(1)

Upslope wind characteristics
The air in the valleys becomes warmer than the air
on the mountain top and thus rises, producing the
upslope wind.
•

The greatest upslope wind speed occurs
about mid-afternoon.

•

Speeds generally range between 3 and 8
mph and can be gusty.

•

East facing slopes receive solar energy at
sunrise, thus the downslope to upslope
change takes place first on east aspects.
This change can be gradual and may be
characterized by a relative calm for an hour
or more as the slope heats.

•

South and west facing slopes receive heat
later in the morning; therefore, the
downslope to upslope takes place usually
by late morning.

2C.17

(2)

Downslope wind characteristics
The air along the mountain tops at night cools
faster than the air in the valley. The cool air sinks,
producing the downslope wind.
•

The greatest downslope flow occurs after
midnight.

•

Speeds generally range between 2 and 5
mph.

•

Relative calm takes place before the
downslope wind begins.

•

Because east aspects lose solar energy first,
the change from upslope to downslope
occurs on east aspects early in the
afternoon.

•

Southwest and west facing slopes receive
solar energy through much of the afternoon,
thus downslope wind typically begins just
after sunset.

•

The change in wind from downslope to
upslope can rapidly change wildland fire
behavior from inactive to active in a matter
of minutes.

•

Though the steepness of slope also plays a
role, stronger upslope winds lead to faster
uphill fire spread.

•

Downslope winds seldom produce
dangerous conditions; however, strong
downslope winds, increased by the
steepness of the terrain, can result in
downhill runs.

2C.18

Valley winds
Valley winds are produced by local temperature and
pressure differences within the valley or between a valley
and a nearby plain.
Though there are exceptions, valley winds flow up-valley
during the day and downvalley at night.
(1)

Up-valley wind characteristics
As air in the valley warms, temperature and
pressure differences within the valley or valley to
adjacent plains results in an up-valley wind flow.
•

The greatest up-valley winds occur mid to
late afternoon.

•

Up-valley wind speeds typically range
between 10 and 15 mph.

•

Because of the large amount of air heated in
the valley, up-valley winds develop after
the upslope winds.

•

Up-valley winds typically continue after
sunset.

2C.19

(2)

Down-valley wind characteristics
As the valley loses solar heating, the air in the
valley cools. The cool air drains down-valley,
resulting in the down-valley wind.

•

The greatest down-valley winds occur after
midnight.

•

Down-valley wind speeds typically range
between 5 and 10 mph.

•

Because of the large amount of air cooling
in the valley, down-valley winds typically
do not develop until a few hours after dark,
and well after the development of the
downslope winds.

Sea and land breezes
(1)

Sea breeze
A daytime breeze in which cooler air from high
pressure over the coastal waters moves onshore to
replace heated air rising above the warmer land
mass.
Typical wind speed is between 10 and 20 mph.
However, wind speed can attain 20 to 30 mph
along the California, Oregon, and Washington
coasts.

(2)

Land breeze
A light nighttime breeze which originates over the
relatively cool land, flows out over the warmer
coastal waters. Typical wind speed is between 3
and 10 mph.

2C.20

Exercise 3. Winds
1.

General winds are:
A.

Found at lower levels of the atmosphere and are induced by smallscale (local) differences in air temperature and pressure.

Large scale upper level winds caused by high and low pressure
systems.

Local winds that develop in mountainous terrain where the differences
in heating and cooling occur.

The different types of local winds include (circle all that apply):
A.
B.
C.
D.
E.

upslope wind
downslope wind
upvalley wind
jet stream
sea-breeze

2C.21

VI.

CRITICAL FIRE WEATHER CONDITIONS
Fire seasons occur at different times of the year in different regions of the
country, depending on seasonal variations in weather.
The typical fire season at any given location has numerous hot and dry days,
yet wildfires are usually clustered within relatively short periods.
These periods are characterized by one (or a combination of) critical fire
weather conditions:
•

Strong and shifting wind

•

Very low relative humidity

•

High temperature

•

Unstable atmosphere

•

Dry lightning

Examples of weather phenomena in which one or more of these critical fire
weather conditions may occur:
•

Cold fronts

•

Foehn winds

•

Thunderstorms

•

Dust devils

•

Firewhirls

2C.22

Cold Fronts
A cold front is the boundary line between two different air masses,
with cooler air behind the front and warmer air ahead of the front.
The two differing air masses result in pressure differences that can
lead to moderate or strong wind speed.
Frontal winds associated with frontal passages are particularly
dangerous, not only for the strength of the wind, but also the shift in
direction as the front approaches and passes through the area.
Along with the shifting winds, atmospheric stability ahead of and
behind the front also impacts the fire environment. Unstable
conditions encouraging upward motion are typically found ahead and
along the frontal boundary. Stable conditions discouraging upward
motion are typical behind the front.
Historically, firefighter fatalities have occurred in the pre-frontal
environment where winds are strong and shifting, and the atmosphere
is unstable.
1.

Potentially dangerous cold front characteristics:
•

Light southeasterly winds are common several hundred
miles ahead of the front.
–

•

Just ahead or along the front, moderate to strong
southwesterly winds are common. The strong
southwesterly flow ahead of the front will drive
the fire head to the northeast.

The air mass ahead of the front is typically very warm
and unstable, resulting in an increase in fire behavior.

2C.23

•

Relative humidities can be low or high depending on the
origin or location of the system.
–

•

As the front pushes through, the wind can abruptly shift
from southwest to northwest, driving the fire head to the
southeast.
–

High relative humidities ahead of the front are
more common over the eastern U.S. than the
western U.S.

This can be a great concern to firefighters due to
the increased fire behavior on the south flank of
the fire as the winds shift.

•

The air mass behind the front is cooler, more stable, and
relative humidities are higher, thus fire activity typically
decreases.

•

Wind speeds just ahead, along, and behind the front
typically range from 15 to 30 mph, and can be gusty.

Cold front indicators:
•

A line of cumulus clouds may be seen approaching from
the west or northwest.

•

Large clouds of dust can precede the arrival of a cold
front.

•

Winds normally shift from the southeast to the south, to
the southwest, and increase in velocity before the arrival
of the front.

•

Winds will be strongest and gusty as the front reaches
you.

•

Winds will continue to shift as the front passes,
generally resulting in strong, gusty, cool wind out of the
northwest.
2C.24

Foehn Winds
Foehn winds are strong, dry winds caused by the compression of air
as it flows down the lee side of a mountain range. It is usually, but
not always, warm for the season.
Foehn winds can persists for days and frequently reach speeds of 4060 mph but can be as high as 90 mph. The relative humidity will
usually drop with the onset of foehn winds.
The combination of high wind speeds and low relative humidity
can cause high rates of fire spread. When a foehn wind occurs after
a long period of dry weather, wildland fire behavior can be extreme.
Common foehn winds in the western U.S. are:
1.

Chinook wind: Found along the east side of the Rockies and
east side of the Sierra Nevada.

Wasatch wind: Found on the west side of the Wasatch Range
in Utah.

Santa Ana and Sundowner: Southern California.

Mono and North wind: Central and Northern California.

East wind: Western Washington and Western Oregon.

2C.25

Thunderstorms
A storm is produced by a cumulonimbus cloud and always
accompanied by lightning and thunder.
1.

Thunderstorms can also produce:
•
•
•

Strong gust winds
Heavy rain
Hail (sometimes)

Thunderstorms are usually of short duration, seldom over 2 to 3
hours for any one storm.
2.

The direction of thunderstorm movement is generally in the
direction of the winds aloft.
•

The direction of thunderstorm movement can be
determined by the direction the anvil shaped top is
pointing.

Downdraft winds from thunderstorms that reach the ground
usually spread radially in all directions.
•

These wind velocities will often be 25 to 35 mph and
can reach as high as 70 mph.

•

Surface winds from a thunderstorm will be the strongest
in the direction the thunderstorm is moving.

•

Thunderstorm wind speed and direction can be altered
by topography and vegetation.

2C.26

Thunderstorms are potentially dangerous to firefighters
because:
•

Wind associated with thunderstorms, whether indrafts or
downdrafts, can change direction and speed, resulting in
sudden changes in the rate and direction of a fire, as
well as fire intensity.

•

Heat rising from a fire can form a convection column
strong enough to trigger the development of a
thunderstorm, even on an otherwise cloudless day.

•

Thunderstorms, as a result of a convection column, can
produce dangerous downdrafts.

•

Thunderstorms produce dangerous lightning that results
in new starts. Lightning is also a safety problem and can
result in death.

2C.27

Dust Devils and Firewhirls
Dust devils are one of the most common indicators of unstable air.
They occur on hot days over dry ground when skies are clear and the
winds are light.
Under intense heating, air near the ground rises in upward-spiraling
motions in columns or chimneys.
Firewhirls, generated by intense fires, have been known to twist off
trees more than 3 feet in diameter.
They can pick up large burning embers and spew them far across the
fireline causing numerous spot fires.
In some extreme cases, firewhirls and dust devils have moved across
safe zones, and burned and turned over vehicles.
•

The size of dust devils can range from 10 feet to over 100 feet
in diameter with heights from 10 feet to 3,000 or 4,000 feet.

•

Wind speeds in dust devils are often more than 20 mph and in
some extreme cases have exceeded 70 mph.

•

A favorite area for firewhirl development is on the wind
sheltered (leeward) side of ridges.

2C.28

Exercise 4. Critical Fire Weather
1.

Chinook and Santa Ana winds are examples of:
A.

A foehn wind

A cold front wind

A thunderstorm wind

A sea-breeze

Cold front winds:
A.

Are strong, dry winds caused by the compression of air as it flows
down the lee side of a mountain range.

Are winds associated with a boundary between two dissimilar air
masses.

A and B.

2C.29

VII. DIFFERENT TYPES OF FIRE WEATHER FORECASTS AND
OUTLOOKS AVAILABLE
A.

Predictive Services
Predictive Services is a combined group of Interagency Land
Management Fire Intelligence Coordinators or Fire Behavior Analysts
(FBAN), and Fire Meteorologists.
1.

Predictive Services monitors, analyzes, and predicts:
•

Fire weather

•

Fire danger

•

Interagency fire management resource impact

Predictive Service products and services:
•

Seasonal assessments

•

7 Day Significant Fire Potential

•

Monthly Fire Weather/Fire Danger Outlook

•

Weather briefings

•

Daily summaries of National Weather Service fire
weather forecasts, both graphical and text.

•

Long term precipitation monitoring

•

Smoke management summaries

2C.30

National Weather Service (NWS)
There are over 120 National Weather Service offices nationwide that
provide a variety of different types of forecasts.
Another major NWS program includes the fire weather program.
NWS standardized products include:
1.

Fire Weather Planning Forecasts (FWF)
These forecasts can be in tabular or narrative format.
They include a discussion of the upcoming weather and
highlights of any critical fire weather events, as well as many
different forecasted elements including:

•

Sky/weather

•

Temperature

•

Relative humidity

•

Wind

Spot forecasts
A spot forecast is a site specific 24- to 36-hour forecast issued
to fit time, topography, and weather of a specific location.

2C.31

Fire Weather Watches / Red Flag Warnings
A fire weather watch or red flag warning is issued when the
combination of dry fuels and weather conditions support
extreme fire behavior or ignition is occurring or expected to
occur.
a.

Fire Weather Watch
Issued when there is a high potential for the development
of a Red Flag Event.
A Fire Weather Watch is normally issued 24 to 72 hours
in advance of the expected onset of criteria.

Red Flag Warning
Is used to warn of an impending or occurring Red Flag
event.
Its issuance denotes a high degree of confidence that a
Red Flag event will occur in 24 hours or less.

Exercise 5. Types of Fire Weather Products
1.

The seasonal assessment product is issued by:
A.
B.
C.

The National Weather Service
Predictive Services
Both A and B

Red Flag Warnings are issued by:
A.
B.
C.

The National Weather Service
Predictive Services
Incident Management Teams

2C.32

Introduction to Wildland Fire Behavior, S-190
Unit 3 – Wildland Fire Behavior and Safety

OBJECTIVES:
Upon completion of this unit, students will be able to:
1.

Identify indications that fire behavior may be increasing.

Describe combined influences that may cause extreme fire behavior and
safety concerns.

List seven fire environment factors to be aware of while monitoring fire
behavior.

3.1

3.2

MONITORING FIRE BEHAVIOR
A.

Large Fire Indicators
Fires rarely just go from small fires to extreme “blow ups.” There are
indicators that, if monitored, will show when a fire is starting to
transition from problem fire behavior to extreme fire behavior.

Problem vs. Extreme Fire Behavior
1.

Problem fire behavior
Fire activity that presents potential hazard to fireline personnel
if the tactics being used are not adjusted. The prediction or
anticipation of fire behavior is the key to good tactical decisions
and safety.

Extreme fire behavior
The highest level of problem fire behavior can be described
with specific elements:
•
•
•
•

Rapid rate of spread
Intense burning
Spotting
Crowning

Incident Response Pocket Guide
Use the Incident Response Pocket (IRPG) as a field reference to help
monitor changing conditions. The pages of the Operational section
are green and placed in the front for quick reference in the field.
The Operational section is where the seven fire environment factors of
Look Up, Down and Around are located. Each factor lists indicators
to help individuals monitor the fire environment and become more
aware of changes that are occurring.

3.3

II.

THE SEVEN FIRE ENVIRONMENT FACTORS OF LOOK UP, DOWN
AND AROUND
The fire environment is the conditions, influences, and modifying forces that
control fire behavior. The fire environment has been described with a
triangle showing weather, fuels, and topography (terrain).
There are seven factors within this fire environment that fireline personnel
must monitor:
•
•
•
•
•
•
•

Fuel Characteristics
Fuel Moisture
Fuel Temperature
Topography (Terrain)
Wind
Atmospheric Stability
Fire Behavior

These seven factors and their corresponding indicators help provide clues
when monitoring the fire and anticipating what might happen.
A.

Fuel Characteristics
1.

Continuous fine fuels
Fire is able to change and spread rapidly in these fuels,
especially when combined with slope and/or wind.

Heavy loading of dead and down
Large amounts of readily available fuel.

Ladder fuels
Allow the fire to readily spread into the canopy, launching
firebrands (spots) into the air.

3.4

Tight crown spacing
Allows fire to move from bush to bush (or tree to tree) easier.

Special conditions
a.

Firebrand sources
Burning material that is carried by the wind ahead of the
fire or outside of control lines.
Potential firebrand sources are:
•
•
•
•
•

pine bark plates
manzanita leaves
eucalyptus leaves
maple leaves
oak leaves

Numerous snags
Fire can become established in these dead or partially
dead trees, making them very hazardous. These can
launch firebrands into the air as well as fall across control
lines.

Frost and bug kill
More available fuel to be consumed by the fire.

Preheated canopy
Caused by a lower intensity fire burning the fuels near
the ground. Heat from the fire dries the fuels above it,
making those fuels available to burn.

3.5

Unusual fine fuels
Light flashy fuels mixed with high energy fuels, such as
continuous grass mixed with sage.

High dead to live ratio
Greater amounts of potentially available fuel.

Fuel Moisture
1.

Low RH (<25%)
The lower the humidity, generally, the more available the fine
fuels are to carry fire.
The 25% RH indicator is a general threshold for much of the
U.S. In the East, Alaska, and Hawaii, the threshold is generally
higher.

Low 10 hr FMC (< 6%)
10 hour fuels are just one good indicator of how available fuels
are to burn.

Drought conditions and seasonal drying
Both are indicators that fuels are more receptive to ignition and
carrying the spread of fire.

3.6

Fuel Temperature
1.

High temps (> 85° F)
Increasing the temperature of the fuels closer to the point of
ignition.

High percent of fuels with direct sun
On any given slope, are a majority of the fuels in the sun or the
shade?

Aspect fuel temperature increasing
Which slopes tend to have higher fuel temperatures in the
morning? Which are higher in the afternoon? Why is this
important?
a.

South and southwest slopes:
•

Are normally more exposed to sunlight.

•

Generally have lighter and sparser fuels.

•

Have higher temperatures, lower humidity, and
lower fuel moisture.

•

Are the most critical in terms of start and spread
of fire.

North facing slopes have more shade, which causes:
•
•
•
•

Heavier fuels
Lower temperatures
Higher humidity
Higher fuel moistures

Being aware of which slopes are “hotter” throughout the day
allows firefighters to monitor where the potential for the
greatest fire behavior is.
3.7

Topography (Terrain)
1.

Steep slopes (>50%)
Provides for rapid rates of fire spread due to convective heating
and increased potential for rollouts below the fire.

Chutes – Chimneys
Provides potential for rapid rates of fire spread by combining
steep terrain with updrafts of air.

Saddles
Fire is pushed faster through these during uphill runs.

Box canyons
All provide for rapid rates of fire spread due to the channeling
of wind and heat.

Narrow canyons
a.

Radiant and convective heating could increase spotting
across the canyon.

Fire can burn down to the bottom of the canyon and then
crossover to the other side. This is known as “slope
reversal.”

3.8

Wind
Wind is the primary factor that influences fire spread.
1.

Surface winds above 10 mph
These winds help determine the direction of fire spread, help to
carry firebrands ahead of the fire, and increase the supply of
oxygen to the fire.

Lenticular clouds
Indicates high winds aloft with the potential to surface.

High, fast moving clouds
Indicates a potential for wind shifts.

Approaching cold fronts
Wind will increase in speed and change direction with the
advance of a cold front.

Cumulonimbus development
Indicates possible wind speed and direction and potential for
erratic winds.

Sudden calm
Be alert for a wind change.

3.9

Battling or shifting winds
Winds that change direction and go back to the original
direction are battling. This is an indication of a probable
change in wind speed and direction.
Changes in wind speed and direction affect everyone on the fire
from individual firefighters to fire managers on many portions
of the fire.
A sudden change in wind direction can cause firebrands to cross
control lines. Increasing winds could cause previously quiet
parts of the fire to increase in intensity.

Atmospheric Stability
Indicators of an unstable atmosphere and the potential for large fire
growth:
1.

Good visibility

Gusty winds and dust devils

Cumulus clouds

Castellatus clouds in the a.m.

Smoke rises straight up

Inversion beginning to lift
This is an indication of a transition from a stable to an unstable
atmosphere and the potential for fire growth.

Thermal belt
An area of lower nighttime relative humidity and higher
temperatures. Fires will generally burn more actively in these
areas at night.

3.10

Fire Behavior
Indicators of a rapidly changing, wind-driven fire with intense
burning:
1.

Leaning column

Sheared column

Well-developed column

Changing column
Fire behavior is usually increasing.

Trees torching
The fire is beginning to transition from a surface fire to a crown
fire. Observe if just one tree is torching or small groups of trees
are catching fire. Note if there is wind present and how fast it is
blowing.

Smoldering fires picking up
Fire behavior is increasing. What else might be occurring to
cause this? It is possible that the:
•

Inversion is lifting

•

Wind is increasing

•

Shading has decreased on that aspect and temperature is
increasing

•

Relative humidity has decreased

3.11

Small firewhirls beginning
The fire is increasing in intensity.

Frequent spot fires
The fire is spreading and increasing in complexity

Review the Seven Fire Environment Factors
•

Important to not just monitor one or two factors but all of them.

•

Equally important to monitor the trends of each indicator as
well.

EXERCISE.
List the seven fire environment factors.
1.
2.
3.
4.
5.
6.
7.

3.12

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S 190_sw Preface 190 Student Workbook

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