Introduction
to Wildland
Fire Behavior
S-190
Unit II
Unit 2 Objectives
• List the indicators of an approaching
cold front and describe what wind
changes to expect
• List three common foehn wind
conditions and the areas in which they
occur
• Identify a thunderstorm and describe
how and when it is dangerous
02-01(1 of 2)-S190
Unit 2 Objectives (cont.)
• Describe the daily cycle of slope and
valley winds
• Describe the effect relative humidity
has on wildland fire behavior
• Identify the wildland fire environment
indicators that can produce problem
and extreme fire behavior
02-01(2 of 2)-S190
Differential Heating
Group 2 — Weather
• Temperature
• Wind
– Increases supply of oxygen
– Drives convective heat into adjacent fuels
– Influences direction of spread and spotting
– Carries away moist air replacing it with drier air
– Dries fuels
– Raises fuel moisture if the air contains moisture
• Relative humidity
– As RH increases, fuel moisture increases
• Precipitation
– Increases fuel moisture
01-14-S190
General or Gradient Winds
• Large scale winds caused by high and low
pressure systems, but generally influenced
and modified in the lower atmosphere by
terrain
• Everyone is most familiar with them from
the nightly news weather reports
• In short & simple terms, classed into warm
fronts and cold fronts
02-02-S190
Warm Front
Cold Front
• A cold front is the boundary line between a
cooler air mass which is replacing a warmer
air mass
• Cold air is more dense and will hug the
ground more than a warm front
02-03-S190
Cold Front
Cold Front Winds
02-04-S190
Indicators of a Cold Front Passage
• Line of cumulus clouds approaching from
the west or northwest
• Large clouds of dust can precede the front
• Shift in winds from the southeast, south, to
the southwest, and increase in velocity
• Winds will be strongest, erratic, and gusty
as the front reaches you
• Winds will continue to shift as the front
passes, generally resulting in strong, gusty,
cool winds out of the west and northwest
02-05-S190
Cold Fronts are Potentially Dangerous
• Wind direction will abruptly change
• Strong southerly winds ahead of front will
drive the fire head to the north or northeast
• Winds shifting to west or northwest after
front passes will drive fire head to the east
or southeast
• Rapid drop in relative humidity within 24
hours of front passage
02-06-S190
Fire Before Cold Front Passage
02-07-S190
Fire After Cold Front Passage
02-08-S190
Foehn Wind
• Also called gravity winds, dry wind with
strong downward components,
characteristic of mountainous regions
• As the wind moves over long distances,
air friction warms the moving air up and
loses even more moisture
02-09-S190
Subsidence
02-23-S190
Examples of Foehn Winds
02-10-S190
Thunderstorm
• A storm produced by a cumulonimbus
cloud
• Always accompanied by:
– Lightning
– Thunder
– Strong gusty winds
02-11-S190
Visual Indicators of Thunderstorm
• Tall, building cumulus cloud
• Cauliflower appearance of cloud
• Dark, flat base
• Virga or rain falling from bottom of cloud
• Ice crystal top usually in anvil shape with
fuzzy appearance
02-12-S190
Thunderstorm
02-13-S190
Why a Thunderstorm is Dangerous
• Produces strong, gusty surface winds
affecting direction of fire spread
• Downward winds will be erratic, but
always away from the thunderstorm
• Thunderstorms also produce lightning,
which can be dangerous
02-14-S190
Sea Breeze
Land Breeze
02-15-S190
Slope Winds
Day
Night
02-16-S190
Valley Winds
Day
Night
02-17-S190
Temperature/Relative Humidity Chart
Noon
Midnight
Noon
Maximum
Relative
Humidity
Temperature
Minimum
02-18-S190
Visual Indicators of Unstable Air
Relatively Cold
• Clouds grow
vertically and
smoke rises to
great heights
• Cumulus-type
clouds
Relatively Warm
• Gusty winds
• Good visibility
• Dust devils and
firewhirls
02-19-S190
Cumulus Clouds
Dust Devil
02-24-S190
Obvious Unstable Air
Visual Indicators of Stable Air
Relatively Cold
• Clouds in layers,
no vertical motion
• Stratus-type
clouds
• Smoke column
drifts apart after
limited rise
Relatively Warm
• Poor visibility in
lower levels due to
accumulation of
haze and smoke
• Fog layers
• Steady winds
02-20-S190
Stable Air Clouds
Inversion
02-21-S190
40°
Thermal Belt
45°
50°
45°
40°
02-22-S190
Indicators of Problem and Extreme Fire
Behavior: Fuel Indicators
• Unusually dry fuels
• Large amount of light fuel
– shrubs, grass, needles
• Fuels exposed to direct sunlight
• Fuels dried by prolonged drought
• Ladder fuels that allow a surface fire to
move into the crowns of shrubs or trees
• Crown foliage dried by surface fire
• Concentration of snags
02-26-S190
Fire in Timber Areas
• Fire normally stays on
the surface unless
there is enough heat or
ladder fuels to move
into the aerial fuels
• When the fire moves
into the crowns, the
situation changes
dramatically
Torching vs. Crowning
• The term “Torching”
refers to a single tree or
a group of trees that
rapidly burn off
• Torching stays in one
spot; it’s isolated aerial
fuels that cant jump to
other aerial fuels
Torching vs. Crowning
• “Crowning” or a
“Crown Fire” is where
the fire is able to take
all of the aerial fuels in
the area, and keep
going
• Moves very fast,
extreme heat, and will
continue until fuels,
weather or topography
make a change
Indicators of Problem and Extreme Fire
Behavior: Topography indicators
• Steep slopes
• Chutes, saddles, and box canyons
which provide conditions for “chimney
effect”
• Narrow canyons may increase fire
spread by radiant heat and spotting
02-27-S190
Steep Slopes
• Slopes with the wind
direction going uphill
with the direction of the
slope is a recipe for a
fast moving hot fire
Canyons and Valleys
• Remember, narrow
canyons + steep slopes
= death traps!
Indicators of Problem and Extreme Fire
Behavior: Weather Indicators
• Strong Wind
• Sudden changes in wind direction and velocity
due to weather fronts
• High, fast-moving clouds may indicate unusual
surface winds
• Unexpected calm may indicate wind shift
• Thunderstorms above or close to the fire
• Unusually high temperatures and low relative
humidity
• Dust devils and whirlwinds developing
• Bent smoke column
02-28-S190
Aftermath of a wind-driven fire
Firewhirls
• Generated by intense fires
• Can pick up large burning embers and
toss them far across the fireline causing
spot fires
02-25-S190
Indicators of Problem and Extreme Fire
Behavior: Fire Behavior Indicators
• Keep an eye on the smoke column
– Indicates direction of fire spread, location
of spot fires, and changes in fire intensity
• Many simultaneous fires starting or smoldering
fires beginning to pick-up in intensity
• Fire begins to torch small groups of trees
or shrubs
• Frequent spot fires occurring
• Firewhirls beginning to develop inside
the main fire
• Crown fires
02-29-S190
Unit 2 Objectives
• List the indicators of an approaching
cold front and describe what wind
changes to expect
• List three common foehn wind
conditions and the areas in which they
occur
• Identify a thunderstorm and describe
how and when it is dangerous
02-029 (1 of 2)-S190
Unit 2 Objectives (cont.)
• Describe the daily cycle of slope and
valley winds
• Describe the effect relative humidity
has on wildland fire behavior
• Identify the wildland fire environment
indicators that can produce problem
and extreme fire behavior
02-30 (2 of 2)-S190