S290 Unit 11

11-1-S290-EPUnit 11 Extreme Wildland Fire Behavior

Unit 11


Unit 11 Objectives1. Describe the four common

denominators of fire behavior on tragedy wildland fires.

2. Describe extreme fire behavior characteristics and recognize fire environment influences that contribute to extreme fire behavior.


Unit 11 Objectives3. Describe the three stages of crown

fire development and identify the key factors and indicators leading to crown fire development.

4. Identify the three factors that contribute to the spotting problem and describe the conditions associated with each factor.


Unit 11 Objectives

5. Define the probability of ignition, describe its use, and determine it using tables.

6. Define firewhirls (vortices), the conditions under which they are likely to develop and their implications to wildland fire behavior.

7. Explain the difference between wind-driven and plume-dominated fires.


The Four Common Denominators of Fire

Behavior on Tragedy Fires


Common Denominators

• On relatively small fires or deceptively quiet areas of large fires.

• In relatively light fuels, such as grass, herbs, and light brush.

• When there is an unexpected shift in wind direction or in wind speed.

• When fire responds to topographic conditions and runs uphill.


Why are firefighters dying on these types of fires?

• Sudden alignment of key elements in the fire environment.

• Recent examples:– South Canyon:

brush fuel type– Cramer:

brush fuel type– Tuolumne:

light flashy fuels (grass, leaves,

light brush)


Extreme Fire Behavior Characteristics and Fire

Environment Influences That Contribute to Extreme Fire

Behavior



Extreme Fire Behavior• Precludes suppression actions

• High rate of spread and frontal fire intensity

• Crowning

• Prolific spotting

• Presence of large fire whirls

• Well established convection column

• Erratic manner


Contributing Factors Extreme fire behavior results from a

combination of environmental factors:– Available fuels– Wind– Low fuel moisture– Unstable

atmosphere


Available Fuels

• The micro-climate and soil conditions

• Vegetative stage of development

• Seasonal and diurnal changes


Fuels Characteristics

• Continuous fine fuels

• Heavy loading

• Ladder fuels

• Tight crown spacing (<20 ft)


Wind Extreme fire behavior has been

associated with strong winds including:– Frontal– Thunderstorm– Foehn winds


Wind• Surface winds above 10 mph

• Lenticular clouds

• High, fast moving clouds

• Approaching cold front

• Cumulonimbus development

• Sudden calm

• Battling or shifting winds


Low Fuel Moistures and Relative Humidities

• Fine fuel moistures

• 1000-hr fuel moistures

• Live fuel moistures

• Daily RH’s and nighttime recovery


Unstable Atmosphere

An unstable atmosphere contributes to the vertical

motion of the air.


Primary Unstable Atmosphere Indicators

• Good visibility• Gusty winds and

dust devils

• Cumulus clouds• Castellanus

clouds in the morning

• Smoke rising straight up

• Inversion beginning to lift


Combining Influences• Fuels are dry and plentiful (drought).

• Atmosphere unstable or was unstable for hours, possibly days prior to the fire.

• Free air wind speeds at or slightly above the elevation of the fire is 18mph or greater.


The Three Stages of Crown Fire Development and

Identify the Key Factors and Indicators Leading to Crown Fire Development


Types of Crown Fire(wind driven)

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Passive• One to a few trees• Commonly called

“torching”• Dependent on surface fire

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Active• Spread through aerial fuels• Dependent on surface fire• Surface fire can precede and vice-

versa• Pulsating spread rate

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Independent• Will outrun the reinforcing surface fire.• Combustion process and heat transfer

mechanisms take place in the aerial fuels.• Surface fire spread results from crown fire

spread.

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Conditions Contributing to Crown Fires

• Crown flammability

• Surface to crown heat transfer

• Crown to crown heat transfer


Crown Flammability

• Fine dead fuel moisture

• Live foliar moisture

• Foliage flammability

• Crown closure (“compactness”)– >75% will improve heat transfer

mechanisms of convection and radiation– less closure allows heat to be lost

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Surface to Crown Heat Transfer

• Surface fire intensity

• Vertical arrangement

• Steepness ofslope

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Crown to CrownHeat Transfer

• Crown spacing (20’ or less)

• Crown level winds (20 mph or greater 20 ft. above the surrounding vegetation)

• Steepness of slope (similar in crown fuels to its effect on surface fuels)

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Does this stand have the potential to crown?


Factors That Contribute to the Spotting Problem and the Conditions Associated

With Each Factor


The eight contributing factors fall into the following three areas:

• Firebrand source

• Transportation

• Receiving fuels and environment


Firebrand Source

• Probability of production

• Number of firebrands

• Type of firebrands

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Transportation• Convective lifting

• Wind field

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Receiving Fuels and Environment• Receptive fuel• Probability of ignition• Environmental conditions

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Short-Range vs. Long-Range Spotting

• Wind field and convective lifting dictate the maximum spotting distance.

• Spotting distances recorded inexcess of 15 miles!


Short-Range SpottingStrong surface winds and limited convective lifting.

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Long-Range Spotting

• Large, aerodynamic firebrands

• Strong convective lifting• Wind field enabling

maximum height and transportation (running crown fires, large fire whirls)

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Determining Potential Spot Fire Locations

• Observe convective column or ash “fallout”

• “Where there is one, there are probably more.”

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Numerous Spots• Getting frequent spot fires across the line

is one of the 18 Watchouts.• What is “frequent”?

– Generally faster than you can pick them up


The Probability of Ignition

(PGI)


Probability of Ignition Rating of the probability that a glowing

firebrand will cause a fire, providing it lands on receptive fuels.


Probability of Ignition TableDry-Bulb

Shading Temp 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17(Percent) (oF)

100+ 100 100 80 70 60 60 50 40 40 30 30 20 20 20 20 10100-109 100 90 80 70 60 60 50 40 40 30 30 20 20 20 10 1090-99 100 90 80 70 60 50 40 40 30 30 30 20 20 20 10 10

80-89 100 90 80 70 60 50 40 40 30 30 30 20 20 10 10 10Unshaded 70-79 100 80 70 60 60 50 40 40 30 30 30 20 20 10 10 10

<50% 60-69 90 80 70 60 50 50 40 30 30 20 20 20 20 10 10 10

50-59 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10 1040-49 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10 1030-39 80 70 60 50 50 40 30 30 20 20 20 10 10 10 10 10

100+ 100 90 80 70 60 50 50 40 40 30 30 20 20 20 10 10100-109 100 90 80 70 60 50 50 40 30 30 30 20 20 20 10 1090-99 100 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10

80-89 100 80 70 60 60 50 40 40 30 30 20 20 20 10 10 10Shaded 70-79 90 80 70 60 50 50 40 30 30 30 20 20 20 10 10 10

>50% 60-69 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10 10

50-59 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10 1040-49 90 80 60 50 50 40 30 30 30 20 20 20 10 10 10 1030-39 80 80 60 50 50 40 30 30 20 20 20 10 10 10 10 10

FINE DEAD FUEL MOISTURE (PERCENT)


Dry-BulbShading Temp 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17(Percent) (oF)

100+ 100 100 80 70 60 60 50 40 40 30 30 20 20 20 20 10100-109 100 90 80 70 60 60 50 40 40 30 30 20 20 20 10 1090-99 100 90 80 70 60 50 40 40 30 30 30 20 20 20 10 10

80-89 100 90 80 70 60 50 40 40 30 30 30 20 20 10 10 10Unshaded 70-79 100 80 70 60 60 50 40 40 30 30 30 20 20 10 10 10

<50% 60-69 90 80 70 60 50 50 40 30 30 20 20 20 20 10 10 10

50-59 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10 1040-49 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10 1030-39 80 70 60 50 50 40 30 30 20 20 20 10 10 10 10 10

100+ 100 90 80 70 60 50 50 40 40 30 30 20 20 20 10 10100-109 100 90 80 70 60 50 50 40 30 30 30 20 20 20 10 1090-99 100 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10

80-89 100 80 70 60 60 50 40 40 30 30 20 20 20 10 10 10Shaded 70-79 90 80 70 60 50 50 40 30 30 30 20 20 20 10 10 10

>50% 60-69 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10 10

50-59 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10 1040-49 90 80 60 50 50 40 30 30 30 20 20 20 10 10 10 1030-39 80 80 60 50 50 40 30 30 20 20 20 10 10 10 10 10


Exercise1

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Dry-BulbShading Temp 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17(Percent) (oF)

100+ 100 100 80 70 60 60 50 40 40 30 30 20 20 20 20 10100-109 100 90 80 70 60 60 50 40 40 30 30 20 20 20 10 1090-99 100 90 80 70 60 50 40 40 30 30 30 20 20 20 10 10

80-89 100 90 80 70 60 50 40 40 30 30 30 20 20 10 10 10Unshaded 70-79 100 80 70 60 60 50 40 40 30 30 30 20 20 10 10 10

<50% 60-69 90 80 70 60 50 50 40 30 30 20 20 20 20 10 10 10

50-59 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10 1040-49 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10 1030-39 80 70 60 50 50 40 30 30 20 20 20 10 10 10 10 10

100+ 100 90 80 70 60 50 50 40 40 30 30 20 20 20 10 10100-109 100 90 80 70 60 50 50 40 30 30 30 20 20 20 10 1090-99 100 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10

80-89 100 80 70 60 60 50 40 40 30 30 20 20 20 10 10 10Shaded 70-79 90 80 70 60 50 50 40 30 30 30 20 20 20 10 10 10

>50% 60-69 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10 10

50-59 90 80 70 60 50 40 40 30 30 20 20 20 10 10 10 1040-49 90 80 60 50 50 40 30 30 30 20 20 20 10 10 10 1030-39 80 80 60 50 50 40 30 30 20 20 20 10 10 10 10 10


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Firewhirls (Vortices), the Conditions Under Which

They are Likely to Develop and Their Implications to Wildland Fire Behavior


Fire Vortices A firewhirl/vortex is

defined as a spinning, moving column of ascending air rising from a vortex and carrying aloft smoke, debris and fire.

Firewhirls belong to the

same family as tornadoes and dust devils.


Two Types of Vortices

Vertical Vortices or Firewhirls

Horizontal or Roll Vortices

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Horizontal Vortices• Rare, exemplifies

extreme fire behavior

• Note “finger” (FOD) moves 100m at 100mph and retreats within 3 seconds

• Not well understood

*Looking down


3 Types of Vertical Vortices

Thermally driven – Similar to the dust devil which results from

some form of horizontal wind shear associated with convective activity in an unstable atmosphere.

Convection column– This form of firewhirl originates high in the

convection column.

Wake type– Occur on lee sides of physical obstructions


When and Where to Expect Firewhirls

• Are the result of local events or processes.

• Occur more frequently when the air mass is unstable to a considerable height.

• Assess the potential for firewhirls by watching for evidence of dust devils and light winds.


Factors Contributing to Firewhirl Formation

• Sun perpendicular to slope• Minimum cloudiness• Low RH• Dry exposed soil or

burned area• Light winds• Unstable atmosphere• Smoke rising to great

heights• Clouds growing vertically


Other Considerations These conditions can increase the

chance of firewhirls:

• The start of upslope winds

• Wind blowing acrossridges

• Up and down canyon winds at corners and spurs

• Hot spots in fire area• Changing air mass


Implications to Wildland Fire Behavior

• Can increase or alter wind flows.

• Carry firebrands up into transport winds (long range spotting).

• Can wander over fire lines and collapse (short range spotting).

• Can cause severe damage and threaten life and property.


The Difference Between Wind-Driven

and Plume-Dominated Fires


Wind-Driven Fire

Power of wind >Power of fire


Plume-Dominated

Fire

Power of fire > Power of wind


Wind-Driven Fires• Often those that escape initial attack

and become the largest

• Easier to predict direction of spread

• Wind shift poses a problem

• Smoke column bent over by wind

• Spotting downwind

• Flanks and heel generally safe


Plume-Dominated Fire

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Plume-Dominated Fire• Fire activity result of convective activity

of the plume

• Spread rate and direction very unpredictable

• Spotting can be in all directions

• Generally low windspeeds

• Generally pulses – can build, collapse, build, etc.


Functions Like a Thunderstorm

• Indrafts– Can be from all directions– Provides oxygen, increases preheating

• Downbursts– Rising air is cooled and can rush forcefully

to the ground– Downburst winds spread out in all

directions– Sudden calm, presence of virga or rain


Virga/Downburst


Review Unit 11 Objectives1. Describe the four common

denominators of fire behavior on tragedy wildland fires.

2. Describe extreme fire behavior characteristics and recognize fire environment influences that contribute to extreme fire behavior.


Review Unit 11 Objectives3. Describe the three stages of crown

fire development and identify the key factors and indicators leading to crown fire development.

4. Identify the three factors that contribute to the spotting problem and describe the conditions associated with each factor.


Review Unit 11 Objectives

5. Define the probability of ignition, describe its use, and determine it using tables.

6. Define firewhirls (vortices), the conditions under which they are likely to develop and their implications to wildland fire behavior.

7. Explain the difference between wind-driven and plume-dominated fires.

Education

S290 Unit 11