Forestry Science I Unit 4: Wildfires Lesson 2: Fire Behavior

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Forestry Science IUnit 4 WildfiresLesson 2
Fire Behavior

• Prepared by Chad Bass
• December 5, 2001
• To accompany Georgia Agriculture Education
  Curriculum Lesson

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Objectives

• 1. Explain the fire triangle.
• 2. Explain how air movement affects fire
  behavior.
• 3. Explain how topography affects fire behavior.
• 4. Explain how fuel types affect fire behavior.
• 5. Analyze the factors affecting a prescribed
  fire.

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The Fire Triangle

• To produce fire, three things must be present at
  the same time- fuel, heat, and oxygen.

FUEL
HEAT
OXYGEN
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The Fire Triangle

• The fire triangle is like a three-legged stool.
  If you remove one leg, the fire goes out.

heat
fuel
oxygen
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How is the triangle broken?

• Some of the ways used to break the fire triangle
  are
• fire rake
• fire flap or pine top
• backpack water pump
• tractor with a fire plow
• backfire torch
• airplane water bomb
• prescribed burning

Substance Eliminated FUEL OXYGEN HEAT FUEL FUEL HE
AT FUEL
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What is fire behavior?

• When we talk about fire behavior, we are thinking
  in terms of the intensity of the fire itself, and
  the rate of spread or speed of the fire.

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What determines fire behavior?

• Fire behavior is affected by a wide range of
  conditions.
• Three important factors of fire behavior are
• Time of occurrence of the fire season
• air movements
• topography (slope of the land, presence of roads,
  fields, streams, etc.)

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Fire Seasons

• In the south, fire seasons occur primarily in the
  fall and spring of the year.
• In the north and western part of the U.S., fire
  season occurs in the summer.
• During fire seasons, there is a buildup of fuel,
  and extended dry periods.

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Fire Seasons

• The hottest wildfires generally burn in the
  middle of the day on sunny days.
• However, when humidities remain extremely low
  along with steady winds, a hot wildfire can burn
  all night.

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Air Movement Affects Fire Behavior

• Horizontal and vertical air movements determine
  the duration and speed of a fire.
• Wind is an important factor in drying out
  vegetation.

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Air Movement Affects Fire Behavior

• Since hot air rises, fires tend to create their
  own winds.
• This up draft type wind carries sparks up into
  the air. Then, upper winds scatter the sparks
  over unburned areas and cause spot fires.
• These spot fires increase the spread and
  intensity of the fire.

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Topography Affects Fire Behavior

• Steep slopes help put fuel in contact with the
  fire more quickly. This causes the fire to spread
  faster.
• These slopes also increase the updraft winds
  created by the fire.
• Highways, fields, streams, and other areas act as
  barriers that help control the spread of fires.

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Topography Affects Fire Behavior

• Some weather conditions can also reduce the rate
  of spread and help with fire control
• Rain on the fire.
• Wind reversal blowing the fire back over the
  burned area.
• An increase in relative humidity (the amount of
  moisture in the air).

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Fuel Types Affect Fire Behavior

• There are two basic types of wildfire fuels. They
  are
• 1. Ground fuels
• 2. Aerial fuels

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Fuel Types Affect Fire Behavior

• Ground fuels include
• 1. Peat 6. Low
  shrubs
• 2. Duff 7. Tree
  seedlings
• 3. Tree roots 8. Dead limbs
• 4. Leaves 9. Weeds
• 5. Dead grass 10. Bushes

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Fuel Types Affect Fire Behavior

• Aerial fuels include all burnable material
  located in the trees canopies above six feet
  from the ground.

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Summary/Review

• Identify the parts of the fire triangle.
• Identify the fuels available in the forest.
• Review the weather information available from the
  Georgia Forestry Commission.

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Credits

• International Paper corporation. International
  Paper School Forestry Guide.
• Georgia Forestry Commission
• Georgia Ag. Ed. Curriculum Resource CD