USU Geo3100 -Forestfires

Used by humans for hundreds of
thousands yrs.
-Allowed migration into colder climates
-Diverse successful civilizations
-Increased number and quality of foods
-Aid in hunting and in agriculture
-Hardening properties pottery, weapons, etc. smelting, metals
-Sterilization public health
-Controlled inside machinery provides energy for
civilization industry, domestic power, travel

Destructive powers of fire:
Denying enemies their prize – scorched earth policy
– Bombs creating firestorms in World War II
• Natural world
– 1,800 thunderstorms active on Earth each hour
– Lightning starts 15% of U.S. fires
– Most fires started by humans

Constructive powers of fire:

What is Fire?
Rapid combination of oxygen with carbon, hydrogen,
and other elements of organic nature in reaction that
produces flame, heat and light

Fire reaction:
– C 6H12O6+ 6 O2 6 CO 2+ 6 H2O + released heat
• Is reverse of photosynthesis reaction:
– 6 CO 2+ 6 H2O + heat from Sun
C 6H12O6+ 6 O2
• Solar energy stored by plants during growth is
returned to atmosphere during fire

Wildfire as a Process
Self-sustaining, rapid, high temperature biochemical reaction
• Requires
– Fuel
– Oxygen
– Heat

The Fire Triangle
• Fire may begin only when fuel, oxygen and heat are present
in the right combination
• Need all 3 for fire to occur -take one away and the fire goes out
• Knowing this -one can fight fire with fire
• light a backfire: the main fire draws in oxygen and the backfire
which eliminates fuel and the main fire should go out
• Any combustible material can be fuel (organic or human-made): grasses, shrubs, trees, slash (organic debris left on ground after logging or storms), houses
-Vegetation of varying heights makes it easy for the fire to climb into trees. Remove brush -but our history is to put out most fires and not allow the brush to be removed by smaller fires. Lots of brush = bigger fire

The Fire Triangle
• Firefighting:
– Water reduces heat
– Reddish-orange viscous fluids block oxygen from plants
– Bulldozing vegetation or setting backfires removes fuel
Fighting fire with fire -backfire in front of fire -will burn fuel in front of fire
and cause it to go out.

Three Wildfire Phases: Preignition
Fuel achieves temperature and humidity favorable to ignition.
• Preheating
– Fuel loses water and other chemical compounds
• Pyrolysis
– Processes that chemically degrade fuel
– Products include volatile gases, mineral ash, tars, etc.
• These processes produce the fuel gasses.

Three Wildfire Phases: Combustion
• Begins with ignition.
– Preignition absorbs energy, combustion releases energy.
• External reactions liberate heat and light.
– Lightning, volcanic activity, and human action.
• Ignition doesn't always lead to wildfires.
– Sufficient fuel must be present.
• Ignition is not a single process but occurs repeatedlyas
wildfire moves.

ree Wildfire Phases: Combustion, cont.
• Flaming combustion
– Dominates early fire
– Rapid high temperature conversion of fuel into heat
– Characterized by flames and large amount of unburned material
• Smoldering combustion
– Takes place at lower temperatures
– Does not require pyrolysis for growth
Figure 12.3
• slowly along the ground – glowing combustion dominates
• wall of fire -flaming combustion dominates
• fire in tree tops -crown fire

Depends on:
-Types of fuel
-Weather and strength of winds
-Topographyof land
-Behaviorwithin fire itself
How a Fire spreads
• Wind -pushes fire, oxygen
-cold-front winds
-Distributes heat
• Transports flaming debris to start new fires
• Type of vegetation (fuel) burning & how much
-Oils in wood (eucalyptus)
-branches close to ground or not
-lots of shrubby understory

• Moisture -wet or dry and for how long
Topography of land -hills -fire burns faster up slope -(heat rises so
• Positive feedback -fire creates unstable air and maycreate fire
tornado-carry burning debris and start new fires
Convection in Wildfires
• Transfer occurs as convectionand radiation.
• Radiationheat increases surface temperature of fuel.
• Gases become less dense and rise.
• Rising gases remove heat and combustion products from zone
of flaming.
• Pulls in fresh air to sustain combustion.

Three Wildfire Processes:
• Point at which combustion ceases.
• There is no longer heat and fuel to sustain fire.

•Regions that experience wet and dry seasons
•wet=plant growth
•Drought areas
•These yield lots of dry grass and shrubs. Dying trees are also fuel for
•Drought also reduces the amount of moisture in living trees
Chaparral -ubiquitous, very flammable shrub over S. Cal.

Fire Environment: Fuel
• Leaves, twigs, decaying material, grass, shrubs, etc.
• Peat-Unconsolidated deposit of partially decayed wood, leaves, or moss.
• Size affects ignition and movement.
• Landslides, hurricanes, and tornadoes can arrange debris to facilitate fires.
• Organic materials can dry out during droughts to become fuel.

The Fuels of Fire Grasses
Cover much of prairies of central U.S. and Canada
• Late summer, early fall: dry grasses ignite easily, lightweight fuels
• Fast, tall grass fires can kill and destroy property
• Loose layering allows easy burning
• High content of natural oils (palmetto, snowberry, chaparral) promote fires
• Florida: 1998 wildfires after warm weather, heavy rainfall, excess plant growth, record-breaking drought, lightning without rain
• California: scarce rain reduces plant growth except chaparral (rich in flammable oil) chaparral -perennial evergreen shrub that has high oil content -makes it one of the most flammable plants in the world. Common in southern California

The Fuels of Fire
• Affected by amount of slash on ground beneath
• Scarce litter: fires pass through quickly, little harm to trees
• Abundant, dry litter:
• fires kill trees by burning hot and slow,
• or slash is ladder to treetops, becomes crown fire
Fire Environment: Topography
• Fuel moisture content is affected by location.
• Drier fuels are found.
– On south-facing slopes in Northern Hemisphere
– Slopes exposed to prevailing winds
• Mountainous areas circulate winds up canyons during
• Wildfires preheat fuels upslope, making it easier to spread.

Fire Environment: Weather
• Fires common following droughts.
– Can bring "dry thunderstorms" with lightning to start fires, but rain
evaporates and can't extinguish them
• Fires burn more when humidity is lowest.
• Wind direction and strength help preheat unburned materials.
• Winds carry embers to ignite spot fires ahead of front.

Fire Weather
Fire hazards greatest where biggest differences between wet and dryseasons
– Rainy season promotes plant growth
Dry season or drought dehydrates living dead plants easier to burn
Dry, windy patterns affect large regionmajor fires break out in bunches

Winds of Fire
Large-scale movements of air-mass fronts
• Small-scale local winds (temperature and topography differences)

Cold-front Winds
Cold fronts move at 30 to 50 km/hr with gusty conditions for hours
• Dry in summer

Foehn Winds
• High-pressure air massspills over mountain range at up to 160 km/hr and
descends as warm, dry wind toward low-pressure zone -caused by pressure
gradient, warms adiabatically
• Occur in September through April in western U.S. when high-pressure air
sits over Great Basin and Rocky Mountains
• Different names for foehn winds in different places

Fire Weather-Types of Fires: Ground Fires
Types of Fires: Ground Fires
• Creep along under ground surface
• Little flaming, more smoldering

Types of Fires: Surface Fires
• Move along surface
• Vary in intensity
• Burn slowly with smoldering, limited flaming

Types of Fires: Crown Fires
Flaming is carried via tree canopies.
• Driven by strong winds and steep slopes.

Yellowstone National Park
Oldest national park, about 15 lightning fires per year
• Policy from 1880s to 1970s:
– Extinguish all fires as soon as possible
• Shift in 1970s to natural management changed policy to:
– Extinguish human-made fires, let lightning fires burn
– Between 1976-1987: 235 lightning fires, about 100 acres each
– Policy judged successful
• Winter of 1987-88 was dry
– Many trees had been killed by mountain pine beetles
– 90 years of fire suppression built up dead wood on ground
– Moisture levels in wood dropped from 15-20% to 2-7%

Yellowstone National Park
• Summer of 1988:
-Lightning fires began as usual
-Not followed by usual June-July rains
-By late July, over 17,000 acres had burned
-New policy enacted to extinguish all fires
-High temperatures and high winds of August allowed fires to burn out of control until mid-Septembersnows weakened them, then November winter conditions
extinguished fires
Fire Suppression
– At fire's conclusion: 1.4 million acres burned, almost half of Yellowstone
– In previous 116 years, only 146,000 acres burned

Long-term effect of short-term fire suppression:
California vs. Baja California: Pay Now or Pay
• Long-term effect of short-term fire suppression:
in U.S.,fires fought energetically and expensively, to
not let fire interfere with human activities
-Chaparral allowed to grow older, more flammable
-Hot, dry Santa Ana winds unleash unstoppable firestorms
-Fewer fires, but more large ones
• In Baja California, Mexico: fires allowed to burn with
little or no interference
-Older chaparral surrounded by younger, less-flammable
-More numerous fires, but smaller
Fire Suppression
The Cedar Fire in San Diego County, October 2003
• Huge areas of old chaparral fuel, dried by five
years of drought
• Lost hunter started signal fire, flames pushed
westward by Santa Ana winds burned 282,000
acres, destroyed 2,232 buildings, killed 15 people
• Fire stopped when it encountered areas recently
burnedin 2001 Viejas fire and 2001, 2002 and 2003
prescribed burns would have burned more than
400,000 acres otherwise
Fire Suppression
Fire in the hills outside San Diego 2003
California Fires 2003
Berkeley -CA
Fires in other places Colorado 2000
Idaho 1996
Montana 2000

Effects of Wildfires on Geologic Environment
• Soil changes
– Water-repellent hydrophobic layer
– Increases runoff and erosionand flood events
• Soil erosion and landslides
– Removal of anchoring vegetation on steep slopes
– Precipitation often exaggerates the effect of fires on landslides
Need erosion control here!
After a fire -to prevent mass wasting the slopes need to be stabilized
Prevent erosion on burned hillsides

Effects of Wildfires on Atmospheric
• Create their own clouds
• Release smoke, soot, and gases
contributing to pollution
• Contribute to smog formation
– Formation of ground-level ozone

Soot affects the cryosphere
Black carbon, or soot, is created primarily by either natural sources such as forest fires, or man-made sources, such as industrial pollution caused by burning fossil fuels like coal. Ice core samples taken from Greenland show that between 1880 and the 1950s, the amount of soot that feel on Greenland’s glaciers and ice sheets dramatically increased. There was seven times more soot from industry than from forest fires in Greenland snow during this per.

Linkages of Wildfires with Climate Change
• Climate change increases intensity and frequency of wildfires.
• Caused by changes in temperature, precipitation, and the
frequency and intensity of severe storms.
– Increases in temperature, decreases in humidity.
– Grasslands replacing forests creating more fuel.
– Lightning strikes increase ignitions.
– Insect infestations make trees more vulnerable to fire.

Effects of Wildfires on Biological Environment
• Vegetation
– Fire can destroy some vegetation.
– Weakens others.
– Some plants use fire to propagate.
• Animals
– Most animals may flee unharmed.
– Habitats are altered.
• Humans
– Water quality is affected.
– Smoke and haze produce eye, respiratory, and skin problems.
– Destroys personal property.

Natural Service Function of Wildfires
• Benefits to soil
– Increases nutrient content
– Reduce populations of microorganisms
• Benefits to plants and animals
– Reduces the number of species of plants
– May trigger a release of seeds in some species
– Removes surface litter for grasses
– Recycles nutrients in system
– Animals benefit from increased plant life

Minimizing the Wildfire Hazard: Fire Management
Task is decide when fires should be allowed and when suppressed
• Science
– Fire regime for site
• Types of fuel available
• Fire behavior
• Fire history
• Education
– Educating people to reduce their risk

Minimizing the Wildfire Hazard: Fire Management, cont.
• Data collection
– Mapping vegetation and potential fuel
– Moisture content
– FPI (Fire Potential Index) maps
• Prescribed burns
– Controlled burns to manage forests
– Reduces fuel for more catastrophic fires
– Necessary to predict the behavior of the fire and control it
Prescribed burn got out of control -high winds came up
50,000 acres burned
235 houses burned
115 building on Los Alamos National Lab damaged or destroyed
25,000 people displaced
Los Alamos 2000

Perception of the Wildfire Hazard
• People do not adequately perceive risk
of wildfires.
• Example: California
– Development on brush-covered hill slopes
– Demand for hill property increases
– Insurance may give people false sense of

Adjustments to the Wildfire Hazard
• Fire danger alerts and warnings
– Red-flag warnings
• Fire education
• Codes and regulations
• Fire insurance
• Evacuation

Home Design and Fire- Poor decisions:
– Home of wood or roofed with wooden shake shingles
– Wooden decks extending over steep slopes Natural or planted vegetation from yard right up to house
or draping over roof
• House can ignite by:
Flames travel through vegetation or along wooden fence
– Flames generating enough
radiant heat to ignite exterior
– Firebrands carried by wind
dropped on or next to house
Fire and homes in the woods don't mix!
This house survived = metal roof helped but not a good
This one didn't survive even though it was on main highway

Home Design and Fire- Safer decisions
Safer decisions:
-Fire breaks of cleared vegetationextending at least 9m from house, farther if on aslope
-if on a hill, build 100 feet back
-no vegetation within 30 feet of house
-use non-flammable materials
-keep flammables away from house
-spark arrestor on chimney
-have good access

Before You Leave Home:
• Check with authorities at your camping location for outdoor burning restrictions. During especially dry seasons even recreational andcooking fires can be restricted. Check the weather forecast.
• Ask if there is wood available or if you should bring your own. Also bring supplies to start your fire, do not use flammable liquids. During dry weather conditions, consider an alternative to a camp fire.
• Keep first aid supplies and emergency telephone
numbers accessible at all times.
• Know where the nearest telephone or ranger station
is located, and carry a cell phone if possible.
Are children included in your group? -Talk with them about
safety around camp fires.
• Supervision is paramount in keeping children safe. Ensure the
close supervision of children whenever a campfire or hot
coals are burning.
• Set a safety zone around the camp fire where all can sit and
relax around the fire. Designate a play area to keep bikes and
other toys out of the safety zone.
• Set family boundaries; only those designated should light the
campfire, add wood or put out the fire.
• Be sure all matches and lighters are out of reach and sight of
children. Matches and lighters should be stored in a secured
location to prohibit access by children.
• Teach children to 'Stop, Drop and Roll' if their clothes catch on

Preparing your camp fire site:
Avoid areas with overhanging branches, steep slopes or dry grasses. Clear area of all debris,down tobare soil.
• Construct a fire ring from rocks, keep the ring under four
feet in diameter with a ten foot clearance around the
• Have a bucket of water, and shovel nearby to put out the
fire when finished. Never leave a fire unattended.
• Burn only natural vegetation such as purchased cut wood
or fallen dead wood. Be prepared to pack your garbage.
• Never use matches or lighters inside tents. Never burn
charcoal, or use portable camping heaters, lanterns or
stoves inside tents, campers or vehicles

When putting a campfire out
drown it with
water. Stir the fire with water and dirt until all
the ashes are cold to the touch.
• Before leaving the area double check the camp
fire to ensure it is out cold.

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