S-190 Intro to Wildland Fire Behavior
Pockets of a fire
Unburned indentations in the fire edge formed by fingers or slow burning areas
Chain
Unit of measure in land survey. 1 chain=66 feet. 80 chains=1 mile. 10 square chains=1 acre.
Radiation
Radiant heat can dry surrounding fuels and sometimes ignite them. Like standing close to a camp fire warms you up.
3 methods of heat transfer
Radiation, convection, conduction
Horizontal Arrangement
Affects fire's rate of spread Uniform- include all fuels distributed continuously over the area Patchy- include all fuels distributed unevenly over the area, or areas of fuel with definite breaks or barriers present
The Basic Principles and Concepts of Fire Weather as They Relate to Wildland Fire Behavior
Air Temperature Relative Humidity (RH) Precipitation Atmospheric Stability Wind
Nighttime (Radiation) Inversions
Air cooled at night, primarily by contact with cold surfaces, gradually deepens as the night progresses and forms a surface 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
Downslope Wind Characteristics
Air cools and sinks producing downslopping winds Greatest flow occurs after midnight Speed between 2-5 mph The change in wind from downslope to upslope can rapidly change wildland fire behavior from inactive to active in a matter of minutes
Ground Fuels
All combustible materials lying beneath the surface-deep duff, tree roots, organic material Smoldering fire Important to check in mop-up
Surface Fuels
All combustible materials lying on or immediately above the ground Linked to fire running
Aerial Fuels
All green and dead materials located in the upper canopy Torching, crowning, spotting
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
Timber-Understory
Found throughout most areas Provides ladder to aerial crown fuels
Shrub
Found throughout most areas Some highly flammable shrubs: -Palmetto/gallberry in the SE -Sagebrush in Great Basin -Chaparral in SW and California
3 elements of the fire triangle
Fuel to burn Air to supply oxygen for the flame Heat to start and continue the combustion process
What three characteristics of fuel is fire behavior dependent on?
Fuel type, fuel loading, fuel availability
Wet Fuels
Fuels that have a high moisture content because of exposure to precipitation or high relative humidity
Dry Fuels
Fuels that have low moisture content because of prolonged exposure to sunshine, dry winds, drought, or low relative humidity
General Wind
Large scale upper level winds caused by high and low pressure systems. Typically flow W->E Jet stream winds
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
Creeping fire
Fire burning with a low flame and spreading slowly
Smoldering
Fire burning without flame and barely spreading
Spot fire
Fire ignited outside the perimeter of the main fire by a fireband
Firewhirl
Spinning vortex column of ascending hot air and gases rising from a fire and carrying aloft smoke, debris, and flame. Can move all over and are unpredictable.
Periods that are characterized as critical fire weather conditions:
Strong and shifting wind Very low RH High temperature Unstable atmosphere Dry lightening
Foehn Winds
Strong, dry winds caused by the compression of air as it flows down the lee side of a mountain range. It is usually warm for the season Can persist for days and frequently reaches 40-60mph RH will drop upon onset of winds COMBINATION OF HIGH WIND SPEEDS AND LOW RH CAN CAUSE HIGH RATES OF FIRE SPREAD
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.
Crown fire
Fire that advances from top to top of trees or shrubs more or less independent of surface fire. Sometimes classes as running or dependent to distinguish the degree of independence from the surface fire.
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. Don't be at the top of these!
Fuel Size Classes
1-hr fuels: 0-.25 inch in diameter; grasses 10-hr fuels: .25-1 inch in diameter; sticks, brush, smaller shrubs 100-hr fuels: 1-3 inches in diameter; small trees, bigger shrubs 1000-hr fuels: 3-8 inches in diameter; trees
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
Inversions
A layer where temperature increases with altitude (warm air over cold air) Under an inversion, fuel moisture content is usually higher, thus decreasing fire spread rates and intensities
Land Breeze
A light nighttime breeze which originates over the relatively cool land, flows out over the warmer coastal waters
Cold Front Indicators
A line of cumulus clouds may be seen approaching from W or NW Large clouds of dust can precede the arrival Winds normally shift from SE to S, to SW, and increase in velocity Winds will be strongest and gusty as the front reaches you
How do unstable atmospheres contribute to fire activity?
Allowing convection columns to reach greater heights, producing stronger in-drafts 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
Anchor point
An advantageous location, usually a barrier to fire spread, from which to start constructing a fireline. Used to minimize the chance of being flanked while line is being constructed.
Unstable Atmosphere
An atmosphere that encourages upward motion Vertical motions increase Cold air over warm air represents an unstable condition
Control line
An inclusive term for all constructed or natural barriers and treated fire edges used to contain a fire
3 main objectives to fight a fire
Anchor, Flank, Pinch
Barriers
Any obstruction to the spread of fire, typically an area or strip lacking any flammable fuel
Flare up
Any sudden acceleration in the rate of spread or intensification. Relatively short-duration. Junipers cause a lot of these.
Island
Area of unburned fuel inside the fire perimeter
Up-valley Wind Characteristics
As air in the valley warms, temp and pressure differences within the valley or valley to adjacent plains results in an up-valley wind flow The greatest occur mid-late afternoon Range between 10-15mph
Down-valley Wind Characterstics
As the valley loses solar heating, the air in the valley cools. The cool air drains down-valley, resulting in the down-valley wind Greatest occur after midnight Speeds of 5-10 mph
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
Running fire
Behavior of a fire spreading rapidly with a well defined head
Common Foehn Winds in the Western US
Chinook wind: Along the E of rockies and E of Sierra Nevada Wasatch wind: W side of Wasatch Range in Utah Santa Ana and Sundowner: SoCal Mono and North wind: Central and NorCal East wind: Western Washington and Western Oregon
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
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
Common weather phenomena in which critical fire weather conditions occur:
Cold fronts Foehn winds Thunderstorms Dust devils Firewhirls
Predictive Services
Combine group of Interagency Land Management Fire Intelligence Coordinators and Fire Meteorologists Seasonal assessments 7-Day significant fire potential Monthly outlook Briefings Daily summaries
Ladder Fuels
Combustible materials that aid the spread of fire from the surface to the upper canopy Linked to torching or crowning
Slash-Blowdown
Debris left after natural events or human activities like logging, pruning, wind, snow, fire, shrub cutting Debris may include: Logs, bark, branches, stumps, shrubs Provides fuel for fire spread
Stable Atmosphere
Define 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 in-drafts into the fire at low levels and fire intensity
Local Winds
Developed when skies are clear and general winds are weak. Terrain has a strong influence Types: Upslope Downslope Up-valley Sea-breeze
Fire perimeter
Entire outer edge of a fire
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
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
Firewhirls
Generated by intense fires Can pick up large burning embers and spew them far across the fireline causing numerous spot fires A favorite area for development is on the win sheltered (leeward) side of ridges
Fuel Types
Grass; Grass-Shrub; Shrub; Timber-Understory; Timber littler; Slash-Blowdown
Vertical Arrangement
Ground Fuels Surface Fuels Ladder Fuels Aerial Fuels
Relationship of Temperature and RH
Have an inverse relationship When temp increases, RH decreases When temp decreases, RH increase
Conduction
Heat is conducted from one fuel particle to another by direct contact.
Inversion Break/Lift Indicators
Increase in temperature Decrease in RH Increase and/or shift in wind
Potentially dangerous cold front characteristics:
Light southeasterly winds are common several hundred miles ahead of the front The air mass ahead of the front is typically very warm and unstable, resulting in an increase in fire behavior As the front pushes through, the wind can abruptly shift from SW to NW, driving the fire head to the SE 15-30mph and gusty
Fingers of a fire
Long narrow extensions of a fire projecting from the main body
Timber Litter
Most dominant in mountainous topography, especially in the NW Provides fuel for ground fire
Aspect in terms of N, S, W, E slops
N- have more shade; lowest rate of spread; late snow melt; later curing of fuels; heavier fuels; higher humidity; lower temps S- More exposed to sunlight; lighter and sparser fuels; high temps; higher rate of spread; early curing of fuels; lower humidity; these are most critical in terms of start and spread of wildland fires W- Later heating and cooling E- Earlier heating and cooling
Thermal Belts
Nighttime inversions in mountainous regions increase in depth during the night. The cold layer gradually deepens, with the nighttime inversion coming in contact with and reaching farther up the slow below the main ridges Characterized by the highest minimum temperature and the lowest nighttime RH Region of warmer air, typically found on the middle third of the slope
Dust Devils
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
Rear of a fire
Portion of a fire spreading directly into the wind or down slope. Opposite the head. Slowest spreading portion of a fire edge. Also called the heel
Precipitation Amount vs. Duration
Precipitation duration has a greater impact on fuel moisture than precipitation amount
Wide Canyons
Prevailing wind direction can be altered by the direction of the canyon. Strong differences in fire behavior will occur on north and south aspects because of fuel loading on the north side.
Surface Area to Volume Ratio
Relates to the amount of the outer surface of the fuel that is expose to the air Smaller (fine) fuels have a high surface area to volume ratio than larger (heavy) fuels Bigger surface area=longer lasting burn time
Upslope Wind Characteristics
Speed generally range 3-8mph and can be gusty Greatest speeds occur about mid-afternoon
Backing fire
That portion of the fire with slower rates of spread. Also called the heel
Flaming front
That zone of a moving fire where the combustion is primarily flaming. Light fuels typically have a shallow flaming front. Heavy fuels have a deeper front.
Fuel Loading
The amount of fuel present expressed quantitatively in terms of weight of fuel per unit area This may be available fuel or total fuel and is usually dry weight
Relative Humidity
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 a percent.
Fuel Moisture
The amount of water in a fuel, expressed as a percentage of the over-dry weight of that fuel Determines how well a fuel will ignite and burn
Slope
The amount or degree of incline of a hillside Fires burn more rapidly uphill than downhill because fuels above the fire are brought into closer contact with the upward moving flames.
Cold Fronts
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.
Torching
The burning of foliage of a single tree or a small group of trees from the bottom up
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
Topography
The configuration of the earth's surface including its relief and the position of its natural and man-made features
Atmospheric Stability
The degree to which vertical motion in the atmosphere is enhance or suppressed. Stability is directly related to the temperature distribution of the atmosphere Wildfire are greatly affected by atmospheric motion and the properties of the atmosphere that affect its motion
Aspect
The direction a slope is facing (its exposure in relation of the sun
Elevation
The height of the terrain above mean sea level
Subsidence Inversion
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 Skies typically clear or cloudless under these high pressure systems
Fuel Arrangement
The manner in which fuels are spread over a certain area -Horizontal -Vertical
Fireline
The part of a containment or control line that is scraped or dug to mineral soil
Flank of a fire
The part of a fire's perimeter that is roughly parallel to the main direction of spread.
Point of origin
The precise location where a competent ignition source came into contact with the material first ignited and sustained combustion occurred.
Timelag
The rate at which dead fuel gains or loses moisture Categories the same as Fuel size classes
Head of a fire
The side of the fire having the fastest rate of spread
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.
Convection
Think of this as a smoke column above the fire. Occurs when lighter warm air moves upward. The hot gases and embers which compose the smoke column can dry and ignite other fuels
When is a fire controlled?
When its expected to hold under any foreseeable conditions
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.
Thunderstorms are potentially dangerous to firefighters because:
Winds 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 Can produce dangerous downdrafts Lightening
