S-190 Intro to Wildland Fire Behavior

Point of origin

The precise location where a competent ignition source came into contact with the material first ignited and sustained combustion occurred.

Head of a fire

The side of the fire having the fastest rate of spread

Flank of a fire

The part of a fire's perimeter that is roughly parallel to the main direction of spread.

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

3 main objectives to fight a fire

Anchor, Flank, Pinch

Fire perimeter

Entire outer edge of a fire

Fingers of a fire

Long narrow extensions of a fire projecting from the main body

Pockets of a fire

Unburned indentations in the fire edge formed by fingers or slow burning areas

Island

Area of unburned fuel inside the fire perimeter

Spot fire

Fire ignited outside the perimeter of the main fire by a fireband

Smoldering

Fire burning without flame and barely spreading

Creeping fire

Fire burning with a low flame and spreading slowly

Running fire

Behavior of a fire spreading rapidly with a well defined head

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

Torching

The burning of foliage of a single tree or a small group of trees from the bottom up

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.

Flare up

Any sudden acceleration in the rate of spread or intensification. Relatively short-duration. Junipers cause a lot of these.

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.

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.

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.

Control line

An inclusive term for all constructed or natural barriers and treated fire edges used to contain a fire

Fireline

The part of a containment or control line that is scraped or dug to mineral soil

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

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.

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

Chain

Unit of measure in land survey. 1 chain=66 feet. 80 chains=1 mile. 10 square chains=1 acre.

3 elements of the fire triangle

Fuel to burn

Air to supply oxygen for the flame

Heat to start and continue the combustion process

3 methods of heat transfer

Radiation, convection, conduction

Radiation

Radiant heat can dry surrounding fuels and sometimes ignite them. Like standing close to a camp fire warms you up.

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

Conduction

Heat is conducted from one fuel particle to another by direct contact.

When is a fire controlled?

When its expected to hold under any foreseeable conditions

Topography

The configuration of the earth's surface including its relief and the position of its natural and man-made features

Aspect

The direction a slope is facing (its exposure in relation of the sun

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

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.

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!

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.

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.

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

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.

Elevation

The height of the terrain above mean sea level

Barriers

Any obstruction to the spread of fire, typically an area or strip lacking any flammable fuel

What three characteristics of fuel is fire behavior dependent on?

Fuel type, fuel loading, fuel availability

Fuel Types

Grass; Grass-Shrub; Shrub; Timber-Understory; Timber littler; Slash-Blowdown

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

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

Shrub

Found throughout most areas

Some highly flammable shrubs:

-Palmetto/gallberry in the SE

-Sagebrush in Great Basin

-Chaparral in SW and California

Timber-Understory

Found throughout most areas

Provides ladder to aerial crown fuels

Timber Litter

Most dominant in mountainous topography, especially in the NW

Provides fuel for ground fire

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

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

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

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

Fuel Arrangement

The manner in which fuels are spread over a certain area

-Horizontal

-Vertical

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

Vertical Arrangement

Ground Fuels

Surface Fuels

Ladder Fuels

Aerial Fuels

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

Ladder Fuels

Combustible materials that aid the spread of fire from the surface to the upper canopy

Linked to torching or crowning

Aerial Fuels

All green and dead materials located in the upper canopy

Torching, crowning, spotting

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

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

Timelag

The rate at which dead fuel gains or loses moisture

Categories the same as Fuel size classes

The Basic Principles and Concepts of Fire Weather as They Relate to Wildland Fire Behavior

Air Temperature

Relative Humidity (RH)

Precipitation

Atmospheric Stability

Wind

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.

Relationship of Temperature and RH

Have an inverse relationship

When temp increases, RH decreases

When temp decreases, RH increase

Precipitation Amount vs. Duration

Precipitation duration has a greater impact on fuel moisture than precipitation amount

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

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

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

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

Inversion Break/Lift Indicators

Increase in temperature

Decrease in RH

Increase and/or shift in 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

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

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

Unstable Atmosphere

An atmosphere that encourages upward motion

Vertical motions increase

Cold air over warm air represents an unstable condition

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

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

General Wind

Large scale upper level winds caused by high and low pressure systems. Typically flow W->E

Jet stream winds

Local Winds

Developed when skies are clear and general winds are weak. Terrain has a strong influence

Types:

Upslope

Downslope

Up-valley

Sea-breeze

Upslope Wind Characteristics

Speed generally range 3-8mph and can be gusty

Greatest speeds occur about mid-afternoon

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

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

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

Land Breeze

A light nighttime breeze which originates over the relatively cool land, flows out over the warmer coastal waters

Periods that are characterized as critical fire weather conditions:

Strong and shifting wind

Very low RH

High temperature

Unstable atmosphere

Dry lightening

Common weather phenomena in which critical fire weather conditions occur:

Cold fronts

Foehn winds

Thunderstorms

Dust devils

Firewhirls

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.

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

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

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

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

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

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

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

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