S-290 Unit 1: The Fire Environment

Crown fire

A fire that burns in the elevated canopy fuels

Surface fire

A fire that burns loose debris on the surface, including dead branches, leaves, and low vegetation

Ground fire

A fire that consumes the organic material beneath the surface litter (such as a peat fire), usually exhibits smoldering

Running fire

A fire that spreads rapidly with a well defined head and is usually associated with surface fires

Closed fire environment

A sheltered environment (for example, a fire burning under a forest canopy)

How does the stability of the lower atmosphere affect the vertical development of a fire's convective smoke column?

A stable atmosphere suppresses convection An unstable atmosphere allows for more vertical movement of air, which can contribute to the development of potentially dangerous thunderstorms or large fire plumes

Firebrand

Flaming or glowing fuel particles

Elements of weather that influence fire behavior

Temperature Relative humidity Atmospheric stability Wind speed and direction Precipitation

Flame height

The average height of flames as measured on a vertical axis

Flaming front

The zone of a moving fire where the combustion is primarily flaming; behind this flaming zone, combustion is primarily glowing or involves the burning out of larger fuels (greater than about 3 inches in diameter)

3 component so of the fire triangle

heat, oxygen, fuel

3 components of the fire environment

weather, topography, and fuels

Conduction

The transfer of heat from one molecule of matter to another (direct contact)

Open fire environment

An environment in which the fuels and fire are exposed to the weather and elements (winds, solar radiation)

Smolering

Combustion that immediately follows flaming combustion

3 methods of transport of firebrands

Convection Wind Gravity

Elements of topography that influence fire behavior

Elevation Position on slope Aspect Shape of the terrain Steepness of slope

3 factors influencing vertical development of a fire's convective smoke column

Fire intensity Stability in the lower atmosphere Winds aloft

Primary factors affecting fireline intensity

Fuel loading Compactness or arrangement of fuels Fuel moisture content The rate of spread

Elements of fuel that influence fire behavior

Fuel loading Size and shape Compactness Horizontal continuity Vertical continuity Chemical content Fuel moisture and temperature

Field conditions affecting the spread pattern of wildfires

Heterogeneous fuel complexes that cause fingering Barriers that stop or partially stop the spread The effect of slope reducing or increasing fire spread at the head or flanks Spotting ahead of or down slope from a fire

Radiation

The transmission of heat energy by electromagnetic waves passing from a heat source to an absorbing material

Ignition temperature

The temperature a material must attain to produce self-sustaining combustion (600o for wildland fuels)

How does fire intensity affect the vertical development of a fire's convective smoke column?

Low intensity fires create weak indrafts at the fire's edge that feed a short, weak smoke or convective column over the fire; the environment mostly controls the fire, the fire's influence is very small, and causes only slight modification of weather elements in the proximity of the fire High intensity fires create much stronger indrafts at the fire's edge, which can help feed a convective column that may build many thousands of feet into the atmosphere; the fire can control the environment to a marked extent and its influence can significantly modify weather elements near and adjacent to the fire (such as wind speed and direction and temperature); high intensity wildland fires, whether large or small in size, usually have a considerable effect on the atmosphere vertically; the associated well-developed convection column and the development of fire whirls illustrate the vertical effects of the fire on the atmosphere

Pattern of wildfires spread

Near-elliptical patterns, influenced by wind speed; as wind speed increases, the ellipse elongates

Convection

The transfer of heat by the motion of air (or any other fluid)

Primary factors affecting ignition

Size and shape of fuels Compactness or arrangement of fuels Fuel moisture content Fuel temperature

Flame length

The distance measured from the tip of the flame to the middle of the flaming zone at base of the fire

Spotting

The mass transport of firebrands

Heat transfer

The physical processes by which heat energy moves to and through unburned fuel Preheat fuel prior to ignition, thus contributing to fire spread

Fireline intensity

The rate of heat released during combustion per foot of fire front, the heat that a firefighter would feel at the fire's flaming front (Btu/feet (ft)/second) Measured visibly by flame length

Rate of spread

The relative activity of a fire in extending its horizontal dimensions

Primary factors affecting rate of spread

Wind speed Steepness of slope Changes in fuel type (grass to timber litter, or surface fuels to aerial fuels)

What are winds aloft and how do they affect the vertical development of a fire's convective smoke column?

Winds aloft are winds affecting the convective column Strong winds tend to discourage vertical development of convective columns and can bend the smoke column so that it stays near the ground as smoke travels away from the fire. In a very unstable environment, the winds can be more erratic, and during times of lighter winds, vertical development of the plume can strengthen, thus increasing the potential for spotting with stronger winds that follow

Wildland fire behavior

the manner in which fuels ignite, flames develop, and fire spreads and exhibits other characteristics