Fire Weather

Weather is the state of the atmosphere and is often described in terms of temperature, humidity, stability, pressure, wind speed and direction, clouds and precipitation. The interaction of these weather elements control many aspects of fire behavior. For example, atmospheric moisture directly effects fuel flammability, and, by its relationship to other weather factors has indirect effects on other aspects of fire behavior. General winds and local winds affects wildfire in many ways. Wind carries away moisture-laden air and hastens the drying of forest fuels. Light winds aid certain firebrands in igniting a fire. It aids fire spread by carrying heat and burning embers to new fuels, and by bending the flames closer to the unburned fuels ahead of the fire. The direction of fire spread is determined mostly by the wind direction. Atmospheric stability is closely related to fire behavior. For example, winds tend to be turbulent and gusty when the atmosphere is unstable, and this type of airflow causes fires to behave erratically. Thunderstorms with strong updrafts and downdrafts develop when the atmosphere is unstable and contains sufficient moisture. Their lightning may set wildfires, and their distinctive winds can have adverse effects on fire behavior.

Fire weather constitutes examining the current state of the atmosphere between the surface and 5 to 10 miles above the surface and how this state will change and impact fires. When knowledge of fire weather is combined with information on fuels and topography, assessments of fire danger and potential fire behavior are possible. Both the fire-control plan, in the case of wildfire, and the burning plan, in the case of prescribed fire, must be based on past and expected weather conditions.

• Air Temperature: Temperature directly influences many other weather elements, such as moisture, stability, and wind speed and direction. This section discusses temperature and heat; the role of solar radiation in creating diurnal and seasonal variations in air temperatures; how topography, water bodies, and vegetation influence air temperatures; and, vertical variations in temperatures (lapse rates and inversions).

• Atmospheric Moisture: Atmospheric moisture has direct effects on the flammability of forest fuels, and, by its relationship to other weather factors, it has indirect effects on other aspects of fire behavior. This section discusses sources of atmospheric moisture; evaporation, dew points, absolute humidity, and relative humidity; how temperature, wind, and other factors influence relative humidity horizontally and vertically; means of measuring relative humidity.

• Atmospheric Stability: Atmospheric stability is the resistance of the atmosphere to vertical motion. This section discusses methods for determining atmospheric stability; various processes that cause air to rise; how and why stability varies diurnally and seasonally; how cloud-types and other visual clues can be used as indicators of stability; and, what subsidence is and conditions that lead to its development.

• General Winds: General winds are produced by the broadscale pressure gradients but may be modified considerably by friction or other topographic effects. This section discusses laminar flow, eddy formation and the conditions that create mechanical and thermal turbulence; winds created by frontal air-masses; effects of mountains on surface winds; and methods used to measure surface winds.

• Convective Winds: Convective winds originate from small-scale pressure gradients produced by local temperature differences and may be strengthened or weakened general winds. This section discusses land and sea breezes, slope and valley winds and their interaction with general winds; whirlwinds and thunderstorm winds.

• Air Masses and Fronts: Air masses have characteristic weather in their source regions. As they leave their source regions, air masses are modified according to the surface over which they travel. In frontal zones, where two or more air masses meet, considerable weather is concentrated. This section discusses the formation and modification of air masses and their associated weather, warm fronts, cold fronts, and stationary fronts.

• Clouds and Precipitation: The amount of precipitation and its seasonal distribution are important factors in controlling the beginning, ending, and severity of local fire seasons. This section discusses saturation, condensation, sublimation, and precipitation processes as well as types of precipitation and clouds.

• Thunderstorms: The heat energy released in condensation provides the energy for thunderstorms and the violent winds associated with them. Thunderstorms are important in fire control because they start fires by lightning, blow them out of control with the downdraft and outflow, or put them out with rain. This section discusses thunderstorm development, types of thunderstorms, lightning, and tornadoes.

• Fire Climate Regions: This section discusses the general temperature, precipitation, and pressure/circulation patterns in North America and how these climate factors are influenced by broad-scale topographical features and oceans. The general fire climate of the Southeastern United States and 14 other regions are summarized.