Weather Key to Fire

July through September is the peak of fire season, when periods of drought, high pressure, low humidity, winds, and frequent lightning create ideal conditions for fire.

The weather for a given location has a great influence on fire behavior. In the mid-elevations, cold air drains down into the valleys and the mid-slope areas stay warmer at night. This "thermal belt" can allow a fire to keep burning at night, whereas in lower elevations where the air cools down a lot at night, fires will calm down.

The "aspect" of a slope -- which direction it faces -- also has a big impact on how a fire will burn. A south-facing slope that gets sun all day and is warmer and drier is likely to feature open areas or brush and grass. North-facing slopes are usually cooler and damper and more likely to have heavier timber on them. There is an interlocking effect between the fuel (what grows there), the microclimate, and a specific weather event such as a storm or dry spell.

How steep a slope is contributes another important factor in fire weather. Winds are driven by heating of the slope. You usually get an up-slope wind in the afternoon as the day's warm air rises. Then the wind reverses and blows downhill in the evening as the air cools and sinks. Canyons and draws also channel the wind, creating high winds in very specific spots.

What often happens is that a period of high pressure and warming dries fuels, and then the weather changes -- a lightning storm will start a fire and winds caused by a storm front moving through will fan the flames. Sometimes a lightning-caused fire can just smolder until the wind comes up to push it around.

There's really no one single cause of fires -- how a fire behaves is a combination of long-term conditions and local factors.

We can create several types of forecasts to help managers decide how to handle a wildland fire or a prescribed burn. First there's the long-range forecast for an entire area. That comes from models, maps, and satellite images. These models factor in observed and predicted large-scale changes in the upper atmosphere. The satellite images show us where precipitation is likely, as well as where the fronts are moving.

Then we'll do a spot forecast for the burn site using data from a local weather station, as well as what people who live in the area know about wind patterns.

With a prescribed fire, we work with the manager and fire behavior specialist to define ahead of time what the weather conditions should be to allow the burn to go or not. We come up with a range of humidity, temperature, and precipitation. If it's too cool and wet, the fire can't burn. If it's too dry and hot, then it's likely to get out of control.

Jean Hoadley
meteorologist