Published Tuesday, May 25, 2004 in the San Jose Mercury News

Weather Corner
Jan Null, CCM

Rainfall deficit sets stage for early start to fire season

As a rather benevolent rainfall season winds down, California's malevolent fire season is already off to a roaring start. There are predictions for a ``bad'' fire season across the state, though this is usually the case because of the area's Mediterranean climate with its natural ``summer drought.''

In years when there is abundant rainfall, the grasses, shrubs and other ``fuels'' grow more and the fire danger is heightened. In years when it is drier than normal, the fuels are dried-out and more combustible.

Most of Northern California has had close to normal rainfall for the season, but there are larger rainfall deficits farther south in the state. (See ).

Compounding the problem is the damage to many pine forests in the West from the bark beetle, which makes them more susceptible to fire. Even the lagging economy is a factor as fire agency budgets and resources are cut.

Despite these natural conditions, about 70 percent of wild land fires in California result from human activity. Approximately 26 percent of fires result from vehicle and equipment use, 21 percent from runaway campfires and the burning of debris, 9 percent from arson, 6 percent from electrical lines, 4 percent from children playing with matches and 2 percent from illegally discarding smoldering cigarettes.

A major fire can also be the result of a relatively short period of significant weather. It only takes a few days of hot weather, low humidity and gusty winds to raise the fire danger significantly.

Once a fire has begun, the ambient weather and climate conditions are only part of the equation. As a fire rages, strong updrafts are created, causing an area of low pressure within the fire zone. Air from surrounding areas of relatively high pressure then rushes in to fill the lower pressure in the form of strong, gusty winds. These winds can intensify the fire. The force of these winds can be enhanced by the surrounding terrain as they are funneled through canyons and over ridge tops.

Fire can also create clouds and rain. As the columns of hot air rise, they carry aloft tiny particles as well as moisture from burned trees and vegetation. When they cool off, they can condense into clouds that sometimes produce rain. Called pyrocumulus or ``fire cumulus,'' they can be a blessing with fire-dampening rain or a curse with lightning that may trigger additional fires.

To monitor conditions in remote areas, there are automated weather stations that send back real-time weather via satellite. Called RAWS, or Remote Automated Weather Stations, there are more than 200 of these devices scattered mostly in the wilderness areas of California. To access RAWS data for the Bay Area and adjacent regions, see .

Q. I am a serious photographer and read your columns about colorful sunsets with great interest. I understand how clouds can create a dramatic sunset. However, I have seen the sky turn brilliant red-orange at sunset on the Monterey coast without a cloud in the sky. Are there temperature, humidity or barometric pressure conditions that can cause brilliant sunsets? I would like to be able to predict them and have time to drive to take a picture. John Herrgott - Sunnyvale

A. Probably the best thing to look for would be a major fire or volcanic eruption. When there is a high concentration of particles (i.e., smoke or dust) in the air larger than air molecules, the shorter wavelengths of light like violet, blue and yellow are scattered away. This leaves the longer wavelengths (orange and red) to reach your camera and make that perfect picture.

Q. Sinking air typically tends to warm. But in thunderstorms, downdrafts are cold. Why? Jared Stephenson - Santa Rosa

A. Sinking air does indeed warm, but the air within a thunderstorm can often start out below freezing. These downdrafts occur as falling rain drags this cold air earthward with it. The cooling is exacerbated due to evaporation of some of this moisture, and this in turns cools the environment around it.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail them to or fill out a form online at