Published Tuesday, August 13,  2002 in the San Jose Mercury News

The Weather Corner

Weather plays large role in growth of fire

Special to the Mercury News

As numerous fires continue to burn the forests and grasslands of the West, weather plays an important role in their development, growth and demise.

Certainly a contributing factor has been the dry rainfall season, with most states west of the Rockies having only 70 to 90 percent of their normal precipitation. The only exceptions are the Southwest, which has gone as low as 10 percent of normal in some areas, and the coastal section from San Francisco to Seattle, which was near normal. For a map showing this distribution, see

While these dry conditions exacerbate fires after they have started, approximately 70 percent of the wildfires in California are the result of careless or destructive human activities. These activities range from vehicle and equipment use, which triggers about 26 percent of fires, to campfires and burning debris at 21 percent, arson at 9 percent, 6 percent from electrical power lines, children playing with fire at 4 percent and 2 percent from the illegal discarding of smoldering cigarettes.

Not only can wild land fires be affected by the weather, but they make their own weather. As a fire burns, strong updrafts are created and an area of low pressure develops. Air from surrounding areas of high pressure then rushes to fill the void, with strong gusty winds resulting. The ferocity of these winds can be enhanced as they are funneled through canyons and over ridge tops.

Another byproduct of the strong updrafts produced by a fire are clouds that even have their own name -- pyrocumulus or ``fire cumulus.'' These arise when moisture from trees and other burned objects is pulled high into the sky by updrafts and condenses into clouds. Under the right conditions, these clouds can produce rain to dampen a fire, or lightning that can cause additional fires.

On large wild land fires, a meteorologist and a fire behaviorist join the firefighting crew. The meteorologist monitors current and forecast weather conditions, paying particular attention to local microclimates. The fire behaviorist uses the weather data plus information about the terrain and vegetation to predict how a fire will spread.

LAST REMINDER: The ``Weather & Climate'' course (Meteorology 10, section 7) that I will be teaching at San Jose State University begins Aug. 27. It will be held Tuesday evenings from 6 to 8:45 p.m. If you would like more information, contact Open University at SJSU, drop me a note at or visit

Q I am a fire captain with the California Department of Forestry and Fire Protection, stationed in the Santa Cruz Mountains. Recently we got numerous calls from people smelling smoke and seeing an ill-defined smoky haze in the area. We knew of no large fires anywhere near the Bay Area. Is it possible that we smelled smoke from fires in Oregon? Those are the only fires we are aware of right now other than ones in the Sequoia forest and other places south. In general, how far do you think smoke can drift before dissipating or before it no longer has an easily recognized odor? Are there some conditions that lend themselves to long-distance smoke transport? Bill Tomkovic - Fire captain, California Department of Forestry and Fire Protection

A It's doubtful that what was seen and smelled came from the Oregon fires, as the winds up there were taking most of the smoke inland. Several weeks ago, when there was a large fire near Sequoia National Forest in the southern Sierra Nevada, we did see some of the smoke in the Bay Area, but, again, the winds were wrong on the occasion you cite.

In general, there can be pretty noticeable effects from smoke that originates a few hundred miles away if the weather conditions are right. For the Bay Area, this happens when there are light east winds in the middle of the atmosphere between about 5,000 and 20,000 feet.

Q Is there any way to find the high/low temperatures for a specific location, such as Little Yosemite Valley, or other places out in the boonies? James Erbes - San Jose

A While most weather observations come from airports that are in the flatlands, the forest service does have a network of Remote Automated Weather Stations, or RAWS, that send back hourly readings of temperature, humidity and wind. There are even some in the more remote regions of the Bay Area. For a clickable map showing RAWS sites in Northern California, see, and for central California check out

Q Why is the heat index figure 5 degrees lower than the actual temperature? Even with low humidities, that doesn't make sense.  Richard Cochran - Morris, Ill.

A The heat index and wind-chill index are both theoretical values and sometimes do not make sense when compared to ``real'' temperatures. The heat index is a function of the temperature and humidity, and when both are high the index can be higher than the actual temperature. Conversely, with a very low humidity, the heat index can indicate slightly cooler weather than a thermometer would read. This is supposed to represent that the body would only ``feel'' this comfortable or uncomfortable, not represent the actual air temperature.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o WeatherCorner, 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 Please indicate in your e-mail what city you live in.