The Furies of October--Heat, High Wind, Low Humidity

The images of fires leaping like demons through Southern California communities easily lend themselves to the impression of a haphazard and random destruction.

To those who know fire through study and experience, however, its behavior is depressingly predictable.

The conditions of heat, high wind and low humidity tend to arrive in the fall, caused by a vast dome of high pressure centered over the Great Basin area to the northeast. Air circulating clockwise around this dome whips across the deserts, over the mountains and into the coastal valleys, heating and drying out by compression as it sweeps down canyons to the sea.

The winds, called Santa Anas, blow generally from the northeast to the southwest, whistling through neighborhoods that wend up into the San Bernardino foothills.

The low humidity, made more severe this year by the lack of an October rain, increases the flammability of the brush.

Temperatures in the upper 90s and relative humidity below 10% were common in the fire areas Sunday. Forecasters don’t expect the winds to abate until tonight, with highs in the 90s through Tuesday.

This confluence of conditions strikes regularly. In October 1993, fires driven by the winds destroyed more than 550 homes in Altadena, Laguna Beach, Thousand Oaks and other dry corners of Southern California.

Stoked by similar winds, flames charred almost 3,000 structures in Oakland and Berkeley in October 1991.

San Bernardino, where hundreds of homes burned Saturday, also lost hundreds of homes in the 1980 Paramount fire.

The biggest variable in the pattern is where the fire strikes. Because so many fires are started by arson or accident, it is often a human who determines the location.

Once ignited, the fires spread where natural fuels are abundant. For San Bernardino, it took 23 years for dry brush to mature to the point that it could feed a second catastrophic fire. More than two decades of growth also left a thick tangle of volatile dead twigs and branches that did not decompose in Southern California’s dry climate, said Richard A. Minnich, a wildfire expert at UC Riverside.

For San Bernardino, that means brush fire is not likely to return soon.

“For the next 20 years, we don’t have any fuel to worry about from San Bernardino to San Dimas,” Minnich said.

The effect of past burns became apparent Sunday when the Grand Prix fire, burning west from Rancho Cucamonga, ran up against the boundary of last year’s Williams fire.

U.S. Forest Service spokesman Mark Whaling said the new fire stopped there, its fuel exhausted. But it continued west along the base of the mountain, below the line where firefighters had contained the Williams fire.

The hot, dry Santa Ana winds that are fanning brush fires across Southern California are forecast to continue into this morning, gusting to 50 mph before easing off sometime this afternoon.

The National Weather Service predicted an onshore flow of cool, moist air later in the week, with foggy, overcast mornings in most of the coastal valleys by Wednesday. High temperatures Wednesday should be in the 80s, dropping to the 70s, or even the 60s, by the weekend.

For experts like Minnich, the very predictability of Southern California’s fires is cause for alarm. Their frequency, size and destructiveness could be reduced by preventive measures that have not received general support, he said.

Minnich advocates a policy of allowing small summer fires to burn themselves out .

He argues that, left alone, the average small summer fire, under less extreme conditions of wind and dryness, would burn slowly up the mountain canyons before expiring, causing little threat to houses.

“Once they get big,” Minnich said, “the only reason they lay down is that the weather changes or they run out of fuel.”