Santa Ana winds alone do little harm Human 'sparks' prove devastating

November 05, 1993|By Frank D. Roylance | Frank D. Roylance,Staff Writer

If it weren't for arsonists and accidents, scientists say, the hot, dry Santa Ana winds would gush into Southern California as they have in the autumn for thousands of years, with little effect.

Mixed together, however, the winds and the human sparks have ignited a nightmare.

Santa Ana winds are born in the high deserts of Southern Nevada and Central Utah, where autumnal jet stream patterns deliver cold, dry high-pressure systems from Canada.

The dome of high-pressure air has to go somewhere and seeks out the lower pressure prevailing over coastal Southern California.

The desert air begins to pour like water from an overflowing tub, down through the canyons of the San Bernardino and San Gabriel mountains that shelter the northern and eastern flanks of the Los Angeles region.

But instead of cooling, the winds are a breath from hell, said Nick Leivers, a National Weather Service forecaster in Los Angeles. "Air, as it descends, heats up about 5.5 degrees per 1,000 feet," he said. That's due to its compression at lower altitudes, like air in a rapidly inflated tire.

"Our typical mountain range here is 5,000 feet, so we're talking a 25-degree warm-up from the top to the coast," said James Murakami, a meteorologist at the University of California, Los Angeles. "That's why you can get temperatures in the coastal areas in the upper 80s to the 90s."

The cold desert air also holds little moisture, and as the temperature climbs, its relative humidity gets very low. Wherever it blows, "it sucks out any moisture coming from [the] ground or the plants," he said.

If it weren't for the mountain barriers, these hot, dry winds would breeze across Southern California at a relatively sedate 10 or 15 mph, Mr. Murakami said.

But the mountains funnel the air through narrow passes and canyons, forcing it to accelerate to 50 and 60 mph. Santa Anas as fast as 100 mph have been recorded. On the eastern slope of the Rocky Mountains, similar winds are called the Chinooks.

The high winds fan any smoldering fires into fast-moving blazes and blow hot embers, igniting new fires. The flames can easily outrun both victims and firefighters.

Settlers along the Santa Ana River, near present-day Anaheim, got the credit for naming the winds they first encountered there.

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