Pressure Cooks Up Santa Anas

Raymond Chandler immortalized them:

“There was a desert wind blowing that night. It was one of those hot, dry Santa Anas that come down through mountain passes and curl your hair and make your nerves jump and your skin itch. On nights like that every booze party ends up in a fight. Meek little wives feel the edge of the carving knife and study their husbands’ necks. Anything can happen. . . . “

Chandler, in his short story “Red Wind,” may have told of the effects of these winds they call the Santa Anas. But Dan Bowman explained the cause.

Bowman, a meteorologist for WeatherData Inc., which provides forecasts for The Times, said Wednesday that a Santa Ana wind is basically a “northeast wind . . . caused by high pressure.”

Late Tuesday, such a high pressure weather system developed over northern Nevada, Bowman said. That combined with a low pressure system over New Mexico. And, as they know very well in Nevada, high beats low. The result: winds blowing across the Southland from the northeast to the southwest.

“Probably the easiest way to explain it in laymen’s terms is that winds flow from high pressure to low pressure,” Bowman said. “It’s kind of like in a bathtub. You know how the water swirls in toward the drain? That’s kind of the same effect we had (Wednesday).”

A wind doesn’t have to attain a minimum speed to qualify as a Santa Ana, Bowman said. But the ones that ripped across Orange County late Tuesday and early Wednesday were helped by the Venturi effect--which means that as the wind flows through canyons and passes, “it speeds up,” he said.

As if that isn’t enough, as the winds descend from higher altitudes, they get warmer, generally by five degrees for every thousand feet. They get drier too.

Thus, at one point Wednesday in downtown Los Angeles, the humidity hit an incredibly low 5%, Bowman said.

This meteorological explanation could come in handy again on Friday when Bowman foresees much the same wind conditions.