Santa Anas Blow Into Town : Weather: Firefighters are wary, surfers are stoked over the the desert winds. By Thursday, gusts may reach 25 m.p.h., and the thermometer could reach 100.

SANTA ANA —  Like surfers and firefighters, Jeffrey Thorstenson has been monitoring the warm Santa Ana winds, ready to shift flight patterns at John Wayne Airport if they intensify today as forecast.

Thorstenson, the manager of the air traffic control tower, will alter flight patterns 180 degrees if offshore winds hit 5 knots. Typically, Orange County’s prevailing winds are onshore--from the ocean toward the mountains--and airplanes arrive and depart the airport from north to south.

“It’s a basic principle of aerodynamics: You want to land and take off into the wind,” Thorstenson said Tuesday from the tower 64 feet above the airport. “The Santa Ana winds can turn our direction around from south to north.”

The desert winds, which are expected to gust to 25 m.p.h. by Thursday, also are sure signs of fire season, said Capt. Dan Young of the Orange County Fire Authority.

“Historically, our Santa Ana winds blow and our worst fires occur in October and November,” said Young, whose department put out two fires believed to have been started by arsonists Monday. “Right now is when we need to be most cautious.”

The current mild Santa Ana condition started Monday, when the winds began to blow and temperatures reached 100 degrees in Anaheim, said Curtis Brack, a meteorologist for WeatherData, which tracks weather conditions for The Times.

The offshore winds slackened Tuesday and the county’s highs dropped slightly, Brack said. The highs included 93 degrees in Santa Ana, 89 in Anaheim and El Toro and 80 in San Juan Capistrano, Brack said.

Expect slightly higher temperatures and similar wind conditions today, with the trend intensifying into Thursday, Brack said.

“We’re expecting it to be a bit warmer [today], with temperatures in the mid-70s along the coast and up to the mid-90s inland. But the highs could reach the upper 90s and 100 degrees again on Thursday,” Brack said.

Santa Anas are generated by a strong ridge of high pressure in the upper atmosphere, Brack said. Temperatures rise because the high pressure pushes a dry wind from the mountains and desert down as much as 8,000 feet to sea level, he said.

“The air moving down in the atmosphere gets compressed and warmed,” Brack said. “A clear sky and low humidity allows the temperatures to skyrocket.”

It is a condition that can be a boon for surfers, particularly when combined with an incoming swell such as the one expected late this week from the Southern Hemisphere, said Sean Collins, a wave forecaster with Huntington Beach-based Surfline/Wavetrak.

Collins predicts a three- to five-foot swell building into Thursday and Friday, with waves up to seven feet at the best spots, such as Trestles, Newport Beach and Huntington Beach, where the beaches face south.

The winds hold up the faces of the surf and blow graceful plumes off the wave lips, he said.

“For surfing, it’s great when these weather conditions happen,” Collins said. “The offshore winds groom the waves. They make them much cleaner.”

Collins said surfers all over the county can count on the action:

“It’s guaranteed to be good. This is a great time of year to get out there and get wet.”

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Hot Winds

Santa Ana winds occur at all times of the year, but they are most frequent in the fall. This week, the air has flowed from a high pressure area over southern Idaho and northern Utah toward a low pressure area off the California coast. Expect a respite by Friday.

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Effects of the Wind

The wind can cause widespread damage, including downed trees, power poles and the spreading of brush fires.

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Heating of the Air

As a general guide, air warms about 5.5. degrees for every 1,000 feet it drops. As the air moves form the Great Basin region elevation of about 4,500 feet toward the coast, the air will warm by at least 25 degrees.

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1. High Pressure Area

Air moving in a clockwise motion over Idaho. Utah is drawn toward a low pressure area off the Southern California coast.

2. Dry Desert Air

The dry desert air removes much of the moisture as the air flows from the high pressure area to the low pressure area.

3. Mountains

The air is compressed as it moves down through the mountains surrounding the Los Angeles Basin toward sea level. As the density of the air increases, its temperature goes up and its humidity goes down. This helps create the hot dry conditions of the Santa Anas,

4. Canyons and Passes

The canyons and passes surrounding the Los Angeles Basin are well suited for increasing the speed of the Santa Anas. The winds tend to be from the north-northeast and the canyons are mainly oriented in the same direction. As the air enters the canyons it is squeezed much as a stream whose banks have narrowed. As the winds are funneled they accelerate often causing two to three times the wind speed in the canyons than in the basin.

5. Coastal Zone

The coast is one of the warmest areas since the air is compressed down to sea level and it has the highest density and temperature. Smog and dust particles are blown out to sea.