Arson Fire Stokes the Heat Wave : Weather: 4 are injured as firefighters battle the blaze. Forecasters predict cooler weather, slight chance for rain.

A deliberately set grass fire burned 34 acres near Irvine Lake Friday, as sweltering temperatures in Orange County reached a record 99 degrees.

More than 200 firefighters, aided by three helicopters and two planes, contained the fire within two hours to an area half a mile east of Irvine Lake along Santiago Canyon Road, said Orange County Fire Capt. Dan Young. Three of the firefighters suffered injuries and one man was critically injured when a truck he was riding in swerved to avoid slow traffic near the fire.

“We’d sure like to know who started this thing,” Young said.

The fire heated up an already sizzling afternoon, as dry air blown in from the desert shattered records in Orange County for the second consecutive day.

But moist, tropical air will push into Southern California today, stirring up scattered thunderstorms and replacing that desert heat. Forecasters say today will be a bit cooler, with some clouds and a 20% chance of rain.

Although Friday’s high of 99 degrees in El Toro set a record for the date, it seemed almost pleasant compared to the 107-degree heat experienced by Anaheim residents the previous day.

The desert-to-coast flow that spawned nearly smog-free skies and high temperatures was created by a high-pressure system that hovered Friday in two areas, along the Montana-Idaho and Colorado-Utah borders.

The offshore flow gave Southern California a breath of fresh air Friday as it swept pollutants out to sea off Orange and Los Angeles counties. Smog concentrations have been low to moderate throughout the region, and visibility has been unusually good.

“It’s a wind shift. Normally you have a southwest wind, and when you’ve got the San Gabriel and Santa Ana mountains to your east, it blows all the pollutants up against the mountains. So it just sits there,” said Rick Dittmann, a meteorologist with WeatherData Inc., which provides weather information to The Times. “But in this case, it blows (pollutants) out to sea instead. And Catalina Island deals with it.”

But that weather pattern was breaking up, and a low-pressure area off Baja California was gaining strength by mid-afternoon Friday, cooling off the region. Forecasters said it would come to shore today, perhaps bringing scattered showers. High temperatures at Orange County beaches are expected to be in the upper 70s and lower 80s, and up to the lower 90s inland.

“We’ll have some relief as we head into the weekend, but we’ll probably still see above-normal temperatures,” Dittmann said.

Rivaling the 101-degree peak in the Southern California deserts, Friday’s highs elsewhere in Orange County were 97 in Santa Ana, 86 in San Juan Capistrano and 78 in Newport Beach. Orange County’s old record for that day was 93 degrees in Orange in 1939.

“When you take air from the high deserts that is already very warm, and you bring it down over the San Bernardino Mountains and the San Gabriels, that air warms and dries as it moves downward. It compresses, like blowing air into a tire pump,” Dittmann said.

Classrooms in central Orange County with no air conditioning were so stifling on Friday that students were sent home early.

But at the beaches, lifeguards said Friday’s weather was a relief after a long summer of dreary skies. Beach-goers, however, found plenty of room to enjoy the sand and sea, since state lifeguards found attendance fairly light during much of the day.

“We don’t have much of a crowd, and not much surf either,” said Huntington State Beach lifeguard Lon Graham. “The normal surfer, wave-riding crowd is not out today. Just a few tourists.”

The Fullerton Unified School District allowed 15 of 17 schools to end the day early, while the Santa Ana Unified School District excused nine of 27 schools as well as two middle schools and two high schools.

At Remington Elementary School in Santa Ana, students and teachers endured a full day because school officials were unable to notify parents of a shortened day, since school was out of session Thursday for teacher training.

“Teachers are easing up on the assignments because everyone is grumpy and hot,” said Ann Leibovitz, principal of Remington.

Leibovitz said teachers are packing about 60 students into the school’s air-conditioned classrooms during the afternoon where they watched movies, played educational games or sang songs.

“When they were standing in line waiting for lunch, they all complained, ‘Oh, it is so hot!’ ” a playground supervisor at Remington said. “But when they begin playing, they don’t care.”

Orange County firefighters, meanwhile, are seeking the public’s help in identifying whoever set the Irvine Lake fire. The arsonist--on foot or in a car--might have been near the entrance to Irvine Lake on Santiago Canyon Road about 2 p.m.

Anyone with information should contact the Fire Department at (714) 744-0455. Callers need not identify themselves, Young, the fire captain, said.

The first flames moved up a slope on the south side of Santiago Canyon Road, Young said. When the flames reached the crest of the hill, winds gusting up to 20 miles per hour blew hot embers north, across the street causing three more fires. The winds pushed those fires across a flat grassy area, Young said.

Traffic was blocked on Santiago Road between Silverado Canyon Road on the east and Chapman Avenue on the west. There were about a dozen people within half a mile of the fire, but no evacuations were necessary, fire officials said.

Firefighters from Anaheim, Orange, the state Department of Forestry and the U.S. Forest Service also helped contain the blaze.

The fire contributed to a truck accident that ejected a man riding in the back of a fast-moving stake-bed truck about 7:15 a.m. when the vehicle encountered slow traffic near the fire, the California Highway Patrol said.

Francisco Cruz, 40, of Santa Ana was in critical condition at UCI Medical Center after being thrown from the truck, which had skidded into a ditch south of Silverado Canyon Road, CHP dispatcher Joe David said. Another passenger, Jose Lopez, 55, of Santa Ana, suffered minor injuries.

Times staff writer Robert Elston contributed to this story.