By midafternoon yesterday, Cliff Scott had repaired 19 car radiators, as opposed to the seven or so he tackles on an ordinary day.

"Cars are overheating everywhere," said Scott, a mechanic at Charlie's Radiator Shop in Capitol Heights. "We're getting calls right and left: Do we have a towing service, can we take care of the car today, 'I'm going to Ocean City for the weekend, and I need the car.' "

It was one heck of a hot one yesterday, with a high of 100 degrees recorded at National Airport shortly before 3 p.m. -- and again at about 5 p.m., after the mercury had dipped to 99, the National Weather Service said.

Although the mercury fell short of the 104-degree record for the day set in 1926, it was the highest recorded temperature so far this summer, according to the Weather Service. The last time it was 100 degrees or above was Aug. 20, 1983, when it was 101.

The Monday overnight low was 82 degrees, the highest minimum ever recorded for that date; the old record was 77, set in 1952. Yesterday's humidity at midafternoon was only 42 percent, or slightly below normal, said forecaster Bob Oszajca of the Weather Service.

The heat spawned a violent cell of thunderstorms that swept through Prince William County last night with wind gusts up to 80 miles an hour. A storm hit the Manassas Airport around 8:30 p.m., damaging about 70 planes, 18 of which were overturned. No one was injured.

Reports that a tornado touched down could not be confirmed last night, the Weather Service said.

The hot, hazy weather -- the result of a high pressure system sitting on the southeastern states, from Texas to Pennsylvania and Iowa to Florida -- will last at least through Saturday, with expected temperatures ranging between 93 and 98 degrees, Weather Service forecasters said.

Yesterday's heat contributed to an all-time peak hourly demand for electricity at Virginia Power, where 1.6 million customers consumed 11.2 million kilowatts, eclipsing the previous hourly high of 10.8 million kilowatts set on Monday, spokesman William N. Curry said.

At Potomac Electric Power Co., which serves 600,000 customers in the District and Washington suburbs in Maryland, a record high of roughly 5 million kilowatts surpassed the previous record set on Monday of 4.8 million kilowatts, spokesman Tom Welle said.

Both power companies said that despite tight supplies, they had adequate generating capacity to handle demand, barring any unforeseen equipment breakdowns.

At the Manassas Airport, where last night's violent storm damaged about 70 planes, observers believed a tornado had struck. "We have employes of the airport that claim they saw a funnel cloud that touched down," leaving a swath of destruction a half-mile long and 1,500 feet wide, said Officer Marc Woolverton, a Manassas City police spokesman.

"One hangar door was sucked out," he said, providing a "good indication that it was a tornado." But, he said, confirmation would have to await the arrival of Weather Service investigators today.

Prince William fire officials estimated that 500 gallons of aviation fuel spilled from ruptured tanks aboard the damaged aircraft. Sandbags and booms were being used to stem the flow of fuel into a nearby creek.

Woolverton said 350 aircraft are permanently housed at the general aviation field owned by the city of Manassas.

Medical experts warned yesterday that the elderly and those with cardiac or respiratory troubles should take it easy in the heat. Spokesmen for several area hospitals, including Greater Southeast Community Hospital and Fairfax Hospital, said they had noticed no surge of heat-related maladies.

But at the 7th District police station in Washington, half of the detectives, clerks and administrative personnel were sent home early after the air-conditioning system broke down, raising the temperature in some rooms to 104 degrees, said David Israel, a chief steward at the station.

Even after the air-conditioning system was repaired, the officers remained skeptical. "We don't expect it to be on too long," Israel said. "The last time it was on, it lasted 30 minutes."

He said the station was so hot Monday night that a woman prisoner fainted and it took 20 minutes to revive her.

At the Alexandria Animal Hospital, veterinarian Richard Hawe cautioned pet owners about keeping animals in enclosed cars, where interior temperatures can heat quickly to 120 degrees or more. He also suggested that animals remain indoors.

Ken Margolius, a landscape designer at the American Landscaping Inc. & Nursery of Silver Spring, suggested that gardeners keep lawns watered well enough to remain moist up to two inches underground.

Ah, Washington in the summer.

About 70 people lined up to await the 1 p.m. opening of the District's public pool at 2500 Georgia Ave. NW, said pool manager Sam Rue. An estimated 11,000 to 13,000 people visit D.C. pools each day.

In suburban Occoquan, where 180 powerboats are kept at the Prince William Marine Sales piers, owner Carlton Phillips reported heavy business for a weekday. "They're just coming in, getting on their boats and going for a ride."

"You gotta drink plenty of water in this heat," said Stacy McCall, who spent part of yesterday laying an underground gas line on 15th Street NW in the District. "You can't eat too much, neither. You'll get cramps if you eat too much in this weather."

In Arlington, vendor Joe Balsamo sold more sodas than usual, but fewer hot dogs. "Seventy-five-degree weather is optimum for selling hot dogs," he said.

As of midmorning, the District's Beverley Ice Co. had already shipped out a quarter-million pounds, or double the normal rate, said owner Ted Beverley. He has had to turn down the equivalent of 40 tractor-trailer loads of ice orders, and is importing ice from as far as Atlanta and Cincinnati.

"I had one lady call up and say, 'God bless you,' " said Mike Harding, a service manager for the District's John G. Webster Co., which repairs and installs air conditioners.

But at Oxon Hill Travel, no customers were asking to be booked for summer vacations to Alaska.

"As a matter of fact," said owner Diane Kramer, "our bookings for the Caribbean are up, which is really fascinating, because if it's hot here, it's hotter there."

Staff writers Peter Pae, Elizabeth Lazarus and Douglas Stevenson contributed to this report.