It was the hottest day of the year here yesterday and the wettest in three weeks as sultry heat and oppressive humidity combined to create the thunder, lightning and scattered disruption of squalling thunderstorms.

In early afternoon, the temperature soared to 95 degrees at National Airport, tying the record for the date set in 1938. Pedestrians mopped their brows, Montgomery County schools closed early and the Potomac Electric Power Co. asked big customers to curtail power use.

Then came the thunderstorms, part of an hours-long display of atmospheric fireworks that stretched over the entire corridor between New York and Washington and threw airline schedules into near chaos.

In the Washington area the storms knocked out power to the tote board at the Laurel Race Course, to the approach control radar at Baltimore-Washington International Airport and to nearly 10,000 homes in the Maryland suburbs.

The .33 of an inch of rain the storm brought to the official measuring station at National Airport is, by many measures, not much. But by the parched standards of this drought-stricken spring it was a lot.

Yesterday was only the second time any measurable rain fell at the airport this month. The first time was Sunday, when only .06 of an inch fell.

The last rain before that was .57 of an inch recorded May 20 and 21. That was the most since April 15, when the figure was .79 of an inch.

The effects of yesterday's storms were scattered.

Northern portions of the metropolitan area received more rain than southern sections. Dulles International Airport measured more than half an inch by 8 p.m., while a fire department dispatcher in Alexandria said, "We didn't get a drop."

Thunderstorms between New York and Washington delayed airline arrivals in both cities by as much as three to four hours, a Federal Aviation Administration controller reported. It was "pretty wild tonight," the controller said.

The approach control radar at BWI airport was struck by lightning about 4:10 p.m., and repairs were still under way hours later, an airport spokeswoman said.

However, the FAA controller said that was not a major cause of the delays.

He said approach controllers from BWI went to National Airport and used a radar scope there to guide planes into BWI.

The first of the thunderstorms in the Washington area knocked out power around 3 p.m. to about 8,000 homes in Montgomery County, according to a Pepco spokeswoman.

She said the Montgomery Village area and Gaithersburg were most severely affected.

In the evening, other storms cut off power to about 1,800 homes in Prince George's, mainly in the Seat Pleasant and Chapel Oaks areas, another Pepco spokesman reported.

At the Laurel Race Course, lightning knocked out the electricity supply to the tote board, which shows betting information. The last five races of the day were canceled.

A spokesman for Virginia Power said storm-caused disruption in Northern Virginia was minimal.

He said the storm had "blown a few fuses here and there."

Earlier, concerns about the heat and humidity prompted the closing of Montgomery County's 153 schools an hour and 15 minutes early.

Exams are under way, a school system spokeswoman said, and "we wanted to let the schools get as far through exams as they could and still get the children out before the highest heat of the day."

Washington's highest heat of the day was one degree less than Baltimore's 96, which equaled a record set there in 1914.

The almost total absence of rain here this spring despite the frequent presence of both heat and humidity has been one of the season's major frustrations.

"You have to have a triggering mechanism," said National Weather Service specialist Joe Cefaratti.

Yesterday's 95, two degrees higher than the year's previous high, reached Sunday, may have provided that extra volatility that touched off the first storms of the day, he suggested.

He said subsequent storms may have resulted from the arrival of the leading edge of a cool front, which is expected to usher in more comfortable conditions today.