Washington suffered and sweated yesterday through the third stifling day of 90-degree or above temperatures. As the mercury reached 96, work turned into torment and forecasters could promise only more of the same.

"I've got a good idea what hell is like," said John F. Leino, a lawyer with an Alexandria firm where the air conditioning was out of order for much of the day.

With an oppressive high pressure system clamped over much of the Atlantic Coast, humidity here was high, nightfall brought two thunderstorms but little relief and area residents searched for new ways to spell misery.

"It's hard to describe," said Melody L. Palmer, a mail carrier in Alexandria, describing a day spent delivering 35 pounds of mail through what seemed like an open-air sauna. "You had to be out here."

Although a July heat wave is as much a part of Washington tradition as a White House tour, this week's weather took at least one visitor by surprise.

"I had heard that it was kind of hot and humid here," said Amy L. Lewis of San Jose, "but I was still shocked when I got off the plane."

"Can you move the capital somewhere else?" asked C.J. Daniel Loney of Cornwall, Ontario, his stiff upper lip showing signs of melting in the shimmering haze of the Mall.

"The city is interesting," he said, "but the heat is making the sights unattractive."

For Herman J. Berkhout, a tourist from Amsterdam, the blast-furnace ferocity of the day taught a lesson.

"I've already learned that life without air conditioning here isn't worth living."

Thousands of people across the Washington area apparently agree, judging from figures provided by electric utility companies.

At 5 p.m. Wednesday, a day on which the mercury reached 97, the Potomac Electric Power Co. generated a record 4,874 megawatts of power, a spokesman said.

Figures for yesterday were not immediately available.

At one point yesterday, Virginia Power pumped 11,230 megawatts to its 1.6 million customers, coming close to breaking a record set just the day before, according to company spokesman Alan Scaggs.

To defend themselves against such ancient enemies as July, Washington area residents have armed themselves with 736,200 central air-conditioning systems and 257,100 window units, according to census data for 1981, the most recent year for which figures are available.

According to the data, about 180,000 homes lacked air conditioning.

The demand for air-conditioner servicing was rising with the temperature yesterday. George McKenzie, sales manager for Climate Heating and Cooling of Washington, said his company is receiving 60 to 80 service calls a day, double the usual number.

"It is making us extremely busy," he said. But, he added, "we've been waiting for this."

At Metropolitan Heating and Air Conditioning, Jim Payne said his crew has been working 60 to 80 hours a week, trying to do two weeks work in one.

Heat or lightning may have contributed to a cable problem that cut off electricity to the Tyson's Corner shopping center for about two hours starting around 9:30 p.m., a Virginia Power spokesman said.

Wednesday night and yesterday morning presented a particular challenge to anyone trying to recover from the rigors of 90-degree days. The overnight temperature, which usually can be counted on to dip into the 70s on the hottest days, fell no lower than 80.

"Last night was a scorcher," said National Weather Service forecaster Jim Kemper.

He said today would be much like yesterday, with high temperatures in the mid to upper 90s. The record for July 9 is 104, set in 1936.

An extension agent in Roanoke County, Va., said agriculture has not been harmed, noting that rain this month has protected crops.

"As long as we're in the lower 90s, I think we'll be all right," said Lowell Gobble.

For some the heat held few terrors. It "is good for my arthritis," said Allen L. Boyd of Alexandria. "You get used to it," said Milton H. Chichester, a paralegal worker.

"It's not like anyone said it was going to be 75 degrees and mild all summer long," said roofing firm owner David Brummer yesterday in Fairfax City. "Every year you get the same thing, so you're warned . . . ."

Usually, Clayton Harrison, a recreation director in Fairfax County, leads about 75 children in outdoor play. Yesterday, he said, "we just reverted to 'plan B' and played inside."