The furnace of the Southwest blew its sweltering heat into the Washington area yesterday, shooting the mercury up to 103 degrees and breaking a century-old record for the date.

It was the hottest day in nearly 37 years -- hot enough to break the previous record for the date of 102, set in 1879, and hot enough to fry the proverbial egg on a sidewalk. It was sunnyside up, of course, and it sizzled to well done within seconds after being cracked open in front of a building at 52 L St. NE by office workers there.

It was not the hottest day ever in Washington. That was July 20, 1930, when the thermometer registered a blistering 106, according to the National Weather Service.

A line of thunderstorms developed in the Fairfax County area and moved southeast across Washington at dusk, bringing some relief. Heavy but brief rains helped drop temperatures 20 degrees from 103 at 5 p.m. to 83 degrees at 9:30 p.m.

But National Weather Service forecaster John Forsing said the hot, humid weather will remain in the area for at least the next several days.

A spokesman for the Natinal Weather Service said the scorching weather was part of the same system that has baked the Southwest and Midwest United States for the past month. The heat there has caused some 760 heat-related deaths and threatens to damage crops extensively, which could drive up food prices.

A Vepco spokesman reported that increased demand for electricity to run air conditioners and fans led to "brownouts" -- or reduced power levels -- in south Arlington and western Alexandria for about an hour yesterday afternoon.

Pepco reported no brownouts, but a record demand for electricity at 4 p.m., when over 4,142 megawatts were consumed. The previous peak occured July 21, 1977, when 3,857 megawatts were consumed. Pepco's installed capacity is 5,000 megawatts, the spokesman said.

Montgomery County announced that 11 schools that do not have air conditioning would be closed for summer school today and probably Friday. Schools in Prince George's and Fairfax counties, and Alexandria are expected to be open as usual. District of Columbia school officials said a decision on whether to close would not be made until this morning.

Local hospitals reported seeing more respiratory ailments, heart attacks or heat prostration, but no heat-related deaths were reported here yesterday.

"Because there are no clouds . . . and the air masses are hot all the way up to 30,000 or 40,000 feet, we are having record temperatures," the spokesman said.

As thermometers climbed from 79 degrees at 6 a.m. to the record at 4 p.m., residents throughout the metropolitan area sweated and complained.

"All it needs down here is a little bit of brimstone and you can call it Hell," said construction worker David Thompson as he slapped a last brick into a staircase at 301 Seventh St. NW before breaking for lunch.

"Yeah, nobody cares about the man who's gotta work in the sun all day," Thompson said. "All these other people worry about is getting from an air-conditioned office to an air-conditioned car. Me? I got five minutes to sit out here, then I got to climb back into Hell," he said jerking a thumb toward the building where he guessed the temperature was at least 150 degrees.

But office-bound workers in buildings that were supposed to be air conditioned had their own problems. Callers from all over the city reported that they were sweltering in their chairs, dripping sweat onto paper work they were too hot to push.

Employes at the District's Department of Employment Services at 3308 14th St. NW had to close up shop at noon when the mercury in the theremometer in their office thermostat climbed out of sight. Two large fans spinning in the corner of the room did little to diffuse the heat.

"The air conditioner's been broken all summer and we can't get nobody to fix it," said one employe who asked not to be named.

At least one jury trial was moved from the U.S. District Courthouse across the street to the D.C. Superior Court when the District court's air conditioner broke down shortly before noon.

Superior court officials told federal court officials they could make up to six court rooms available, and business, after a short delay, continued as usual.

The heat problem was compounded when the Fairfax City Water Authority placed a mandatory ban on unnecessary use of water for washing cars, filling pools beyond the amount required to keep filters running, or watering shrubs. Customers failed to curtail water use under an earlier voluntary ban.

The ban originally was imposed Tuesday because of the installation of a new raw water pump, part of a $5 million water expanison project at Goose Creek.

About 75,000 water customers in Fairfax City, Herndon, Sterling Park and Sugarland Run are affected by the ban, which FCWA officials said would remain in effect in some degree until Aug. 1. Fines of $100 are provided for violations.

The heat didn't faze Richard Grubic, a 50-year-old ramp serviceman at National Airport as he flagged in an Eastern Airlines jet taxing onto the sizzling tarmac.

"You get used to the heat," said Grubic, drops of sweat-trickling down his face. "Myself, I find the cold the worst. Today there's a breeze so things aren't too bad out here."

Coworker Chuck Worrall, a deeply tanned 38-year-old, said the intense heat of the asphalt runways didn't bother him either."After work I'll go home, maybe cut the grass, stay out in the yard," he said.

Meanwhile, at a construction site on Georgia Avenue in the District, David Sivert and a crew of eight laid cinder blocks. "I wish it wasn't so hot," he said, squaring up a 25-pound block, "but we're used to it."

At the Mircle Cleaner's laundry, at 1212 13th St. NW, Tim (The Tailor) Chisley wiped his forehead with one handerchief and waved anotehr at his face, taking a break from his alterations.

If Chisley thought he had it rough, he should have been in Pepco's Potomac River Electric Plant in Alexandria. Inside the plant are five 11-story coal burners. The temperature inside them, where a steady flow of coal dust keeps huge turbines pumping electricity to Washington residents, hits 8,000 degrees.

About 125 workers are stationed inside the plant, where temperatures reach 125 degrees. Hot air rises through steel-grid floors and steel handrails burn naked hands.

"I've been doing this for 30 years," said plant manager Bob Lewis as he examined a thermometer on the boiler, sweat rolling down his cheeks and off his mustache. "People who work outside think they have it rough. But hell, the hot work's not outside it's in here."

But not everyone was bothered by the heat. Ted Beverley, of the Beverly Ice. Co., 4515 14th St. NW, said he hoped the weather stayed just like it was. "Right now we're moving one million pounds of ice a week. If it stayed like this all year I could retire in three," said Beverley, who "would like to do for ice what Perdue did for chickens."

Leading a reporter past his five 30-foot-high ice makers and into a storage room, he introduced Walter Perkins, who was wearing a heavy winter parka and gloves against the 25-degree temperatures inside.

"It's cold in here, man," said Perkins, stamping his feet. "But it's not too bad as long as I keep moving. Anyway, it's not as cold as unemployment." t