Washington underwent its second day of muggy weather and record-breaking temperatures yesterday as the mercury reached 102 degrees. Late afternoon thunderstorm knocked down the temperature briefly, but caused several fires and cut off power to thousands of homes and businesses.

The high temperature yesterday was recorded at 3:23 p.m., breaking the record for the date of 99 degrees set in 1888. Today is expected to be only a little cooler and not much more comfortable, with highs in the mid-90s.

As storm clouds gathered yesterday afternoon, the temperature plunged 13 degrees in an hour, from 99 degrees at 4 p.m. to 86 at 5 p.m. At the peak, the storms, which brought between one and two inches of rain to many spots, knocked out power to more than 42,000 homes and businesses.

By late last night, about 7,500 were still without electricity, mostly in the northern part of Prince George's County.

Storm - related power failures brought the Metro subway system to a standstill for about 15 minutes at the start of evening rush hour, and delayed operations on part of the system for hours afterward, according to Metro spokesmen.

The halt and the delays, which added to the discomfort of thousands of commuters, were blamed on a loss of power in Metro's automatic control system. After the halt, trains were restarted under manual control in the affected sections, but this caused delays between Rhode Island Avenue and Silver Spring on the Red Line and between Cheverly and New Carrolton on the Orange.

Meanwhile, in Prince George's County, where the storm appeared to strike most heavily, officials reported at least seven fires caused by lightning strikes. In the most serious, a house at 4811 Lottsford Vista Rd., West Lanham, suffered $23,000 in damage. Three persons were injured in the blaze.

In 79 minutes at the storm's height, the county fire department logged 101 emergency calls reporting such storm related problems as lightning strikes, fallen electic wires, elevators immobilized between floors, and, in one case, a television set that suddenly switched on and, apparently because of a lightning-caused short-circuit, could not be switched off.

Several lightning strikes were also reported within a few square blocks in south Arlington. A fire that was ignited at 3563 S. Stafford St. spread to four other units in the Fairlington Village development, causing a total of $180,000 in damage, according to fire officials.

The triple digit temperatures that have afflicted Washington for the past two days stemmed from a high pressure system that had its origins in the heavily heat-burdened Southwest, weather forecasters said.

The high pressure system possesses a self-sustaining mechanism, a process of atmospheric circulation in which hot days make possible even hotter days, according to the experts.

"Once it is established," forecaster John Forsing of the National Weather Service said, "It is hard to get rid of."

Elsewhere nearly half the country baked in the merciless heat. With the national death toll approaching 1,000, the National Weather Service issued a warning for Texas, where temperatures have exceeded 100 degrees for nearly a month. There, livestock herds and poultry flocks were decimated and industry officials predicted food prices across the country would soon reflect the heat's devastation.

Area hospitals reported no deaths caused by the heat, but did report an increase in visits by patients complaining of nausea, headaches, weakness and respiratory ailments, typical symptoms of heat fatigue.

Though Pepco reported no problems handling the increased demand for electricity, Vepco asked its customers early yesterday to reduce their consumption after Wednesday's record demands for power caused brownouts in parts of Northern Virginia.

"We have put the message out to all our customers that it would help us if they would reduce some of their loads, but we do not have an emergency, no voltage reduction.

"We are carrying all the load that is being demanded," said operations supervisor George Brown, adding that the high customer demand had forced Vepco to rely more heavily on their least efficient and more expensive oil -- and coal-burning combustion turbines.

All over the area, residents perspired and complained for the second day in a row as they bore up under the oppressive heat.

Metrobus officials reported that several buses around the area were without air conditioning yesterday.

A Metro spokeswoman could not estimate how many buses were without this service, explaining that "in the last couple of days, we've had many breakdowns. Too many to keep track of really."

The spokeswoman said the air conditioning systems in the buses are designed to cool the air in the bus to a level only 15 degrees below temperatures outside. "When it is extremely hot, they just get overworked and break down," he said.

Officials at the Securities and Exchange Commission authorized 90 percent of 900 employes who work at the 500 North Capitol Street building to take administrative leave at 11 a.m. yesterday, after a break in the water main there left the building without air conditioning or water.

Across town, at the Regency House, a federally subsized apartment building at 5201 Connecticut Ave. NW for senior and handicapped citizens, about 160 residents suffered through their second day without air conditioning. a

"Oh my goodness," said resident Lena Bright, 78. "I've been sitting around in my underwear. It's awfully hot. It just smothers you in here."

A spokesman for the firm that manages the privately owned building said yesterday that an assessment of the air conditioning system was being made yesterday afternoon.