Stifling heat and humidity forced the early closing yesterday of most public schools in the Washington metropolitan area for the first time in recent memory, as a hot western air mass smothered the region with steambath-like conditions that felt like 100 degrees, especially in classrooms that are not air-conditioned.

"It was unbearable to sit there and try to do some work" in most classrooms, said D.C. school spokeswoman Janis Cromer. "Even in schools with air conditioning, the air conditioners are being so taxed we are finding it difficult to keep reasonable temperatures on the second, third, and fourth floors."

"We have kept schools open before when it was this hot," Cromer said, "but when you add the humidity, it became impossible."

Fairfax and Prince George's counties closed schools two hours early because of sweltering classrooms and malfunctioning air conditioners, while Montgomery and Arlington counties and the District closed 1 1/2 hours early. Prince William County closed 12 of its 45 schools early.

Schools in Alexandria and Loudoun County remained open.

In several jurisdictions, some parents were caught off guard by mid-morning announcements of the closings. But school officials said the early exodus of students generally went smoothly, with schools calling parents and parent-teacher associations to spread the word. At some elementary schools, teachers or administrators remained with those children who were stranded until friends or family came to pick them up.

In the District, where only 64 of the 187 public schools are air conditioned, school officials were "inundated with complaints" from parents on Tuesday, when temperatures on the first day of school soared into the upper 90s, Cromer said.

Expecting yesterday's continued heat, Fairfax and Montgomery officials announced Tuesday night that schools would close early Wednesday, but the District, Prince George's, Arlington and Alexandria said they would not announce decisions until early Wednesday.

Cromer said D.C. officials delayed their decision because "the weather is unpredictable. . .We had to see if it was actually going to materialize." She said school Superintendent Floretta D. McKenzie decided at 10 a.m. to close schools after receiving weather predictions and reports of stifling hot classrooms.

Various school systems use different criteria for closings. Arlington officials close schools if classroom temperatures exceed 85 degrees for more than an hour. D.C. schools use a city government guideline in which workers are dismissed if temperatures reach 95 with 55 percent humidity.

Today, schools in several jurisdictions are closed for the Jewish New Year, Rosh Hashanah. Those schools not closing were expected to have full school days because the National Weather Service is predicting some relief today from the sweltering conditions. While temperatures were expected to climb into the low 90s, the humidity was expected to drop to tolerable levels.

Yesterday's high was 95 degrees, slightly lower than the record highs recorded at various locations Tuesday. But yesterday's relative humidity hovered at more than 40 percent through much of the day, making temperatures feel like more than 100 degrees, the weather service said.

Overworked air conditioners broke down at many schools and other offices, and the cooling system shut down at 12:30 p.m. yesterday at Children's Hospital National Medical Center because of a failure in a high-power electrical cable at the nearby Washington Hospital Center. Children's Hospital said its electrical power was unaffected, and that it was using emergency fans to cool the building, which has windows that do not open.

A spokesman for the Potomac Electric Power Co. said the D.C. juvenile receiving home on Mount Olivet Road in Northeast Washington also lost power yesterday.

The Pepco spokesman said the weather created very high demand for electricity, but power usage apparently fell short of the record-high 4,283 megawatts set on July 18.

In Prince George's, where only 35 of 170 schools are fully air conditioned, classroom temperatures were reported at more than 100 degrees, said spokesman Brian Porter. Schools open at 8 a.m. and the heat wasn't overwhelming until late in the day, he said. "We still managed to get most of the school day in."

Porter said the school system makes no special provision for stranded pupils. Many are "latch-key kids" who normally go home to an empty apartment or house anyway, he said. Others go to baby-sitters who are likely to be available all day, while very few have to wait for parents to pick them up, he said.

Although more than half of Fairfax's 159 schools are air conditioned, school officials decided Tuesday that the early closing had to be system-wide because of bus scheduling, according to spokesman George Hamel. In Arlington, school officials said there were numerous malfunctions in the cooling systems of the 13 schools with air conditioners. With maintenance workers repairing the systems yesterday, officials said closings would be decided on a school-by-school basis.

Loudoun County school Superintendent Robert E. Butt said his system did not even consider closing yesterday. "We're out in the country," he said. "The heat has been very tolerable . . .we have a different kind of breeze here."