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The dog days of May?

A dome of hot, humid air parked itself 20,000 feet over the northeastern United States about three weeks ago and just hung around, making the month of May the hottest that New York City, Philadelphia, Baltimore, Washington and many other cities have experienced in meteorological memory. And while the East sweltered, temperatures dropped to record-breaking lows in Southern California and unseasonable snow fell in the Sierra Nevada Mountains.

"For the season, it's weird," Daniel Graf, a meteorologist at Pennsylvania State University in State College, Pa., said Friday.

"It is not unusual to have areas that are very hot and others that are very cold, but the oddest part is that a summer weather pattern has persisted here for three weeks in the spring," he said. "These are the kind of severe weather patterns that meteorologists live for. It's very exciting."

Coastal Maine had nine days over 80 degrees in May, breaking the previous record of seven for May.

Norfolk, Va., set a record for May with 100 degrees Friday.

Baltimore's 81-degree low Friday was higher than the average high at this time of year.

Highs headed for the 90s again Saturday.

In New York, the heat drove many homeless people from their temporary resting spots in the subway system to new resting spots on the street, said Robert E. Slovak, a Transit Authority spokesman.

The buildup of oppressive heat and humidity took its toll in schools as well, driving thousands of students and teachers from their classrooms.

Many school districts closed early Friday, observing a novelty: a heat day.

Every time it gets unseasonably warm, concerns about global warming increase, but Dr. Frederick House, professor of physics and atmospheric science at Drexel University in Philadelphia, said significant effects of atmospheric pollution on the Earth's temperature would not be seen for 10, 20 or 30 years. "It's just abnormally hot," he said Friday.

James Friend, a professor of atmospheric chemistry at Drexel, theorized that the heat wave was caused by a Bermuda high pressure system, named for the location of its center off the Carolina coast.

"What happens is the whole Northeast becomes very stagnant and allows a concentration of heat and pollutants to build up," he said. "That's why it feels so awful."

But in Los Angeles, Friday was the coldest day in May ever recorded, with 62 degrees.