People played golf and tennis; they dined alfresco, wore shorts and drove about in tank tops. It was January, but their behavior was not unusual--the weather was unusual, as the mercury soared into the upper 60s across the Washington area yesterday, setting a record at Dulles International Airport.

The high for the day at Dulles was 67 degrees, eclipsing the old record by 3 degrees. The high at Reagan National Airport was 68 degrees, 3 degrees short of the record for Washington, which was set 124 years ago. The high at Baltimore-Washington International Airport was also 68.

It was the second day of January, frequently a time for donning insulated attire, not tossing it aside, a time for shivering and not for sweating, but despite nature's aberrations, people knew just what to do.

"We were jampacked all day," said Bob Green, one of the facility managers at the East Potomac Park golf course on Hains Point. He said 393 rounds were played, about four times as many as on a more typical January day, with its temperatures in the 30s or 40s.

Many of yesterday's rounds were played by people who, Green said, were clearly "excited about a day like this in January." Some of them, he said, went around the course in no more than short-sleeved shirts and shorts.

Which would have been no surprise to Quentin Butts, a cashier at the Telegraph Road Amoco filling station in Alexandria, who said his customers included "women in tank tops," driving up with windows rolled down, the better to enjoy the balmy breezes.

Lizette Carillo saw the tennis players at Woodmont Country Club in Rockville "running around in shorts" in the bright sunshine of a "beautiful day." It was a day in which Reon Maxwell, a receptionist at the Phillips seafood restaurant on Washington's Southwest waterfront, fielded "a lot of requests for eating outside."

Not necessarily a January request, but aside from the assertions of the calendar and its clockwork consistency, this was not necessarily a January day.

"It was stupendous today," said Douglas Douglas, of Accokeek, who followed church services by shooting "a little basketball" in shorts and a T-shirt.

Someone on a nature hike in Rock Creek Park observed with good meteorologic reason that it was "like the first day of spring," reported U.S. Park Service Ranger Aly Baltrus. She said there was a high turnout for a 1 1/2-mile hike during which not only humans but also many animals--woodpeckers, squirrels and raccoons among them--appeared to be "out and about."

Explanations exist for all this. Now lodged off the East Coast is the same sort of high-pressure system that pumps hot air into the area during the summer, said meteorologist Dewey Walston, of the National Weather Service. "Warm air from well down in the Gulf of Mexico" has been streaming into the area on the wings of southerly breezes that gusted up to 25 mph yesterday, he said.

These conditions probably will persist for at least another day, he said, and today may be even warmer than yesterday, with high temperatures expected to near 70.

CAPTION: Esperanza Flores watches her son Merlyn, 6, practice soccer kicks at Meridian Hill Park in Northwest Washington.