Cold, snowy January finally departed yesterday, leaving the Washington area in a fog.

Leftover snow and ice, remnants of more than 15 inches of snowfall since New Year's Day, combined with warm air and light rain to create a thick stubborn fog that hugged the ground through much of the day and made traveling hazardous.

After three weeks of below-normal temperature, the thermometer climbed to a spring-like 60 degrees at National Airport at 5:20 p.m., the highest reading this month, according to the National Weather Service.

Heavy fog shrouded the entire region. Maryland State Police reported more than 20 fog-related accidents, most of them minor, on U.S. 29 in Montgomery County.

"One of the major problems is that people can't see the intersections and are running red lights," said state police Sgt. Kenneth Frick, "It's like a bad dream." State and county police placed flares and parked police cars with their rooftop lights on to alert motorists to intersections.

The worst conditions were at the intersections of U. S. 29 and Fairland, Briggs Chaney and Randolph roads, he said.

While Virginia and other Maryland jurisdictions reported heavy fog, they said few accidents were attributable to it.

Because of the fog, National Airport was closed to incoming flights from 7 to 7:45 a. m. and again from 11:22 a.m. until 2:30 p.m., a spokesman said. National reported visibility of only three-eighths of a mile yesterday afternoon, below the half-mile minimum required to allow landings there. Takeoffs were not affected, but dozens of landings were delayed or diverted to Dulles or Baltimore-Washington International airports.

BWI, however, was closed to landings and take-offs about noon, when visibility dropped to about 1/16th of a mile. The airport did not reopen until after 3 p.m. Dulles reported heavy fog, but visibility remained higher than its one-sixth-mile minimum for operating.