Encountering occasional air, rail and road delays, and one of the longest afternoon rush hours ever, thousands of travelers swarmed out of Washington yesterday as the fog-draped city braced for its coldest Christmas in more than 100 years.

Although the mercury rose slightly last night to near the 40-degree mark, weather forecasters predicted confidently that before sunrise today readings would plunge to a frigid 12 degrees. That would be the lowest Dec. 25 temperature recorded since 1872, when the reading was five degrees.

The cold front knifing down from the northwest will be accompanied by icy winds of up to 20-to-30 miles an hour, which are expected to provide the numbing sensation that on a windless day is offered only by temperatures of 10 degrees below zero, or worse.

With fog and freezing drizzle in the air and a film of ice on the ground in many places for much of the day yesterday, the evening rush hour appeared to begin almost as soon as the morning rush was over.

Long lines of slow-moving automobiles, headlights beaming in the gray murk, choked the approaches to the Potomac River bridges from 2 p.m. to almost 9 p.m. according to D.C. police. Although slow, traffic was smooth and police reported few accidents.

Many of the autos were headed to National Airport, where they found 40-to-50 percent of United Airlines flights from the North and Northeast experienced delays, some of as much as two hours, according to an airline spokesman. A Trailways bus dispatcher also reported delays of under an hour in arrivals here from northern areas.