Dense fog and icy pavement confronted commuters in much of the Washington area yesterday morning, sending cars skidding into one another and forcing the partial closing of three Potomac River bridges and National Airport. Police reported confusion and delays but no major injuries.

Vast banks of fog settled over the city in the early morning hours yesterday. Coursing slowly through streets and alleys after sunrise, the fog brought damp and gloomy semi-darkness before lifting.

Most common in January and February, the fog was a sight of beauty to those on foot. But, combined with ice left by freezing overnight temperatures, it was a menace to motorized travel.

Eleven separate accidents involving 18 vehicles occurred in one hour on the 14th Street bridges, beginning at 7:10 a.m. "You couldn't see a good 50 feet," Officer Frank Strother of the D.C. traffic division said.

Car-pool express lanes in the middle of the three bridges that cross the Potomac at 14th Street were closed at 7:25 a.m. because of ice. Trucks spread sand on the bridges and the lanes reopened just after 9 a.m.

Downstream at the Woodrow Wilson Bridge, a 5:30 a.m., three-car pileup in thick fog on the icy span led police to close lanes crossing into Maryland. After the cars were cleared and the road surface salted, the lanes opened again by 6:30 a.m., police said, although weather conditions continued to slow traffic.

Fog and ice also were blamed for a 20-vehicle collision on Interstate 95 in Baltimore yesterday morning. Wire reports said there were no serious injuries in the 7:45 a.m. accident and traffic was back to normal by 9:15 a.m.

At National Airport, flight operations normally begin at 7 a.m.. But yesterday's fog meant nothing until 11 a.m.

About 80 airplanes normally land between 7 a.m. and 11. a.m. at National. Yesterday, many were held on the ground at their airports of origin, while 12 that arrived in the fog were diverted to Dulles International Airport.

A few planes departed on schedule at National, despite the fog. Takeoff at National is allowed with lower visibility than that required for landing. Yesterday morning, the fog occasionally lifted slightly, letting a few planes leave, then closed in again, forcing a suspension in takeoffs, airport officials said.

USAir diverted seven inbound flights yesterday. Three of the planes went to Dulles. Passengers were bused to National. The other four planes went on to their next stops, according to airline spokesman David Shipley. Seven departures were canceled, although two flights originating in Washington left with only minor delays, Shipley said.

Baltimore-Washington International Airport also was fog-bound for much of the morning, forcing some planes to land elsewhere.

Much of the East Coast was hit by rain and freezing temperatures Sunday. Washington's fog, according to the National Weather Service, was caused by airborne moisture settling toward the ground, then turning to clouds--fog is simply a cloud at low altitude--near the ground.

Winds remained slow through the night, meaning the fog was not blown away as it formed. Then, as temperatures rose yesterday morning, winds began to pick up and the mists dispersed. The weather service said Washington experiences similar fog about eight times year.