Undated (AP) _ The day before Christmas dawned foggy and gray in parts of the West, but a brief clearing permitted some holiday travelers to fly into and out of fog- bound Seattle, while in New York, hundreds of people heading to the Caribbean had their vacations shortened by overbooking.

''The backlog's not as big as you'd think,'' Pete Wise, superintendent of Seattle-Tacoma International Airport, said at mid-morning. ''I think a lot of people have been able to fly out, either from here or at Boeing Field.''

Others have been bused to Portland, Ore., and still more probably just gave up and canceled their travel plans, he said.

''The population of the airport is definitely down,'' he said.

Thousands of passengers hoping to fly in or out of Seattle have been affected since Wednesday, with no quick end to the fog in sight. Neither the Federal Aviation Administration, Port of Seattle nor individual airlines had exact figures on the number of passengers affected.

Fog cleared briefly at the airport late this morning, but instruments showed it rolling in right away again from the north, Wise said.

Only three or four flights were able to land at Seattle by midday Monday as visibility was down to a sixteenth of a mile, said tower supervisor Charles Abnet. Planes on the ground were allowed to depart, he said.

Airports in Missoula, Mont., Sacramento, Calif., and Reno, Nev., also have been socked in much of the time in recent days. Airports in Spokane, Portland, Ore., and Vancouver and Victoria, British Columbia, had fog problems too, but far fewer cancellations or passenger backups than in Seattle.

Dozens of flights were diverted to the smaller King County International Airport, better known as Boeing Field, 10 miles to the north of Sea-Tac, after fog there cleared about midday.

Sea-Tac also was reopened for a time but was closed again late Monday night to all planes except those with the most sophisticated equipment, said FAA watch supervisor Al De Rosa.

Sea-Tac normally records up to 650 takeoffs and landings daily. On Sunday, there were 307, with 55 diverted takeoffs and landings at Boeing Field, the FAA said.

The National Weather Service predicted little improvement in the situation, caused by a stagnant air mass.

''The five-day forecast looks identical to this morning,'' said Tom Ainsworth, meteorologist at the National Weather Service office in Seattle.

Travelers jammed Seattle-Tacoma's terminals, waiting for a break in the fog and a seat on a plane, said Margo Spellman of the port, which runs the airport. Many ended up boarding buses or renting cars to get out of town.

Officials had no reports of incidents caused by frayed tempers, said Ms. Spellman, but many passengers had trying tales.

Holly and Benjie Gualco of Anchorage, Alaska, said they had to wait two days to leave for Washington and then landed at Boeing Field.

''It was really scary,'' Mrs. Gualco said. ''It was like landing through whipped cream.''

At Reno, about 2,000 people were affected when the airport was closed by fog Monday, officials said. Sacramento Metropolitan Airport, which handles about 120 flights a day, has been closed for much of the past two weeks because of fog.

The fog has meant problems for incoming flights as well as outgoing ones. Some 5,000 United Airlines passengers at Los Angeles International Airport had to change plans when flights to the Northwest were canceled, said Laura Dukes, a United reservations operator.

A continent away at New York's Kennedy International Airport, 100 to 200 holiday travelers a day have been stranded over the last five days because of extensive overbookings on Air Dominicana flights, officials said.

''There were a lot of hot tempers,'' Port Authority Police Officer R. Sheehan said today. ''It is not unusual to have overbookings at this time of year, just some airlines do it a little better than others.''

There was no answer at the New York office of Air Dominicana, the national airline of the Dominican Republic.