LONDON (AP) – Relief was in sight for thousands of stranded travelers at Heathrow Airport on Friday after forecasters said the thick, freezing fog that forced flight cancellations and delays for four days should begin to lift.

Dry air from the south was expected to help remove the blanket of fog around Europe’s busiest airport by Saturday morning, forecasters said. “While it may remain gray and misty, the key factor – visibility – should improve considerably,” Meteorological Office spokesman Keith Fenwick said.

Heathrow – built on flat, grassy land and surrounded by reservoirs and canals – is particularly vulnerable to fog.

British Airways canceled 170 incoming and outgoing flights Friday, 84 of them domestic and the rest short-haul to Europe. While the airline’s long-haul services were still operational, some departing passengers were expected to face delays of several hours.

“At this stage we are hopeful that the weather will improve slightly over the weekend and therefore we can get back to operating a full planned Christmas Eve schedule,” Geoff Want, director of ground operations at British Airways, said in a statement.

The airline said it would be able to fly 95 percent of customers today, adding that it was already operating 80 percent of short haul flights. “We’re seeing a gradual improvement,” spokeswoman Amanda Poole said.

Airport operator BAA warned that an immediate resumption of full services was unlikely because the cancellations had stranded jets and cabin crews across Europe.

Hundreds of flights have been canceled since the fog rolled in Tuesday, affecting an estimated 40,000 people. On Friday, the busiest day of the holiday season, 150,000 were estimated to be passing through the airport, about 50,000 fewer travelers than might otherwise have been expected.

Heathrow’s second-busiest airline, bmi, said Friday it expected half of its scheduled flights to be canceled. No-frills airline Ryanair canceled 16 flights.

BAA said it was providing a range of amenities for stranded passengers, including heated tents outside terminals with blankets, ponchos, sleeping mats, children’s packs and food and drink. In one tent, a brass band played Christmas carols.

With Heathrow hotels full, some people slept in chilly terminals overnight while waiting to rebook a flight home.

“I knew that there was going to be trouble, but I’ve never seen anything quite like this,” said Jon Davidson, 22, a student from Austin, Texas, waiting on Friday for a flight to Amsterdam.

Bob Ostreck, 33, waiting for a flight to Paris for a second day, said he had been unable to book a train ticket. “We’re so close, I hope we don’t miss Christmas,” he said.

Virgin Trains, which provides services between London and Scotland, said it added an extra 4,800 seats due to “record-breaking demand.” Eurostar, operating trains from London to Paris and Brussels, reported a 15-percent spike in traffic and said all seats Friday had been booked.

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