Thick fog caused the cancellation of flights at London's Heathrow Airport for a fourth day Friday, forcing thousands of frustrated passengers to scrap or delay their Christmas travel plans.

The fog was expected to continue through the weekend, causing more potential delays for passengers making connecting flights.

"When I arrived this morning I was expecting madness, I got exactly what I expected," said Jason Gourevich, 45, attempting to travel to Paris.

British Airways announced it had canceled 170 flights Friday, 84 domestic and the rest short-haul European ones.

BA said it planned to operate all its long-haul services in and out of Heathrow, but some departing passengers were expected to face hours of delays.

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Hundreds of flights at Europe's busiest airport have been canceled since the fog rolled in Tuesday, affecting an estimated 40,000 people. About 160,000 people use Heathrow on a typical day, but nearly 200,000 were expected to travel through the airport Friday because of the holidays.

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

Heathrow's second-busiest airline, bmi, had scrapped eight flights by early Friday on top of 40 cancellations Thursday. It said it expected half of all its scheduled flights to be canceled.

Airport operator BAA said it was providing stranded passengers with heated tents, blankets, ponchos, sleeping mats, children's packs and food and drink.

BA also had buses to take as many as 3,000 people north from Heathrow to cities such as Manchester, Newcastle, Glasgow, Edinburgh and Aberdeen, where flights were going ahead as normal.

British Airways said the fog was expected to continue to affect services at Heathrow, Gatwick and London City airports into Saturday night.

With Heathrow hotels so full that even service rooms were occupied, around 500 people slept in the chilly terminals overnight while waiting to rebook flights.

"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 straight day, said he had been unable to book a train seat on packed rail services between Britain and France. "We're so close, I hope we don't miss Christmas," he said.

Virgin Trains, which provides services between London and Scotland, said it was providing an extra 4,800 seats due to "record-breaking demand," and Eurostar, the operator of trains from London to Paris and Brussels, reported a 15-percent spike in traffic.

Planes can land using electronics, but reduced visibility means that pilots have difficulty spotting other airplanes — increasing the risk of collision. The need for increased spacing between planes means lowered capacity for both incoming and outgoing flights.