Thick fog caused the cancellation of flights at London's Heathrow Airport for a fourth successive day Friday, forcing thousands of frustrated passengers to scrap or delay their Christmas travel plans. British Airways announced it was canceling 170 incoming or outgoing flights on Friday, 84 of them domestic and the rest short-haul European ones. BA said it was to operate all its long-haul services in and out of Heathrow, but some departing passengers were expected to face delays of several hours. Hundreds of flights have been canceled since the fog rolled in Tuesday, affecting an estimated 40,000 people. About 160,000 people transit through Heathrow on a typical day, but nearly 200,000 are expected to travel through the airport Friday. Heathrow's second-busiest airline, bmi, had scrapped eight flights by early Friday morning on top of 40 cancellations Thursday. It said it expected half of all its scheduled flights to be canceled. The chaos was expected to continue at other British airports, with Gatwick, Norwich, Southampton and Coventry among those predicting further delays. However, BA said Friday morning that it had only canceled one of its Gatwick flights scheduled that day. Airport operator BAA said it was providing a range of amenities for stranded passengers, including heated marquees outside terminals at Heathrow, with blankets and 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. At Heathrow the fog was expected to continue through the weekend, causing more potential delays for passengers making connecting flights. 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 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. "When I arrived this morning I was expecting madness, I got exactly what I expected," said Jason Gourevich, 45, attempting to travel to Paris. Bob Ostreck, 33, waiting for a flight to Paris for a second straight day said he had also 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. With international flights receiving priority and capacity at the airport reduced by nearly half, many passengers' holiday plans were thrown into doubt. 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. Visibility on Thursday had reached a low of 115 meters (377 feet), well below the 1,000 meters (3,280 feet) generally considered disruptive for flights. Heathrow _ built on flat, grassy land and surrounded by reservoirs and canals _ is particularly vulnerable to fog. Long, cool nights and calm winds have led forecasters to warn that the fog could linger into the weekend.