updated 11:10 p.m. ET Dec. 21, 2006

Two of the world’s busiest airports, in London and Denver, were socked with bad weather on Thursday, spreading delays and cancellations to airports around the world and stranding tens of thousands of travelers during the pre-Christmas travel crunch.

As flight after flight was canceled, the situation grew into a logistical horror for fliers, whose vacations were disrupted if not spoiled, and for airlines, who may lose much-needed revenue.

Industry officials said it could take two days to untangle the knot, which is tightest in Denver, where more than two feet of snow kept the airport closed for a second day. Home to one of United Airlines biggest hub operations, it’s not expected to reopen until midday Friday. In London, the weekend forecast is for more fog — and more travel delays.

Jodie and Andy Hartfield of Colorado Springs, Colo., spent a sleepless night at the Denver airport with their three young children. Luckily they scrounged a cot and some blankets from a family that left the airport to stay in a hotel. The Hartfields decided to stick around until Christmas Eve in hopes of catching a flight to Seattle.

“We can’t go home, the highway’s closed,” Jodie Hartfield said. “We can’t get to the car, it’s 10 miles away. And the hotels are not cheap.”

Denver pharmacist Robert Helmer fumed about the delays after spending the night at the Denver airport — on the floor. He boarded a United flight for St. Louis Wednesday morning, only to sit on the runway for four hours, first because of a late-arriving flight attendant and then stuck in the snow.

“This isn’t an act of God,” Helmer said. “It was mismanagement by United.”

On Thursday, he waited angrily for a bus convoy organized by airport officials to take passengers to downtown Denver.

“A lot of people are going to lose their holidays,” said Joe Brancatelli, who runs a Web site for travelers called joesentme.com. “The smart ones may decide to just stay home.”

Inclement weather can make air travel a nightmare under the best of circumstances, and the impact is only magnified around holidays.

But what makes Thursday’s snags so daunting, travel experts said, is that airlines have tightened their belts in recent years to regain financial stability. That means there are fewer employees to help stranded passengers than in years past, and fewer empty seats to offer stranded fliers determined to reach their destinations.

“This is the worst-case scenario,” Brancatelli said.

Gummed-up service in London — where more than 700 flights have been canceled since Tuesday — reverberated across Europe, slowing travel to and from Helsinki, Vienna, Brussels, Paris and Amsterdam. The majority of the cancellations at Heathrow, Europe’s busiest airport, affected British Airways travelers.

NBC VIDEO Bad weather blues Dec. 21: "Today" show travel editor Peter Greenberg has tips if you encounter bad weather while flying this holiday season. MSNBC

Large passenger planes can land using electronics, but reduced visibility means that pilots have difficulty spotting other airplanes, thereby increasing the risk of collision. The need for extra spacing between airplanes means fewer planes can go in and out of the airport.

“It’s bedlam,” said Nicholas Velez, 23, from Washington, D.C. “The whole terminal is so packed you can barely walk.”

With Heathrow hotels so full that even service rooms were occupied, Velez was one of the 500 people who slept in the chilly terminals overnight while waiting to rebook a flight home. Heated tents, sleeping mats and catering stalls were being set up for anxious travelers.

Zubair Qamar, 33, also heading to Washington, was luckier. He was given a hotel voucher after waiting for six hours Wednesday. “I spent some of my vacation in a five-star hotel, which was not so bad,” he said. “But I would have preferred to be back.”

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