Airlines canceled thousands of flights on Tuesday, erasing much of their service in the middle of the U.S. as ice coated runways in Dallas and Chicago braced for a major snowfall.

Flight tracking service FlightAware logged more than 6,000 cancellations on Tuesday and more than 3,000 for Wednesday.

American Airlines operations took a one-two punch, with storms hitting its hubs in Dallas and Chicago. Ice closed Dallas-Fort Worth International for more than two hours in the morning. Even after it reopened, few American flights took off, in part because high winds made it too dangerous to send de-icing crews up in bucket trucks, said American spokesman Tim Smith.

American canceled some 1,900 flights on Tuesday, more than half of its schedule, Smith said. Some flights were diverted to San Antonio or Austin because of the ice. Roughly three-quarters of all American flights at Dallas were canceled, he said.

Blizzard conditions were expected later in the afternoon in Chicago, another American hub.

United Continental Holdings Inc. canceled 1,450 flights, including 850 by United Airlines, many in Chicago. United and its feeder carriers would usually fly 540 departures a day out of O’Hare, but roughly 300 of those were canceled, spokeswoman Megan McCarthy said.

The company’s Continental Airlines unit canceled about 600 flights, including many at its hub in Newark, N.J.

“We expect to have very limited operations at Newark and O’Hare,” she said.

JetBlue canceled flights in and out of New York’s JFK airport after Tuesday morning, with some flights to resume Wednesday afternoon. It planned to shut down its Boston flying Tuesday night through Thursday morning.

Delta Air Lines Inc. canceled more than 1,350 flights on Tuesday. It has already canceled 800 Wednesday flights.

More than half the flights scheduled to leave Milwaukee past 2:30 p.m. were canceled earlier in the morning. That included Angela Margolit’s flight to Newark.

The 57-year-old owner of a software-development company booked a flight to Philadelphia instead, and estimated there was a 50-50 chance it would leave on time.

“It’s no big deal. If you’re a seasoned traveler you know the drill, especially if you come to Wisconsin a lot like I do,” Margolit said as she relaxed with a novel. “Stuff happens, you deal with it.”

The airlines now take a simple approach to bad weather: cancel flights early, if possible. In last week’s Northeast storm, most of the 3,500 cancellations happened before the first snowflake fell.

Cancellations aren’t as expensive for airlines as one might think. It would cost United Airlines roughly $11,000 to fly an Airbus A320 from New York to Chicago, based on costs disclosed in its public filings. The same flight on the same plane would cost JetBlue roughly $8,800.

Canceling a flight eliminates the fuel cost. And many passengers still fly, just on a later flight, so the airline collects its fare, U.S. Airways Chairman and CEO Doug Parker said on a conference call last week. That’s why the storms in late 2010 cost his airline just a few million dollars, he said.

Canceling before the storm hits also prevents an aircraft from being stranded at a socked-in airport when it could be flying passengers in a part of the country with better weather. And pilots and flight attendants are under strict rules that limit their work days. The clock starts when they get to the airport, whether or not planes are cleared for takeoff. A crew on a delayed flight could “time out” and go home before the plane gets off the ground.

Airline consultant Robert Mann said Continental and Delta don’t have to pay for crew time or passenger rebookings if they tell one of their feeder carriers to cancel a flight.

“Half of all cancellations don’t cost big airlines a dime,” Mann said.

In big storms, airlines often waive the fees usually associated with changing a booking. They tell passengers to check their flight status before coming to the airport. They’re not required to pay for hotel rooms, food or other expenses for passengers stuck overnight. Travel advisors urge travelers to book a hotel room if they think they’re likely to be stranded.

In late December a blizzard in the Northeast caused more than 10,000 flight cancellations. Hordes of holiday travelers were stranded in airports. In some cases it took days to get them on a new flight. Passengers fared better in recent storms because there’s less traffic in January and February, so it’s easier to rebook.

Associated Press Writer Dinesh Ramde in Milwaukee contributed to this report.