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Superstorm Sandy has grounded more than 18,000 flights this week across the Northeast and the globe, and it could be days before some passengers can get where they're going.

According to the flight-tracking service FlightAware, more than 7,000 flights were canceled on Tuesday alone. Delays rippled across the U.S., affecting travelers in cities from San Francisco to Salt Lake City to Atlanta. Some passengers attempting to fly out of Europe and Asia also were stuck. By Tuesday morning, more than 500 flights scheduled for Wednesday also were canceled.

The three big New York-area airports were closed on Tuesday by the Port Authority of New York and New Jersey.

New York has the nation's busiest airspace, with about one-quarter of all U.S. flights traveling to or from there each day. So cancelations here can dramatically impact travel in other cities.

Delta Air Lines Inc., the dominant carrier at Salt Lake City International Airport, has canceled close to 3,000 flights from Sunday night through Wednesday. Delta said it will resume limited flying to New York's LaGuardia Airport and Newark Liberty International Airport in northern New Jersey for a Thursday morning restart.

It will restart limited flying at Kennedy on Wednesday afternoon. Flying has resumed at other East Coast airports.

How soon Salt Lake city passengers may catch a flight to Kennedy or Newark will depend on how quickly the airlines re-establish service elsewhere.

Travelers are urged to contact the airlines directly, not the airport, about flight status.

Delta passengers can rebook without penalties as long as they are ticketed and rescheduled for travel on or before Sunday.

Passengers whose flights are canceled or significantly delayed at entitled to a refund from Delta. To rebook or ask for a refund, call 800-221-1212.

JetBlue is continuing to waive change and cancel fees, as well as fare differences, to Kennedy — the airport that the carrier serves from Salt Lake City.

Passengers scheduled to travel Wednesday can rebook without penalty through Nov. 14. Call 800-538-2583.

Anyone who wants to cancel their travel plans can seek a refund at the same number.

Hurricane Sandy converged with a cold-weather system and slammed into New Jersey on Monday evening with 80 mph winds. The monstrous hybrid of rain and high wind — and even snow in some mountainous inland areas — has killed more than three dozen people in the U.S.

Airlines anticipated the storm's impact and began canceling flights on Saturday. By Tuesday they had scrapped more than 18,000.

In years past, airlines would have operated many of those flights — and left airplanes and crews stranded in the wrong cities when a blizzard or thunderstorm shut things down.

But airlines have gotten aggressive about canceling because it makes restarting flights easier.

"It's kind of like dominoes — when one aircraft is out of place, it means the flight crew is out of place, and that has a ripple effect throughout the rest of the day," said Lance Sherry, who runs the Center for Air Transportation Systems Research at George Mason University.

The number of cancellations from Sandy was roughly on par with other major storms that airlines deal with.

A major winter storm in early 2011 caused 14,000 cancellations over four days.

Even if storm damage is minor it could be a week before operations are normal at major East Coast airports, said Angela Gittens, director general of the Airports Council International, a trade group for airports worldwide.