By JOSHUA FREED AND SAMANTHA BOMKAMP

AP Airlines Writers

Superstorm Sandy grounded more than 18,000 flights across the Northeast and around the globe, and it will take days before travel gets back to normal.

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 Atlanta. Some passengers attempting to fly out of Europe and Asia also were stuck.

Authorities closed the three big New York airports because of the storm. New York has the nation's busiest airspace, so cancellations there can dramatically affect travel in other cities.

It was possible that John F. Kennedy airport would re-open for flights on Wednesday, according to the Port Authority of New York and New Jersey. It wasn't known when the LaGuardia and Newark, N.J. airports would reopen.

Flying began to resume at other airports. Delta restarted flying from Boston and Washington Dulles and Reagan on Tuesday. Airline spokesman Morgan Durrant said it would resume domestic flights from JFK on Wednesday.

Service was slowly returning to Philadelphia International Airport on Tuesday afternoon.

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.

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.