Superstorm Sandy grounded more than 18,000 flights across the Northeast and 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 cancelled on Tuesday alone. Delays rippled across the U.S., affecting travellers 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.

Traffic from Asia to the East Coast was beginning to resume, with flights from Tokyo's Narita International Airport to New York and to Washington, D.C., resuming as of Wednesday morning.

From Tokyo's Haneda airport, the JAL/American Airlines flight to and from New York was cancelled.

Hong Kong's Cathay Pacific, which cancelled seven flights to Kennedy airport through Wednesday, said service would resume Thursday.

South Korean airlines Korean Air and Asiana Airlines said they will resume normal service to East Coast cities starting late Wednesday or Thursday.

Travellers stuck overseas made the best of the delays they faced.

Peter and Sheryl Knight were scheduled to fly home to Washington from Hong Kong on Wednesday, but their Cathay Pacific Airways flight via New York's Kennedy airport was cancelled. The airline put them on a flight for Chicago on Thursday, where they would board another flight home.

The delay gave them an extra day to enjoy Hong Kong, Knight said.

"We're very anxious to get back and were very much hoping that the flight would be on time, but once we learned it was delayed and once we learned our delay would be only one day, Hong Kong's not a bad place to be for another day," said Knight.

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 cancelling flights on Saturday. By Tuesday they had scrapped more than 18,000.

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