HOUSTON -

Superstorm Sandy grounded more than 18,000 flights 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, almost all related to the storm. Hundreds of flights scheduled for Wednesday also were canceled.

Major carriers such as American Airlines, United and Delta canceled all flights into and out of three area airports in New York, the nation's busiest airspace. About one-quarter of all U.S. flights travel to or from New York airports each day. So cancelations there can dramatically impact 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.

Philadelphia native Gerry Dolan said he thought he and his son, Patrick, would only be in Houston for one night.

"I hoped to get in and out before Sandy hit, but it didn't work that way," Dolan said. "If I knew then what I know now, I would have rescheduled it and stayed home so that I could help out there because it's borderline disaster there."

"They canceled my flight at 3:28. I was on the 5:35 p.m flight," said traveler Marcia Blackwell on Monday. "There's a lady stuck in your airport with no ... don't you think I would have called someone if I knew some people? I'm not from here. I'm on my spring break from school. I'm looking at law schools."

A generous Houstonian heard about Blackwell's situation and offered to pay for a hotel room. She was very thankful.

"Somebody just helped me because I was just stranded in the airport," she said.

Delays rippled across the U.S., affecting travelers in cities from San Francisco to Atlanta. Others attempting to fly out of Europe and Asia also were stuck.

The flight cancelations were on par with a major winter storm in early 2011 that forced 14,000 flights to be scrapped over four days.

Hurricane Sandy converged with a cold-weather system and made landfall over 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 -- killed at least 16 people in seven states, cut power to more than 6 million homes and businesses from the Carolinas to Ohio, caused scares at two nuclear power plants and stopped the presidential campaign cold.

The canceled flights could cost the airline industry millions of dollars. Travel experts said there is no way to know if those losses will be passed along to holiday travels in higher fares.

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