Morrisville, N.C. — Superstorm Sandy grounded more than 15,000 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 6,000 flights were canceled on Tuesday. That brings the tally of flights canceled because of the storm to more than 15,000. By early Tuesday morning, more than 500 flights scheduled for Wednesday also were canceled.

At Raleigh-Durham International Airport, about 40 flights to and from the Northeast were canceled between 5:30 a.m. and noon on Tuesday.

But, airport spokeswoman Mindy Hamlin said, there is a slight improvement over Monday, when 150 flights to and from the Northeast were canceled.

"Some of the airlines are going to initiate limited schedules today, particularly to Boston, Philadelphia and the DC area," she said. "It will take some time for the airlines to get all of the aircraft back into the schedule and completely back on track."

The best advice she could give passengers is to check with their airlines before going to the airport.

Up north, the three big New York airports were closed on Tuesday by the Port Authority of New York and New Jersey. Stewart International Airport remained open, but airlines had suspended operations there.

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.

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.

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 flight cancelations surpassed those of a major winter storm in early 2011 that forced 14,000 flights to be scrapped 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.

"The storm has such a wide swath and so many major airports are involved that it's going to take some time (to recover) because those airplanes are so far away," said Gittens, who served as aviation director at Miami International Airport Dade during several hurricanes from 2001 to 2004.

JetBlue Airways Corp. canceled 1,200 flights for Sunday through Tuesday. Delta Air Lines Inc. has canceled 2,100 flights over the three days. American Airlines has scrapped 1,000 flights, including 260 on regional affiliate American Eagle.

The impact on airline's bottom lines is unclear. Many of the customers on flights currently being canceled will reschedule later on, so the airlines will still collect the fares.