NEW YORK — 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 today. That brings the tally of flights canceled because of the storm to more than 15,000.

This morning more than 500 flights scheduled for Wednesday also were canceled.

The three big New York airports were closed today 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.