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 canceled today 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.

John F. Kennedy International Airport in New York and Newark International Airport in New Jersey will reopen at 7 a.m. Wednesday with limited service, but LaGuardia Airport will stay closed, officials said.

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.

In years past, airlines would have operated many of those flights — and left airplanes and crews stranded in the wrong cities when a blizzard or thunderstorm shut things down.

But airlines have gotten aggressive about canceling because it makes restarting flights easier.

“It’s kind of like dominoes — when one aircraft is out of place, it means the flight crew is out of place, and that has a ripple effect throughout the rest of the day,” said Lance Sherry, who runs the Center for Air Transportation Systems Research at George Mason University.

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.

Airlines face a large task in getting things back to normal. Workers had to clear garbage and downed tree limbs from runways at JFK. Water was on the runway at LaGuardia, according to a letter from United CEO Jeff Smisek to workers. At one point, some airlines hoped to restart some New York flights by late Tuesday, but that idea went out the window right along with the travel plans of their passengers.

Flooded roads and closed subways will keep some workers from the airport. Reservations workers at other airports and at call centers are busy dealing with stranded passengers.

Some travelers hunkered down and waited, while others looked for a new way home.

Orbitz said car rates jumped 14 percent in New York from last week. Rates jumped even higher in Boston and Washington, including a 50 percent spike in Philadelphia.

Orbitz said hotel room rates rose 55 percent in Newark, where cancellations accelerated earlier than other New York-area airports. They rose 9 percent in one week in Washington, but fell 8 percent in Boston and New York City.

Some travelers figured they could do better the further away they got from the coast.

Wedding photographer Josh Saran was in Washington D.C. to shoot a Saturday wedding. His Southwest flight home to Seattle was canceled, so he rented a car and headed toward Columbus, Ohio. When snow closed the highway, he turned his rented Chevy Aveo toward Pittsburgh to catch a US Airways flight.

“I have a really loving and smart girlfriend in Seattle that sits in front of a computer and calls the airlines and sees where I can go,” he said.

Airline reservations systems are so complex that one department might cancel a flight even while a reservations worker is trying to shift a traveler onto that same flight, said Joe Brancatelli, a travel expert who runs a newsletter for business travelers.

Travelers don’t have any choice but to be patient, he said.

“Where are they gonna go? They hate United today, they go to Delta next week,” he said. “Delta screws them, they go to American, and then it’s a big circle.”