Updated

Hurricane Sandy continues to be a nightmare for travelers across the U.S., and as far flung as Europe and Asia.

According to FlightAware, there are over 15,773 flight cancellations so far as of 8:50 a.m. ET, while competitor FlightStats tallied 15,508 in an update at 7:08AM ET.

Most New York City area airports, including John F. Kennedy, LaGuardia and Newark, are closed with no known re-open time. Some airlines have also begun canceling flights on Wednesday.

In Washington, D.C., Dulles International, Reagan National Airport and Baltimore-Washington International Airport are all open but experiencing flight delays.  Philadelphia International Airport and Boston's Logan International Airport are open but they too are experiencing delays.

About one-quarter of all U.S. flights travel to or from New York airports each day. Cancellations there are having a ripple effect in other cities,  from Atlanta to San Francisco and beyond.

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    London's Heathrow Airport has cancelled a total of 84 flights, including British Airways, United and Virgin flights, since the storm took hold on Monday. Narita, the international airport near Tokyo, canceled 11 flights Tuesday — nine to the New York area and two to Washington, D.C. Travelers overseas could wait days to get to the East Coast of the U.S.

    So what should you do if you have a ticket to fly?

    First, ticket holders unable to fly are all eligible for a refund.

    Those who don't opt for a refund will be looking for an empty seat on an available flight. Most airlines are allowing affected customers to make one ticket change without the usual change fees. Those who asked first to be rebooked during calmer weather will have gotten first dibs at seats. But as the weather begins to clear and flights start to resume their schedules, more seat availability will appear.

    Airlines and airports are advising passengers not to go the airport to rebook a flight.  Instead, individual airlines should be contacted directly. Customers should first try changing their tickets online. Passenger are reporting hour-long wait times at airline call centers. (TollFreeAirline.com offers toll-free numbers for most major airlines.)

    Also, try posting a Twitter update about your rebooking needs, with a mention of the airline you're using.  This sometimes prompts a quicker response from customer service than calling by phone. If you try it, just make sure you get a reservation number. Also, sign up for alerts from your carrier and follow them on Twitter.  Most airlines also have apps that will allow you to get the latest information on your flight.

    "If you absolutely have to get to the business meeting or funeral or whatever, and your flight hasn't been cancelled yet, then head for the airport as early as possible on your day of travel," says George Hobica, founder of Airfarewatchdog.com.

    Also to help cut down on the volume, airlines are asking that for those travelers whose flights are cancelled and who no longer need to travel wait a few days before calling and apply for a refund.

    Other things to keep in mind.  Often airlines will not cover the cost of hotel stays for cases of weather-related delays, but some may offer food and beverages, or meal vouchers.

    If you have to stay overnight, some airlines are arranging for hotels may be able to give you a distressed passenger rate.  In addition, hotels such as the Quality Inn, are also offering distressed rates for the next few nights for passengers waiting for their return flights to be rescheduled.