NEW YORK, Feb 23 — Fog reduced visibility at New York’s airports to zero today, blocking many airline flights from reaching the area and contributing to about 7,000 delays across the US

Planes to New York’s LaGuardia and John F. Kennedy airports and New Jersey’s Newark Liberty were held more than two hours in their originating cities, according to the US Federal Aviation Administration’s website. Thunderstorms slowed Newark takeoffs more than two hours and snarled Philadelphia air traffic.

Today’s rain-related delays were a surprise in the US Northeast, where snow from a succession of winter storms has been the chief culprit in tens of thousands of flight cancellations this year. New York’s airspace is the nation’s busiest, with each of the three major US airlines running at least one hub in the region.

“The weather is taking its toll on our operation again today, causing widespread delays across the system,” said Michelle Agnew, a spokeswoman for Southwest Airlines Co. “We’re ready for summer.”

Southwest, the biggest discount carrier, had the most delays, with 1,312, FlightAware reported. In addition to the disruptions in New York and Philadelphia, winds closed two of four runways at Denver International, Agnew said.

United Continental Holdings Inc., with a hub at Newark, had 900 delayed flights, according to FlightAware, while Delta Air Lines Inc., which has hubs at LaGuardia and Kennedy, had 667. American Airlines Group Inc.’s American and US Airways units had 765 delays.

Visibility was at zero miles at about 4 p.m. at all three of New York’s major airports, according to FlightStats, another industry data provider.

More than 1,300 US flights were cancelled, FlightAware reported. Today’s tie-ups built on delays totalling more than 9,460 yesterday and February 19, and on winter storms that disrupted travel in January and earlier this month, according to Houston- based FlightAware. — Bloomberg