Fog reduced visibility at New York’s airports to zero Friday, blocking many airline flights from reaching the area and contributing to more than 6,300 delays across the U.S.

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 U.S. Federal Aviation Administration’s website. Thunderstorms slowed Newark takeoffs more than two hours and snarled Philadelphia air traffic.

New York’s airspace is the busiest in the U.S., with each of the three major U.S. airlines running at least one hub in the region. United Continental has a hub at Newark, had 869 delayed flights, industry data tracker FlightAware reported. Delta Air Lines which has hubs at LaGuardia and Kennedy, has 637 delayed Friday. Almost 1,300 U.S. flights were canceled.

Fridays’ tie-ups built on delays totaling more than 9,460 yesterday and Feb. 19, and on winter storms that disrupted travel in January and earlier this month, according to Houston- based FlightAware.

Save 75% on a Digital Subscription Today

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

Southwest Airlines Co., the biggest discount carrier, had the most delays, with almost 1,300, FlightAware reported.