About 4,000 flights have been cancelled across the nation as much of the United States is facing freezing temperatures due to the polar vortex.

The Midwest, where temperatures dipped as low as 20 degrees below zero, has been the hardest hit in terms of flight cancellations. Chicago’s O’Hare International Airport has had 800 outbound flights cancelled as of 2:40 p.m. EST, according to Flight Aware. Another 63 outbound flights were delayed at O’Hare. Roughly 60 percent of inbound flights, or766 flights, were cancelled.

The polar vortex is to blame for the frigid weather. Accuweather.com describes the polar vortex as a “large pocket of very cold air, typically the coldest air in the Northern hemisphere, which sits over the polar region during the winter season.

The polar vortex pushed unusually far south, which created the freezing conditions over a large portion of the country.

“This is why we’ve had such extreme cold,” said Accuweather meteorologist Brett Anderson. “The polar vortex moves around at times during the course of the winter, but rarely do you see it get pushed this far south.”

More than half of the outbound flights at Cleveland-Hopkins International Airport, where it was -3 degrees with a -29-degree wind chill as of 1:40 p.m. local time, were cancelled. Almost 70 percent of flights originating at Indianapolis International Airport, or 115 flights, were cancelled Monday, with another 16 flights delayed.

The Northeast, where temperatures were chilly but not as cold as the Midwest, has also been affected by the polar vortex. New York’s LaGuardia Airport had 115 outbound flights cancelled Monday and another 97 flights were delayed.

The hardest-hit airline Monday was American Eagle. The Fort Worth, Texas-based carrier, which has hubs at O’Hare and LaGuardia, among other airports, had 531 cancelled flights and 171 cancellations. Georgia-based ExpressJet Airlines had 511 cancellations and 332 delayed flights. New York City-based JetBlue had 424 cancellations and 199 delayed flights.

JetBlue said it expects to be “100% operational” by 3 p.m. EST Tuesday.

“This plan allows for 17 hours of rest for crews, and time for Tech Ops to service the aircraft,” the airline said in its Monday operational update.

“The new year began with the winter storm some called Hercules, shutting down the heaviest-trafficked air corridor in the world during one of the heaviest travel periods of the year. Mother Nature then followed that up with icing conditions over the weekend, causing even more issues and ground stops at the airports,” the airline said on its blog. “Even as airports began to reopen, though, newly launched FAA regulations on pilot duty times caused delayed flights to quickly turn into canceled ones. Now today, less than a week into the year, we’re watching a polar vortex wreak havoc on flight schedules across the industry, as rainy weather prepares to turn airports in the Northeast into ice rinks once again.”

Due to the weather, American Airlines is allowing customers traveling to certain cities to change their plans here.

Not sure if your flight is cancelled or delayed? You can check out that info here.