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Each year, cold winter weather leads to 60,000 flight cancellations in the U.S., costing airlines and airports an estimated $3 billion. But freezing cold temperatures aren’t a problem for planes, which excel in cold, dense air. Rather, the real problems occur at ground level, where clearing snow and ice off of runways and planes can create major delays.

Following is a transcript of the video:

Cold winter weather causes thousands of flight cancellations each year, and the real problems happen before planes even leave the ground.

Each year, about 60,000 flights get canceled because of bad winter weather, which costs airlines and airports an estimated $3 billion. But it’s not the freezing cold temperatures that cause problems for planes. After all, commercial jets fly 10 kilometers up, where temperatures hover around -50 degrees Celsius.

In fact, planes excel in cold weather, since cold air is denser and leads to better thrust. So clearly, the real problem isn’t what’s going on up there. It’s what happens on the ground.

When a nasty polar vortex struck the Midwestern US in January 2019, temperatures dropped to -40 degrees Celsius and airline canceled 3,000 flights nationwide. In these situations, when temperatures start dropping, everything slows down. Cargo doors can freeze up, along with the nozzles that pump fuel into planes, which delays the refueling process.

Even the plane itself can freeze over. Just a quarter-inch-thick layer of ice on a plane can disrupt the way air flows over its wings.

Les Westbrooks: “So the number one reason I would say that the reason flights get delayed in cold weather is going to be because there’s some kind of frozen precipitation, from frost to snow to a sheet of ice, adhering to the aircraft, adhering to the wings of the aircraft particularly."

That’s Les Westbrooks, a retired airline pilot and an associate professor of aeronautical science at Embry-Riddle Aeronautical University. He says that typically, these planes are “de-iced” — but this also delays takeoff. The crew can spray the plane with a special hot water/glycol mixture. It can take around 40 minutes to de-ice large passenger airplanes, so planes often have to wait “their turn” for the de-icing station, which can also trigger major delays.

And ice on the runway creates another set of issues. In 2014, a plane at JFK skidded off an icy runway and into a mound of snow, leading to an hours-long shut down at the airport. And even though crews can remove ice from the runway, scraping it off the pavement can lead to potholes and other imperfections, which makes takeoffs and landings more dangerous. And of course, snow and freezing rain on the ground can affect visibility to the point where officials may decide it’s not safe to fly at all. But if ice and snow aren’t the problem in these extremely cold temperatures, it’s usually another factor: people.

Les Westbrooks: “The airplane flies fine at high altitudes, -60 degrees. It’s made to do that. Humans are not made to be outside in -60 degrees weather. And so the human factor becomes a big, big factor, when it becomes extremely cold.”

Baggage handlers, aircraft fuelers, and mechanics all have to stay warm. Some airports, like O’Hare in Chicago, set up heated shelters for its employees. Of course, with all these breaks to stay warm, not as much gets done, which leads to even more delays and cancellations. Passengers start missing their connecting flights, and that, along with passengers who can’t make it to the airport due to bad road conditions, leads to half-empty planes.

In fact, many airlines might preemptively cancel flights before bad weather even hits. Because it’s simply not worth it to fly with so few passengers. So, in the end, you can still blame cold weather for cancelling your flight. But unfortunately, it’s out of our control. All you can do is stay home, bundled up with a mug of hot cocoa.