Right now, millions of people in the American West and Southwest are embroiled in a heat wave, with excessive heat warnings issued for inland California, southern Nevada, and parts of southwest Arizona through Friday. Temperatures are an upwards of 120 degrees. Forget about cooking an egg on the sidewalk—you could cook a steak.

People trying to fly elsewhere to beat the heat, however, could encounter one major wrinkle: It's so hot that planes are having trouble taking off and landing. Forty-three American Airlines flights alone were canceled on Monday because temperatures were too high, and more cancellations are expected for Tuesday. And with temperatures forecast to climb even higher, we're likely to see continued disruption to air travel throughout the week.

But don't planes fly to destinations from the equator to the Arctic Circle? Why is it that they can't handle the heat? It turns out that planes can handle cold a whole better than they can deal with heat. As long as the inside of the aircraft is kept warm enough, planes can take off and land in temperatures as low as minus 67 degrees Fahrenheit. It's often not much warmer than that outside your window at altitude. If you've ever put your hand on the wall or window when you're flying, you'll note that it's cold, sure. But what you probably didn't know is just how cold it is—likely minus 40 degrees or more.

It's actually extreme heat that causes the most challenges: not so much to a plane's mechanical abilities to fly as to its effect on the air around the plane. Heat creates thinner air, which means it doesn't have the necessary density to give the plane enough lift to take off or land. The heat also limits the plane's ability to climb properly, and the heavier the aircraft—the more people and luggage on it—the more impaired it becomes.

Essentially, the higher the air temperature, the thinner the air, and the more challenging for a pilot to safely get the plane on or off the ground. Anything higher than 104 degrees can be a cause for concern. Smaller regional planes are certified to handle more than that, up to 118 degrees, but with temperatures expected to climb, we may see more airlines canceling or rerouting flights. As for the larger planes? Most Boeing aircraft can operate in up to 126 degree weather, and Airbus, 127 degrees. Pilots can opt to use a higher engine thrust setting to compensate on a hotter day, which gives them equal lift as if it were a normal, lower temperature. Airlines may also need to use more runway where possible or fly lighter planes by capping the number of seats that can be filled or luggage that can be stored to offset the thinner air. With temperatures above these levels, no amount of compensation will help given how hot it is, leaving airlines with no choice but to cancel flights. (According to the BBC, heat is often why countries in the Middle East, and some high-altitude airports in South America, often schedule long flights for cooler times—think evening or nighttime departures.)

This is not the first time airplanes have felt the heat in certain parts of the U.S.—last summer, some planes faced similar challenges, and in past decades, we've seen the same story play out: In June of 1990, temperatures hit 120 degrees, so hot that the asphalt on the tarmac softened, aircraft couldn’t move, and they had to ground flights. Scheduled to fly in to (or out of) the Southwest in the coming days? Be prepared for possible flight disruptions. It may be literally too hot for pilots to handle.

This article was originally published in 2016. It has been updated with new information.