It’s so hot in the southwestern United States that flights out of Phoenix, Arizona are being cancelled because of the extreme heat.

Temperatures on Tuesday were expected to reach 49C, which is too hot for some planes to operate.

American Airlines said it was going to cancel 38 flights leaving from Phoenix’s Sky Harbor airport during the hottest part of the day from 3 p.m. to 6 p.m.

Most of the cancelled flights were shorter distance, regional flights because of the smaller planes they utilize. One commonly used smaller jet, the Bombardier CRJ, has a maximum operating temperature of 48C.

The reason is that hot air is thinner than cold air and requires more speed in order to provide an airplane enough lift to take off. High-altitude airports face similar problems due to the thinner air.

According to a 2016 report from the International Civil Aviation Organization, high temperatures “have severe consequences for aircraft take-off performance, where high altitudes or short runways limit the payload or even the fuel-carrying capacity.”

Larger Boeing and Airbus jets can fly in temperatures as high as 53C.

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