Summer heat can make even the hardiest people wilt and collapse into a pool of sweat. The same is true for the Bombardier Inc. family of regional jets—it can become too hot to fly them when the weather turns extreme.

American Airlines Group Inc. was forced to cancel 50 flights so far this week—43 of them on Tuesday—from its Phoenix hub because the forecast high is 120 degrees Fahrenheit (49 degrees Celsius). An additional seven flights to Phoenix were delayed Tuesday due to the heat. The 50-to-76 seat Bombardier jets flown by American Eagle are certified to operate at 118 F.

Hotter air is less dense, which reduces engine performance and can require lower takeoff weights and longer takeoff distances to produce lift.

The cancelations affected flights scheduled to arrive or depart Phoenix from 3 p.m. to 6 p.m. on Tuesday. American said it doesn’t currently expect more disruptions Wednesday, with a forecast high around 116 F. Phoenix’s record high is 122 F, reached in June 1990. That month also saw similar flight cancelations due to heat.

Extreme heat is less of an issue for larger planes. Airbus SE jets can operate to a maximum of 127 F, one more than Boeing Co. models, according to American.

Climate change poses huge risks for aviation, according to a 2016 report from the International Civil Aviation Organization, a UN agency based in Montreal. Reduced aircraft performance is just one impact from higher temperatures, according to the agency, which also cited heat damage to airport surfaces, increased heating and cooling requirements, greater pressure on local utilities, and greater noise generation due to aircraft performance issues.

As summer high temperatures climb even higher, diminished aircraft performance is likely to generate increased discussion for airlines and jet manufacturers.

For those already weary of the heat, here’s the real kicker: Summer formally starts Wednesday in the northern hemisphere.