You've heard the one about it being so hot you can fry an egg on a sidewalk, well how about it being hot enough to ground a jet?

That was the case in Phoenix on Tuesday, where temperatures were forecast to climb as high as 120 degrees Fahrenheit.

More than 40 American Eagle regional flights scheduled to take off or land at Phoenix Sky Harbor International Airport between 3 and 6 p.m. local time were canceled.

American Airlines said in a statement that the Bombardier CRJ aircraft used on some shorter routes have a maximum operating temperature of 118 degrees.

For bigger jets, the threshold is higher. The carrier says that, for example, Airbus aircraft have a maximum operating temperature of 127 degrees and that for Boeing, it is 126 degrees.

As USA Today reports:

"Extreme heat affects a plane's ability to take off. Hot air is less dense than cold air, and the hotter the temperature, the more speed a plane needs to lift off. A runway might not be long enough to allow a plane to achieve the necessary extra speed."

Bianca Hernandez, a meteorologist with the National Oceanic and Atmospheric Administration, tells NPR that Phoenix is seeing an unusually strong high-pressure system, which is causing the soaring temperatures.

"We are used to 100 to 110 degrees, but once it gets this much above average, it poses a threat to people," Hernandez says. "Heat is the No. 1 weather-related killer in Arizona."

The National Weather Service has issued an "excessive heat warning," effective until Sunday evening local time, for south-central and southwest Arizona.

The city of Phoenix advised residents to take precautions, such as limiting outdoor activity to before sunrise or after sunset, staying hydrated and wearing light-colored clothing.

Phoenix is operating additional cooling centers and other emergency facilities during the heat wave.

But the worst of it might soon be past. "It looks like (Tuesday) is the peak of the heat wave," Hernandez says. But "temperatures are expected to be above 110 degrees for the next week."

The extreme weather extends over much of the Southwest, including parts of California and Nevada, which are also under excessive heat warnings.

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