Flight cancellations due to weather are fairly frequent. But when’s the last time you heard an airline scrub a flight because it’s just too hot?
Nearly 50 flights out of Phoenix, Ariz. were cancelled Tuesday thanks to scorching temperatures across the southwestern United States. American Airlines said that the aircraft used by its regional American Eagle brand have a maximum operating temperature of 118 degrees, the Arizona Republic reports. Phoenix’s forecast for Tuesday included a high of 120 degrees.
So how can mere heat without any precipitation or hazardous wind conditions ground a flight? Extreme temperatures can damage a plane’s internal components, for starters. But the main reason boils down to physics and air density.
Patrick Smith, pilot and author of Cockpit Confidential, explained the science behind the phenomenon. In the book, which Business Insider first quoted in 2013, Smith writes:
Hot air is less dense. This affects the output of the engines as well as aerodynamic capabilities, increasing the required runway distance and reducing climb performance. Therefore the amount of passengers and cargo a plane can carry are often restricted when temps are very high. How much so depends on the temperature, airport elevation and the length of the available runways. And getting off the ground is only part of it: once airborne, planes have to meet specific, engine-out climb criterion, so nearby obstructions like hills and towers are another complication.
With high temperatures in the Phoenix area expected to remain around 118 degrees on Wednesday, travelers in the area should expect further cancellations.
This article originally appeared on Fortune.com