(NEXSTAR) — Sky-high temperatures could leave airline travelers grounded, in extreme cases.

Flights can be delayed or canceled for any number of weather-related reasons. But even on calm, cloudless days, airlines can ground their aircraft simply because of soaring temperatures. It’s a relatively rare occurrence, albeit a necessary one to keep passengers and crew safe.

“During times of extreme heat, the air is less dense and generates less lift,” the Federal Aviation Administration said in a statement. “Less lift” is exactly what it sounds like — certain planes with heavier passenger or cargo loads simply have a tougher time getting off the ground.

“Airlines may have to reduce aircraft weight in order to take off, which they do by offloading cargo or passengers,” the FAA notes. “Airlines may also decide to reduce the amount of fuel loaded in order to accommodate all passengers, in which case they may add a fuel stop in order to reach their intended destination.”

On especially hot days, dozens of flights can be grounded altogether. On one scorching June afternoon in 2017, for instance, American Airlines canceled 40 flights at Phoenix Sky Harbor International Airport amid temperatures that neared 120 degrees Fahrenheit.

In that instance, an American Airlines representative said the carrier canceled its smaller regional flights operated on Bombardier jets, which were not recommended for operation in temperatures above 118 degrees.

Bigger jets, like those made by Boeing or Airbus, can operate in hotter temperatures — specifically, the mid-to-high 120s.

“All of our airplanes are designed and tested to operate in both extreme cold and hot weather conditions,” a representative for Boeing told Nexstar. “Our maintenance and operations manuals provide guidance and procedures to ensure safe operation of the commercial fleet.”

A spokesman for Boeing also once said the manufacturer sells “hot and high” packages to carriers who anticipate the need for more thrust in hot weather, according to a CNBC report.

Lucky for many passengers taking off from Phoenix Sky Harbor, the hottest temperature ever recorded at the airport was 122 degrees — back on June 26, 1990. Greg Roybal, the public information officer for Sky Harbor, said the facility is equipped with longer runways to accommodate planes that need higher takeoff speeds in hot weather. Employees are also reminded to stay hydrated and carry water throughout the summer.  

“Sky Harbor is well prepared for Arizona summers,” Roybal said.