Weather already has the power to cause airplane delays — currently, bad weather leads to about one-third of all delays.

Another variable is weighing in, though. Heat. According to a study in the journal Climactic Change, rising temperatures could lead to more flight cancelations like the debacle in Phoenix, Ariz. in June, when scores of flights were canceled due to high temperatures soaring above 118 degrees.

Heat makes it dangerous to take off, because it decreases the lift under the wings of a plane. High temperatures can also affect a plane’s weight capacity.

“During the hottest parts of the day, 10 to 30 percent of fully loaded planes may have to remove some fuel, cargo or passengers, or else wait for cooler hours to fly,” the study noted. “Weight must be dumped, or else the flight delayed or cancelled.”

The study showed that, on the hottest days, planes need to decrease weight by as much as 4 percent. Depending on the type of aircraft that could mean 12–13 fewer passengers.

Average global temperatures have been rising, and 2016 was the warmest year on record.