[♪♩INTRO] Back in June, it was almost 50 degrees Celsius in Phoenix, Arizona—120 degrees Fahrenheit.

Like half of the way to boiling. Whatever way of imagining this you want to use, this is way too hot.

I grew up in Florida, and I draw the line at weather that’s hot enough to help you cook a gater sausage on the sidewalk. But the heat wave had an unusual side effect — one that made headlines because it’s so foreign to those of us who don’t live in places that are basically the surface of the sun: more than forty flights were canceled because it was too hot to fly. And it wasn't a problem with the planes themselves, it wasn’t too hot inside.

The air, the atmosphere, just wasn’t dense enough for planes to fly safely. The strength of the lift, the force pushing a plane off the ground, depends on how many air molecules are flowing past the wings. The more molecules there are, the more of a push the plane gets, so planes have big, tilted wings to hit lots of molecules, and they speed down the runway to ram through as many as possible.

If all goes well, enough molecules are pushing past the wings at the end of the runway to get the plane up in the air. Which almost always happens. But if there aren’t enough molecules flowing past the wings, the plane… just won’t lift off.

And that’s no good. Besides the fact that you’re not going to get anywhere, you do not want planes, like, still on the ground when the runway ends. Airports at higher altitudes, where the air is thinner, have longer runways so that planes can speed up more and catch enough molecules to get themselves in the air.

Which is why mile-high Denver, Colorado, has some of the longest runways in the world. But temperature also matters. Air molecules move faster at higher temperatures and slower at lower temperatures, which kind of makes it seem like higher temperatures should help planes — you would think faster molecules would get you more lift.

But the molecules in a gas are also constantly bouncing off of each other. The faster they move, the further away they bounce after each collision, and the more spread out they are. So warmer air in the atmosphere is less dense than cooler air.

And the number of molecules moving past the wings matters more than the speed of those molecules. Wings speeding down a runway through nice, cool, dense air move through lots and lots of molecules, so they get plenty of lift. But when air is too hot, the molecules are too spread out to lift the plane up by the time it’s at the end of the runway.

Higher temperatures also make planes’ engines less efficient, which makes it even harder to catch enough molecules in time. Airlines can lighten planes by removing passengers or baggage, but that only works up to a point. For safety, some commercial airplanes aren’t supposed to be flown at temperatures above about fifty degrees Celsius.

So that’s one more reason I’m glad I do not live in a place like Phoenix. If it gets too hot, you ju-, you like, cannot escape by flying somewhere else. Thanks for watching this episode of SciShow.

If you are in Phoenix, I’m sorry. About the fact that you live in Phoenix, and also about insulting your town. And, if you want to learn more about planes and how they fly, you can check out our video on the ups and downs of air turbulence. [♪♩OUTRO]