# BLOG: Why does heat cause flight cancellations?

On Tuesday many flights were canceled out of Phoenix Shy Harbor International Airport due to the extreme heat. If you follow my Facebook page, it's possible you saw that I spent the past week out in Arizona and Nevada. Yesterday I was one of thousands of people traveling home out of Phoenix.

I was actually lucky as my flight was relatively early. We departed at 9:30 a.m and the temperature at that hour was (wait for it) already at 100 degrees! Now as outrageous as that may seem at that hour of the morning, 100 degrees is still safe for flying. But yesterday afternoon nearly 50 flights were canceled from the heat as temperatures topped out near 118 degrees!

Now why would flights be canceled due to heat? That's where we dive into the physics of it.

Think of the air around you as tiny balls of air (these of course being air molecules) that are continuously moving in all directions and bumping into each other.  As temperatures warm, the tiny balls of air will move faster and expand. As this happens, the density of air will decrease.

Here may be an easier way to understand it. Think of a group of people where each body represents an independent air molecule. When it is cold out side, typically you (as a single body) will want to huddle around your friends or family (other bodies or air molecules) to warm up, right? And when it is hot outside, it is more comfortable to have some space and sprawl out. Well, think of air molecules the same way. As it cools down, the air becomes more dense as the tiny balls of air move closer together. As it warms up, the tiny balls of air will expand and move away from each other, therefore resulting in air becoming lass dense.

This is important because the wings of the airplane need this density to create lift. When it gets 118 degrees outside, the density of the surrounding air is so low that the only way to compensate would be higher speeds before take off. Unfortunately, airports (like Phoenix Sky Harbor) don't have that additional runway length to get enough speed before take off to compensate. This is why flights were canceled yesterday afternoon.

All of this hot air is due in part to a heat dome (otherwise known as a ridge of high pressure) which is stretched across the Desert Southwest through the end of the week. This will allow for the highs will be back near 117 degrees on Wednesday, so additional cancelations are possible in that part of the country.