Why Do Planes Leave White Streaks in The Sky?

[INTRO] Squeaks and I were outside the other day looking for shapes in the clouds. We found some clouds that looked like our friend Ruffles the Squirrel, and one that looked like a turkey, and even one that looked like the Fort! And that’s not all we saw in the sky… We got to watch a jet plane fly overhead! Squeaks noticed something interesting about it: there was a white trail that came out from the back of the airplane. It looked like the plane was leaving a line behind it across the sky! Maybe you’ve seen this too — or maybe you didn’t even see the plane, and only saw the trail it left behind. Sometimes you can see them for a long time after the jet is gone. These trails aren’t really clouds, even though they look like it. But the trails and clouds are made mostly of the same stuff. Do you know what clouds are made of? [ Squeaks squeaks] Right, Squeaks! Clouds are mostly made out of water, and so are the trails that come from airplanes. But where does the water in the trails come from? Well, you might know that jets can fly really high and super fast because of their powerful engines. And just like most car engines need gas, jet engines need fuel to make them run. When engines burn fuel, they let out waste, which we call exhaust. Jet exhaust is made of lots of things all mixed together, like little pieces of dirt, heat, and a type of water that we can’t see because it’s so spread out in the air, called water vapor. You know, you make a kind of exhaust, too — every time you breathe out! It’s not the same as jet exhaust, but it has does have some of the same things in it, like heat and vapor. If you put your hand close to your mouth… ...and breathe out on it gently...how does it feel? Warm, right? You can feel /heat/ when you breathe out. But what about vapor? That’s a little trickier because it’s hard to see. But if you breathe out the same way, gently, on a mirror or window, you’ll see that it gets foggy. And that’s how you know there’s water vapor in your breath. When the vapor in your warm breath touches the cooler window or mirror, it changes into teeny tiny droplets of water. Together, all those little tiny droplets make the fog you see on the window or mirror. Now, you can’t usually see the vapor that’s in jet exhaust, either, just like you usually can’t see your breath when you breathe out. But you can see your breath sometimes, when it’s cold! [Squeaks squeaks] Right, Squeaks! When you breathe out on a cold day, the vapor in your breath changes into droplets of water, which is how you can see where you breathed out in the air. The same kind of thing happens to the vapor coming from the plane. See, the higher you get up in the sky, the colder it gets. And jet planes fly really, really high up, where it’s super duper cold. So, the cold air cools down the vapor in the jet exhaust, turning it into drops, or even tiny pieces of ice. Tons of these little drops of water and ice make the trail that we see in the sky! You might notice that some plane trails are wide and some are thin. Some last a long time and some disappear pretty quickly. All of these things depend mostly on how much vapor there is in the jet’s exhaust, and how high the jet is flying. [Squeaks squeaks] Good question, Squeaks! It’s true that you won’t always see a trail behind a plane. If you see a plane without a trail, it might not be cold enough for the vapor to turn into water droplets or ice. And sometimes it just depends on the kind of engine that the plane has. But if you do see one...now you know why! Come on, Squeaks, let’s go look for more cloud shapes. We can see if we can spot any more planes, too! Thanks for joining us! If you want to keep learning and having fun with Squeaks and me, hit the subscribe button, and don’t forget to check us out on the YouTube Kids app. We’ll see you next time, here at the Fort! [ OUTRO ]