How airplanes create temporary clouds in the sky

HostI was looking up at the sky earlier today and saw those long white streaks that planes leave behind. It's funny because some of them just sit there for hours, while others seem to vanish the second the plane passes by. It actually reminded me of being a kid on a really cold morning, when you blow out a breath and see that little puff of mist for a second. Is that basically what's happening up there, just on a much bigger scale?

GuestThat's the perfect way to think about it. When you breathe out on a cold day, the warm, wet air from your lungs hits the freezing air outside and turns into a tiny cloud. A jet engine is doing the same thing, just much more intensely. As the plane burns its fuel, the burning process releases two main things: carbon dioxide and water vapor. Now, once you get up to where those planes fly, maybe thirty or forty thousand feet up, it's incredibly cold. We're talking forty degrees below zero or even colder. When that hot, moist exhaust hits that freezing air, the water vapor cools down fast. But there's a catch. To turn into ice, that water needs a surface to grab onto.

HostSo the water can't just freeze on its own in mid-air? It needs a landing pad?

GuestIt needs a seed. And the engine provides those seeds in the form of tiny bits of soot and sulfur. These are just microscopic leftovers from the fuel burning. The water vapor sticks to those tiny particles and freezes instantly into tiny ice crystals. So, those white lines you see aren't actually smoke or exhaust fumes. They're human-made clouds made almost entirely of ice. We call them contrails, which is just short for condensation trails. But whether they stay or go depends on a physics rule called the Schmidt-Appleman Criterion. Basically, the air has to be both cold enough and moist enough for a trail to survive.

HostBut I have seen those lines just disappear after a few seconds. If it's all ice, why doesn't it just stay frozen?

GuestThat happens if the plane flies through a patch of air that's very dry. In those conditions, the ice crystals go through a process called sublimation. That's just a way of saying the ice turns straight back into an invisible gas without ever becoming liquid water first. The atmosphere basically gobbles the moisture back up. You might see a tiny, fleeting white tip right behind the engines, like the tip of a pencil, but it vanishes almost as soon as it forms because the air is just too thirsty.

HostWhat about those days when the streaks stay there and even start to grow? I have seen them spread out until the whole sky looks hazy and white.

GuestThat's the really interesting part. That happens when a plane hits what we call a supersaturated zone. In these pockets of the sky, the air is holding way more moisture than it normally can at that temperature. It's like a sponge that's already dripping wet. When the plane leaves a trail in that kind of air, the contrail doesn't just sit there. It actually pulls moisture from the surrounding air and grows larger. Over a few hours, those sharp white lines lose their edges and spread out. They eventually become indistinguishable from natural, wispy cirrus clouds. In places with a lot of flights, a huge chunk of the high-altitude cloud cover is actually just these aged contrails, which can even trap heat and change the local temperature.

HostI have seen something that always confuses me, though. Sometimes you see two planes in the same part of the sky, maybe one is slightly above the other, and one is leaving a massive trail while the other leaves nothing at all. Does one plane just have a cleaner engine?

GuestIt has nothing to do with the engine. It happens because the atmosphere is like a giant layer cake. It isn't just one big block of air with the same weather everywhere. You can have a layer of very moist air sitting right on top of a layer that's bone dry, with only a few hundred feet between them. Since the humidity can change so much in just a tiny bit of altitude, two planes at slightly different heights will look completely different. A pilot might even see their own trail stop and start as they cross these invisible boundaries, making it look like the engines were turned off when it was really just the air changing.

GuestIt's wild to think that a line of clouds stretching from one horizon to the other can start with a piece of soot so small you couldn't even see it.

HostWinter breath on a massive scale is a strange thing to picture, but it explains why the sky can look so different from one minute to the next. Those white streaks are just the atmosphere showing us exactly where the moisture is hiding.