It’s a clear day, the sun is shining, not a cloud in sight… until a plane flies over.

And then suddenly, there’s a long, white line across the sky. Sometimes these clouds, called condensation trails, or contrails, disappear almost as soon as they form.

Other times they stick around long after the plane is gone. It all depends on the conditions and ingredients floating around in the atmosphere. And it turns out that planes aren’t the only accidental cloud machines we have!

All clouds are made of tiny water droplets or ice crystals that form when water vapor condenses out of the air. Now, water molecules are floating around pretty much everywhere in a gaseous state, bumping into other things every so often. But it’s really unlikely that a bunch of these water molecules will just clump together at one place and one time in all the chaos of the atmosphere, to form a liquid droplet.

To make condensation much easier, the water molecules need something to stick to. And this is where these things called cloud condensation nuclei or cloud seeds come in. Dust, sea salt, or even bacteria will do.

Most particles form closer to the ground, though. And any particles that do manage to make it up to lofty heights are too heavy to stay there for long, although sometimes they become a part of a cloud. This is where planes come in with their combustion engines, which burn fuel to generate power.

Even highly efficient jet engines emit some non-combusted carbon that can stick together to form soot. They can also spew out small metallic particles, as well as sulfur and nitrogen compounds that can clump together and work as cloud seeds. All this junk is splendid for a water molecule looking to leave the gas phase and condense somewhere nice.

Because planes today fly at high altitudes where temperatures are really cold, as soon as the water condenses, it freezes, forming a man-made ice cloud. At least… for a little while. If the air behind the plane is too warm or dry, the water molecules will sublimate almost as quickly as they freeze, transitioning straight from a solid to a gas instead of melting.

This creates a short-lived contrail that seems to follow the plane across the sky, never getting any longer. But under cooler, more humid conditions, these ice crystals stick around. They might even travel or grow into bigger clouds.

Today, a lot of air traffic makes contrails pretty common, but they’ve been around since at least the 1920s when early high-altitude planes were flown. Scientists didn’t think much of them until World War II, when long-lasting contrails started messing with stealth missions. After all, it’s hard to be sneaky when there’s a line of clouds pointing towards you.

And nowadays we know that contrails aren’t the only man-made clouds in the sky. There are clouds formed from the exhaust of big shipping vessels called ship tracks. They criss-cross the oceans, making patterns that are visible all the way from space.

Ship engines and the fuel they burn tend to be less regulated than other modes of transportation, so their exhaust contains a lot of condensation nuclei. This means water molecules form many more tiny droplets than they would naturally, creating low, dense, fog-like clouds that look brighter to satellites because they reflect more sunlight. Because of their reflectiveness, ship tracks may also have a small cooling effect on the climate, leading some people to suggest seeding a lot of these types of clouds to help stave off global warming.

But scientists have some big concerns about potential side effects. We aren’t sure how these man-made clouds affect global precipitation, plus they’re made by burning fuels that cause heavy air pollution… which doesn’t help. Now, the most common man-made clouds are actually billowing out of smokestacks of coal and gas-fired power stations.

Hot air, water vapor, and soot rise into the atmosphere, cooling as they go. Eventually, the rising air matches the temperature of its surroundings, which is often cool enough for water to condense onto the soot and form clouds. In extreme cases, a rapid heating event like an explosion can cause a pocket of hot, low-density air to shoot into the atmosphere along with soot.

When this hot plume hits the cooler, denser air above it, the top flattens and bottom curls in on itself, forming what might be the most famous type of man-made cloud: the mushroom cloud. And after a while, all the air will cool and the cloud will start to disperse. If you’re tired of contrails interfering with your cloud gazing, you’ll be happy to know that a recent study by NASA showed that planes that use biofuels produce less soot in their exhaust, which means fewer cloud seeds.

Regulations targeting soot and sulfur emissions by ships could reduce ship tracks, too. And, with any luck, using more renewable energy sources will cut down on the need for smokestacks. So it is possible that nature will monopolize cloud-creation again someday.

Thanks for watching this episode of SciShow, which was brought to you by our patrons on Patreon. If you want to help support this show, you can go to patreon.com/scishow. And don’t forget to go to youtube.com/scishow and subscribe!