Have you ever watched an airplane fly across a clear, blue sky? Sometimes it’s easy to trace an airplane’s path because it leaves behind a white streak across the sky. After a while, though, the plane’s track fades and disappears into thin air.

Those white streaks planes leave behind are actually artificial clouds. They’re called “contrails,” which is a shortened version of the phrase “condensation trail.”

Airplane engines produce exhaust, just like car engines do. As hot exhaust gases escape from a plane, the water vapor in the fumes hits the air. At heights of 26,000 feet or more, the air is extremely cold (sometimes more than -40° F!).

The cold air causes the water vapor to condense. This means the water vapor gases turn into tiny water droplets or even freeze into tiny ice crystals before eventually evaporating. This condensed water vapor and mixture of ice crystals make up the cloud-like trails you see in the sky.

If you’ve ever seen an air show, you may have seen airplanes creating messages in the sky with what look like clouds. Are these contrails? Not exactly…

Skywriters use small airplanes equipped with special smoke machines to fly in special patterns to create written messages visible from the ground. The smoke machines usually consist of pressurized containers full of oil. At a pilot’s command, the machines spray oil onto the plane’s hot exhaust system, where it burns quickly and creates clouds of dense white smoke.

Pilots have to learn how to fly special patterns and work the smoke machine carefully to be able to create their unique messages in the sky. From advertisements to marriage proposals, the messages can be practical, personal or just plain silly.

Skywriting goes way back. There are reports of successful skywriters before World War I, possibly as early as 1915. Today, skywriters take advantage of satellite navigation to program messages before flight, thereby increasing accuracy.