top of page

What Are Contrails? Clouds Made by Planes

Those long, thin clouds you sometimes see crisscrossing the sky aren't really clouds. At least they are not naturally occurring clouds like the big, puffy clouds we sometimes see. They are contrails, which are manmade clouds created from the exhaust of jet planes. 

Contrails crisscrossing the sky with thin cirrus clouds above them.
Contrails crisscrossing the sky. (Photo via Shutterstock)

Contrails are trails of condensation from jet engines that are created when planes pass overhead. The word contrail is a shortened form of condensation trail.

Jet engines create hot, humid air. As this hot, humid air is emitted into the atmosphere as exhaust, it mixes with air with low vapor pressure and a low temperature. When the two air masses combine, it allows these long, thin clouds containing ice crystals to form.

We often see contrails in the sky overhead, so we mostly associate them with jets. However, rockets and spacecraft create contrails too. This is because they also emit hot, humid air into colder, drier air during launch or liftoff.


Words to know

Atmosphere: The envelope of gases surrounding Earth or another planet.

Climate: The weather conditions prevailing in an area in general over a long period.

Condensation: Water that collects as droplets on a cold surface when it is in contact with humid air.

Vapor: A substance diffused or suspended in the air.


The process of how contrails form is similar to what happens when we exhale into the cold air and can see our breath. Our breath is hot and humid, much warmer and more humid than the cold, dry winter air we are exhaling into. Just like the jet exhaust creates a cloud when it mixes with the cold, dry air, so does our warm breath when it mixes with the cold air. 

Have you noticed that your breath doesn't always leave a cloud in the winter air? The same is true of jet planes. Contrails don't always form in their wake. Even when they do form, how long they remain visible varies. 

We most often see contrails from planes flying at a cruising altitude of between 32,000 and 42,000 feet above the ground. This is because the conditions at that altitude are most suitable for contrail formation. 

How long a contrail will remain visible — or how long your breath remains visible — is related to how much humidity is in the atmosphere. The drier the atmosphere is, the less time a contrail will remain visible; the more humid it is, the longer it will last. When humidity levels are too low, contrails won't form at all. 

In humid conditions, contrails also get bigger because ice crystals get larger as they take in water from the surrounding atmosphere. They can also spread out because of wind, turbulence created by the plane and even the effects of the sun. In some cases, a contrail can grow to several kilometers wide, and they can remain visible in the sky for hours. 

When contrails remain in the air for a long time, they contribute to cloud cover, which in turn can affect weather. At any given time, contrails are estimated to cover about 0.1% of Earth's surface. Contrail coverage is greatest in areas where jet travel is most common, particularly the United States and Europe. 

Climate scientists study contrails for a couple of reasons. First, contrails can essentially form cirrus clouds, and the cloud cover generated by these manmade clouds can reflect sunlight away from Earth. In addition, the exhaust from the jets may also change the chemistry of the upper levels of Earth's atmosphere. Over time, this can affect climate. 


Follow Willy's Wilderness on Facebook for more kid-friendly nature stories and activities.


Commenting has been turned off.
bottom of page