As airplanes have become a dominant mode of travel, the large volume of air traffic leads to a unique cloud formation – contrails.
Contrails, short for Condensation trails, are formed from the condensation of an airplane’s hot exhaust. An aircraft’s exhaust contains water vapor and tiny particles (in both gas and solid form) that are a result of the combustion process that drives the engine. The particles serve as a condensation nuclei – a particle onto which condensation can occur. Contrails form high in the atmosphere (usually above 8 km or 4.3 mi) where temperatures well below freezing. This means that contrails are generally ice clouds.
This cloud type can evaporate quickly or last a very long time depending upon atmospheric conditions. As a contrail ages is often spreads laterally due to the winds aloft. It is important to understand contrails because they impact how much of the sun’s incoming rays will be reflected back to space, which can have an impact on climate. Here’s an example of a couple of contrails in a generally clear sky
and more persistent contrails in a cloudy sky
So what happens when a jet flies through an already established cloud? The heat of the exhaust can actually cause the cloud droplets to evaporate, leaving clear (visually) air in its place. You can actually see a line in the cloud where the aircraft transited!
If a cloud layer forms below the contrail and the sun is behind the contrail from your perspective the contrail can cast a shadow onto the lower cloud deck. Don’t confuse this with a distrail. Here’s an example, notice the contrails visible through the thin cloud layer and the dark paths are the projected shadow.