If you watch jets fly high above, you may notice that sometimes white clouds trail them. These condensation trails are called contrails. Most clouds form as air rises. Contrails form by a mixing process that is similar to the cloud you see when you exhale and “see your breath”.
Contrails form when hot humid air from the jet exhaust mixes with environmental air that has a low vapor pressure and is at a cold temperature. One of the byproducts of the combustion in jet engines is a substantial amount of water vapor. We cannot see the water vapor, but when it mixes with the surrounding air it can condense to form tiny water droplets that soon freeze into ice crystals. These particles we see as the cloud lines behind the jet. Whether a contrail will form depends on the temperature and humidity of the air the jet flies through. If the upper atmosphere is very dry, a contrail may not form.
If you pay attention to contrail formation and duration, you will notice that they sometimes rapidly dissipate, but other times they will spread horizontally into an extensive thin cloud layer. How long a contrail remains intact depends on the humidity structure and winds of the upper atmosphere the jet is flying through. If that atmosphere is relatively humid, the contrail may exist for several hours. However, if the atmosphere has a low relative humidity, the contrail will dissipate as soon as it mixes with the environment. Sinking motions cause the air to warm and dry out, which causes clouds to evaporate away. If the air is moving up and down like a roller coaster ride, the descending air will warm and cause the cloud to dissipate. So, the region in the photograph where there is no contrail is likely air that is descending, and air is rising where the contrail is still visible.
