The white condensation trails left behind jet aircraft are called contrails (condensation trails). Contrails usually form higher than 26,000 feet above the ground. Contrails form when hot, humid air from jet exhaust mixes with surrounding air of low water vapor content and low temperature. The clouds that form are similar to the cloud you see when you exhale in cold air and “see your breath.”
If you are attentive to contrail formation and duration, you will notice that they can rapidly dissipate or spread horizontally into an extensive thin cirrus layer. How long a contrail remains intact depends on the humidity level and winds of the upper troposphere. If the atmosphere is near saturation, the contrail may exist for some time. On the other hand, if the atmosphere is dry, the contrail mixes with the surrounding air and dissipates quickly.
Contrails are a concern in climate studies as increased jet traffic may result in an increase in cloud cover. It has been estimated that in certain heavy air-traffic corridors, cloud cover has increased by as much as 20 percent.
An increase in clouds can change a region’s radiation balance. For example, solar energy reaching the surface may be reduced, resulting in surface cooling. Contrails also reduce energy loss from the Earth’s surface, resulting in warming. Averaged over a year, contrails tend to cause a cooling over the regions they inhabit.