Contrails were initially discovered during the first high-altitude flights in the 1920s, although scientists and engineers were not overly concerned with them until WWII, when military bombers could be seen from miles away due to the long lines of contrails left in their wake.

Why are contrails formed?

A contrail will form if, as exhaust gases cool and mix with surrounding air, the humidity becomes high enough (or, equivalently, the air temperature becomes low enough) for liquid water condensation to occur. ... If sufficient humidity occurs in the exhaust plume, water con- denses on particles to form liquid droplets.

How long do contrails stay in the sky?

Satellites have observed clusters of contrails lasting as long as 14 hours, though most remain visible for four to six hours. The long-lived, spreading contrails are of great interest to climate scientists because they reflect sunlight and trap infrared radiation.