Why do vapor trails sometimes form behind jets?
Vapor trails sometimes form behind jets, but at other times they do not. Why is that?
— Jacob Bott
Water vapor added to the air in jet engine exhaust leads to “condensation trails,” those wisps of cirrus-like clouds that we see behind high-flying jet aircraft. Temperature and humidity at flight elevation determine whether condensation trails form and how long they will last. Condensation trials won’t form in very dry air, but when the relative humidity is high enough, they do indeed form. At times, in a saturated atmosphere at flight level, they expand into a cirrostratus cloud shield that can blanket the whole sky when viewed from the ground.
The U.S. Air Force had great interest in jet condensation trails during the Cold War years because they could give away the location of high-flying military aircraft. That concern led to condensation trail forecast techniques.
Temperature and humidity at flight elevation determine whether condensation trails form and how long they will last.