Why do contrails form...and why are they sometimes more persistent?
Contrails form from water vapor released in the exhaust from jet engines. The air needs to be humid enough (supersaturated) and cold enough (mostly forty below zero, or greater) in order for contrails to develop. Obviously this must happen in the upper part of the atmosphere, what we call the 'troposphere'. At lower humidities and higher winds, contrails dissipate more quickly. If humidities are higher...the contrails are more persistent.