On some days I notice long-lasting condensation trails behind high-flying jets, but on other days they are entirely absent. Why is that?
Condensation trails (often abbreviated to “contrails”) form when jet aircraft fly in the upper levels of the atmosphere where temperatures, even in the summer, are generally lower than 25 degrees below zero. The aircraft exhaust emissions are laden with water droplets that instantly freeze into ice crystals, creating an artificial cloud, the contrail. If the air that the jet is flying through is very dry, contrails are thin and short-lived. They evaporate quickly and are invisible to observers on the ground. If the air mass through which the jet is flying is moist, contrails become broad, long trails of cloud and they can last for an extended period of time, sometimes expanding and covering the entire sky.