Why do airplanes leave a white trail in the sky?

A jet engine burns kerosene and produces, among other gases, carbon dioxide and water vapor, along with soot particles. As it leaves the nozzle, that jet at several hundred degrees mixes with the surrounding air. If the outside air is cold and humid enough, the mixture reaches saturation: the vapor condenses onto the particles and, since the temperature is well below zero, freezes into ice crystals. That is a condensation trail, or contrail — the same mechanism behind the breath we exhale in winter.

The condition is described by the Schmidt-Appleman criterion, formulated in the mid-20th century: it depends on temperature, humidity and air pressure, as well as engine efficiency. In practice it is met below about −40 °C, which at mid latitudes sets the threshold between 8,000 and 12,000 m (26,000 to 39,000 ft), the usual cruise altitude. That's why a propeller plane at 3,000 m leaves no trail while a jet does, and why two aircraft at the same altitude can behave differently if they cross air layers with different humidity.

If the air is dry, the crystals sublimate within seconds and the trail measures a few hundred meters. If the air is supersaturated with respect to ice, the crystals grow, the trail widens over hours and can end up turning into a cirrus cloud indistinguishable from a natural one. These persistent trails trap infrared radiation, and several studies attribute a significant share of aviation's climate impact to them, which is why rerouting flights away from the air layers that favor them is being tested.