Why do planes leave a white line?

When burning kerosene, a jet engine produces mainly carbon dioxide and water vapor: for every kilogram of fuel burned, about 1.25 kg of water is released as gas. That vapor leaves the nozzle at several hundred degrees and mixes in fractions of a second with air that, at cruise altitude, is between −40 and −60 °C. The mixture saturates, the vapor condenses onto soot and sulfur particles from the exhaust itself, and the droplets freeze into ice crystals. The result is a linear cirrus cloud manufactured by the aircraft, visible from the ground as a white streak.

Formation depends on the temperature and humidity of the air, not on the aircraft. The Schmidt-Appleman criterion, formulated in the mid-20th century, sets the threshold: below about −40 °C the contrail almost always forms; in the warm air at low altitude it doesn't appear, which is why aircraft taking off or landing don't leave one. If the ambient air is dry, the crystals sublimate within seconds and the contrail measures only a few hundred meters. If the air is saturated with respect to ice, the contrail survives for hours, the wind spreads it out, and it can turn into a layer of cirrus clouds covering dozens of kilometers.

Persistent contrails are not harmless. They trap more infrared radiation emitted by the Earth than they reflect sunlight, so on balance they have a warming effect. Studies compiled by EASA and NASA estimate that their climate effect is comparable to that of the CO2 emitted by aviation itself, though with much greater uncertainty. That's why routes that avoid humid layers are being tested, along with fuels with less sulfur and soot, which generate fewer condensation nuclei. By contrast, there is no basis whatsoever for chemical spraying theories: the composition of contrails has been measured in flight for decades, and it's ice.