contrail, also called condensation trail or vapour trail, streamer of cloud sometimes observed behind an airplane flying in clear cold humid air. A contrail forms when water vapour produced by the combustion of fuel in airplane engines condenses upon soot particles or sulfur aerosols in the plane’s exhaust. When the ambient relative humidity is high, the resulting ice-crystal plume may last several hours. The trail may be distorted by the winds, and sometimes it spreads outward to form a layer of cirrus cloud. On rare occasions, when the air is nearly saturated with water vapour, air circulation at the wing tips of an airplane may cause sufficient pressure and temperature reductions to cause cloud streamers to form.

In the 1990s a popular conspiracy theory arose claiming that long-lasting contrails contained chemicals (“chemtrails”) that were sprayed by governments for purposes such as controlling the weather or dispersing drugs to influence the general population. However, atmospheric scientists have explained that some contrails last longer than others because of factors such as the humidity of the air (which can vary quite sharply over short distances) and the temperature of the airplane’s exhaust.