Streaks of condensed water vapor created in the air by jet airplanes at high altitudes. (Merriam-Websters)
Streamer of cloud sometimes observed behind an airplane flying in clear, cold, humid air. (Encyclopaedia Britannica)
A visible cloud streak, usually brilliantly white in color, which trails behind a missile or other vehicle in flight under certain conditions. (DOD Dictionary of Military Terms)
Contrails can exist in two forms: water droplet and ice crystal.
Under what conditions do Contrails form?
The primary factors in contrail formation are air temperature and moisture content. They are usually seen at the higher colder altidtudes, but will even occur at ground level in Antartica, sometimes causing a visibility problem for jets that take too long to take-off. Contrails started becoming a common sight during World War II, when bombers started flying at altitudes above 30,000 feet. They can exist in two forms: water and ice. A water droplet contrail occurs when an airplane flies though cold and supersaturated moist air and the warm water vapour produced by the engine condenses into tiny droplets. Under colder temperatures the water will freeze creating suspended ice-crystals.
The following graph represents contrail data collected for Houston, TX over several months. See the Trail Research Report for full details.
Temperature and Humidity (degrees of separation between Dew Point and Temperature).