Contrails are the linear stream of fumes that is formed from the exhaust of an aircraft. Contrail is the short for "condensation trails". The line-shaped clouds are produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruise altitudes. This occurs several kilometers above the surface of the earth.
The exhaust from a jet engine becomes visible because of water vapors condensation. In a jet engine, the air taken in at the intake is compressed. Thereafter the temperature goes up. As the petroleum fuel is burnt, the temperature increases further. This hot air, sometimes around 1500 degree centigrade gets expanded when passes through a turbine and then through a nozzle.
By the time the exhaust comes out, it carries a temperature around 500 degree centigrade. This exhaust has water content due to combustion. Due to air friction, the exhaust air slows down, increases in size, mixes with surrounding air and becomes cooler. If there is sufficient water vapor in the jet exhaust to reach saturation when cooled, it condenses and forms tiny ice crystals. Sometimes impurities in the engine exhaust like sulfur compounds aid formation of water droplets. The major component of contrails is water in the form of ice crystals. The water vapor content in the engine exhaust and the low ambient temperature at high altitudes leads to formation of contrails. The white streams become viewable due to the presence of ice crystals. During takeoff or landing, black smoke is seen in the jet exhaust. This exhaust has no role in the formation of contrails. The contrails are formed only high altitudes from 8 to 12 km where the temperature is normally (-)40 degree centigrade or lower.
If the humidity is high, apart from contrails, vortices at the wing tip also become visible. This is due to water vapour condensation due to low pressure and temperature in the vortex. Such short lived trails are formed at low altitudes.
In jet exhaust, it takes some time for the air to cool and form ice crystals. Due to this, contrails are seen after a gap. The duration of contrails depends on the ambient temperature, wind conditions and humidity. Contrails, and other clouds formed as a result of human activity, are collectively named homogenitus.