Aeroplane trails

[thelma2310]

Why do some aeroplanes leave white trails in the sky?  Is it only certain types of plane or is it to do with the height at which they fly?

[Clanad]

The white vapor trails are called contrails and generally form through the injection of water vapor into the atmosphere by exhaust fumes from a jet engine. If there is sufficient mixing between the cold upper tropospheric air and the hot exhaust gases to produce a state of saturation, ice crystals will develop. If sufficient water vapor, in the form of super-cooled water droplets exists in the atmosphere, contrails can be produced simply by the mechanical mixing action of the wing moving trough such saturated air.  this is usually at much lower altitudes and the aircraft need not be jet powered...

[Dacetine]

Interestingly these contrails can seed full-blown cloud formation (perhaps based initially around particulates in the jet exhaust fumes). Aeroplane activity contributes a measurable amount to global cloud cover because of this.