Contrails
The condensation trail left behind jet aircraft.
Illustration of contrails
Tim Festraets 2007
Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapour pressure temperature.
See also: http://profhorn.aos.wisc.edu/wxwise/AckermanKnox/chap15/contrail_applet.html. On this webpage, it is possible to "play" with a small applet to see under which meteorological conditions contrails form.
Belgium is situated right in the middle of the area in Europe where the highest air traffic is concentrated and thus where most contrails are forming. This area is expected to grow and be more intensely covered in the following years (see figure below from "Aviation and the global atmosphere", IPCC 1999).
Evolution of the contrail cover
=> Glossary