Naive question about aircraft 'contrails'

[Robbie J]

I have often seen jets flying at high altitude with a 'double streak' of smoky contrails dragging behind the back of the jet. Sometimes the trails are quite long and do not dissipate for some time.

While I appreciate the visual effect, what is the purpose of these displays?

The jets appear to be quite small, flying at high altitude, and with no visible markings.

[Radi K]

http://en.wikipedia.org/wiki/Chemtrail_conspiracy_theory !!

:-D

[Brenden S]

It is the government spraying chemicals over the earth :D



Really it is just the moisture in the air becoming visible with the heat from the aircraft engine exhaust. It is all dependent on moisture content and also temperature of the ambient air.

[Nathan Long]

It's the water vapour produced by the combustion of fuel condensing into ice crystals.

[Steve S... 2]

As the average Joe would say - wow they are sky writers.

[Philip Argy]

http://en.wikipedia.org/wiki/Contrails :)