Vapour Trails

[Ian King]

Why do somedays you see vapour trails on Passenger jets high in the sky and somedays you do not? How are they formed? Thanks.

[Wraith]

Vapour trails are artificial clouds made by the exhaust of jet aircraft or wingtip vortices which precipitate a stream of tiny ice crystals in moist, frigid upper air.

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Sample from this LINK

[bernardo]

If it doesn't, presumably because the air is not soggy enough.

[Drewhound]

I've noticed they seem to happen more often in an easterly weather stream but I've no idea why

[Ewood27]

There is a layer of air a few thousand feet thick in which con-trails can form. Above and below that layer the conditions are not right. The actual height and thickness of the layer varies from day to day, and the layer is lower in winter than in summer.

Another reason for not seeing them might be that it's cloudy!

[bluedolphin]

I read somewhere that they are a pretty good indicator of weather for the following day....if they linger for more than 30minutes or so u can be pretty sure of getting a drop of rain the next day!!!!!

[thekraut]

Well but you'd be on the safe side with that any day in britain, wouldn't you?