Trails

[Duane Charles]

Morning all,
This comes in 2 parts:

1) At what height do most jetliners start to trail, is it about FL300 (30,000ft)?

2) Why is it that some jet aircraft produce a vapour trail and others don't when at this height and higher, is it weather related?

Apologies for sounding dim at this time of the morning
Duane

[mhm]

http://en.wikipedia.org/wiki/Contrail