Mxsmanic wrote in
:

"Darkwing" theducksmail"AT"yahoo.com writes:

Is there a certain speed an aircraft must be going for contrails to
form?

No. They depend mostly on atmospheric conditions.

I have never seen a 172 with contrails but as a kid I used to go to
Road America with my dad and watch the (then CART Indycars) and in
the morning you could see the contrails off the rear wings, so that
tells me it has something to do with speed/humidity nothing to do
with altitude.

It has to do with all of these. Any combination of conditions that
encourages water to condense out of the air (including water in engine
exhaust).


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