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

BTW, if you never seen an Indycar run at Road America in WI in person I
highly recommend it!

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DW