Darkwing theducksmail"AT"yahoo.com wrote:
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!

--------------------------------------
DW

Highly simplified explanation:

Technically, contrails are formed at altitude from exhaust water
vapor and what you see at low altitudes are vortices usually from
wing tips.

As things like wings move through the air they cause a pressure
drop around them which in turn decreases the air temperature.

If the temperature drop is enough to get below the dew point, the
water vapor condenses any you get visible "fog" for lack of a better
term.

For a more detailed explanation:

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

--
Jim Pennino

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