Dash8 From New Zealand, joined Aug 2005, 1 posts, RR: 0 Reply 1, posted (13 years 4 weeks 20 hours ago) and read 715 times:
The total amount of moisture air can carry depends on it's temperature and pressure. The colder the air, the less it can carry, the lower the pressure, the less it can carry.
When you pass an airfoil through the air at 200 mph it'll create large pressure differences in the air it travels through.
At the top of the airfoil you'll find a shrarp drop in pressure (that's how wings work). The sharp pressure drop causes a natural temperature drop also.'
So if the pressure and temp of the air drop enough, moisture that was already in the air will now become access and therefore start transforming from gas to liquid state. And that is what you see.
Because the air is disturbed for only a short time, moisture is absorbed back by the air after the airfoil passes.