Why do planes leave a vapor trail?

Dear Matt:

Why do some high-flying large planes leave vapor trails that remain long after the plane is out of sight, while others leave vapor trails that dissipate quickly just a short distance behind the plane?

-- G.S., Del Mar

The rain that's on your brain stays mainly behind the plane when it's cold, I'm told. Ya see, when a plane zips through the sky, it creates its own low-pressure system above the wing. This lowers the temp and condenses water out of the air. If the plane's flying high enough and the circumstances are right, the trail of water vapor turns to ice crystals, which will persist for a while.