brainy marble [deleted] says:

Vapour trail - also called Condensation Trail, or Contrail, streamer of cloud sometimes observed behind an airplane flying in clear, cold, humid air. It forms upon condensation of the water vapour produced by the combustion of fuel in the airplane engines. When the ambient relative humidity is high, the resulting ice-crystal plume may last for several hours.

Let's take photos of them. Post your results here.
Originally posted at 8:09PM, 17 August 2007 PDT (permalink)
brainy marble edited this topic ages ago.