What causes the vapor trail from a high-flying jet?
We searched the Net to find the answer to your question and, in the process, we discovered a simple definition, some great photos, an amazing trove of related information, and a little-known community of vigilant vapor-trail watchers. Our first step was to search Yahoo! using the phrase “vapor trail.” That led us to a great little explanation from the University of Illinois that describes the birth of vapor trails, also known as “contrails”: Contrails form through the injection of water vapor into the atmosphere by exhaust fumes from a jet engine. If there is sufficient mixing between the cold upper tropospheric air and the hot exhaust gases to produce a state of saturation, ice crystals will develop. We were pleased to have found such a straightforward explanation, but, as always, we hungered for more. That meant trying another search. This
