When you are outside do you ever take the time to pause and look up into the sky? If you have, and it is clear to at least partly cloudy, you have likely at some time or another seen white streaks which are trailing a hardly discernible spec. Of course, the "spec" is a high flying jet and the white streaks are its contrails. A little internet searching reveals that a plane creates contrails when water vapor quickly condenses around soot from the plane's exhaust and freezes. If you are a frequent sky watcher, you have probably noticed that contrails do not always accompany jets. Sometimes they stretch from horizon to horizon and last for hours while at other times they will trail the jet for a short distance before dissipating after only a few minutes.