Richard in San Jose wanted to know what the white streams are that high-flying jets trail behind them, and how the term came to be. The answer is that the contrails are formed from the water vapor expelled from the engines, which is left over after the fuel has been burned, but this only occurs at certain altitudes and atmospheric conditions. The low temperature at such altitudes causes the water to turn to ice crystals, forming a trail that is essentially a mini cloud. The term is a combination of condensation and trail.
Richard also wanted to know what the similar trails were that spiraled off the wingtips of aircraft. These are caused because there is a pressure drop over the surface of the wing (that’s how it gets lift), which under some circumstances, will cause the air cool enough (remember that lower pressures result in lower temperatures, all other input or output of energy being the same) that the suspended will form ice crystals, again forming mini clouds.
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