When aircraft fly at cool high levels their engines emit water vapor. In addition to water vapor, certain gases emitted from the engine or already present in the air are also important because they are needed for droplets to form (condense) on.

A contrail will only form if atmospheric conditions are right. Exhaust gasses must be warm and moist enough and the surrounding air must be cool enough for the water to condense.

If conditions are right, the droplets rapidly freeze to form the ice particles that make up the contrail.  

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The photograph above and below were taken in Portland, Oregon, April 2009

Once it forms, the life of a contrail also depends on the atmospheric conditions of the moment.  If humidity is low, the contrail will quickly evaporate.  Under these conditions one can observe a short, sharp line-shaped contrail that quickly disappears.

If the humidity is high, as in the photographs on this page, the contrail will continue to develop, with the newly formed ice particles growing in size by accumulating water from the surrounding atmosphere.

Growing contrails will extend long distances behind the producing aircraft and can persist for many hours, growing to several miles in width and as much as 1000-1500 feet in height.