Motion through clouds can expand air and, through this expansion, the air is cooled to such a low temperature that vapor condenses to liquid and even forms ice crystals.
Water Droplets seen in Clouds That Planes Penetrated
Source - http://www.youtube.com/watch?v=pYWAR2OiiNg&feature=mfu_in_order&list=UL
As reported in the LA Times (2 July 2009 issue), a study has been completed by the National Center for Atmospheric Research in Boulder, Colorado, that links planes to precipitation.
The study focused on satellite images taken over a four-hour period of a cloud layer over Texas. The images revealed a series of holes and canals in the cloud layer, that upon analysis, were shown to be made up primarily of liquid water droplets.
Scientists measured the holes and canals, and through a review of FAA records, identified a full array of aircraft that were involved.
Expanding Air Enables Flight and Condensation
Computer models built as part of the study showed that the very same mechanisms that keep airplane aloft had cooled the air to the extent necessary to condense water vapor. Cooling of air was the result of ambient air being forced to expand around propeller tips and over a jet wing.
Condensation Nuclei NOT Needed
Typically, an increase in collisions between water vapor molecules and condensation nuclei is required to convert vapor molecules to liquid molecules. But, as this research shows, rapid air expansion by motion of airplanes can produce precipitation without requiring condensation nuclei.
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