Pick of the pics
Aircraft can seed precipitation from clouds
A hole left in a cloud-layer after an aircraft has passed through, provoking snowfall – or rainfall if the ice crystals melt on the way down to the ground. Photo: hole in altocumulus clouds over Mobile, Alabama, from 35 mm negative film, by Alan Sealls, chief meteorologist, WKRG-TV, Pensacola, Florida.
In 2007, meteorologists noticed a connection between snowfall near Denver, Colorado, and a turboprop aircraft descending through a cloud layer. The snowfall, in a band about 20 miles long and 2.5 miles wide, continued for about 45 minutes, resulting in about two inches of snow on the ground. They eventually figured out that when a turboprop plane flies through such a cloud layer with supercooled water droplets at about -15 degrees C, the tips of its propellers cause the air to expand, cool and freeze the droplets. Since then they have concluded that the expansion of air over a jet aircraft’s wings can have the same effect in colder conditions, -20 to -25C. Aircraft seeding may be particularly common in the northwestern USA and Western Europe.
Andrew Heymsfield, lead author of the report, is from National Center for Atmospheric Research (NCAR) in Boulder, Colorado.
Andrew Heymsfield et al., “Aircraft-Induced Hole Punch and Canal Clouds: Inadvertent Cloud Seeding”, Bulletin of the American Meteorological Society, June 2010
More info and a video here: http://www2.ucar.edu/news/mysterious-clouds-produced-when-aircraft-inadvertently-cause-rain-or-snow