From Scientific American:
Anyone who has ever seen a streaky line of vapor, known as a contrail, behind a high-flying aircraft knows that airplanes can produce their own clouds. But in rarer cases aircraft can also punch round holes or carve long channels through existing, natural clouds.
Those hole-punch and canal clouds arise from the strong cooling effects of airflow past a plane's propeller or over a jetliner's wing, according to a new study. That cooling can spontaneously freeze water droplets in the cloud and stimulate precipitation, the study's authors say.
The phenomenon requires a very specific set of cloud conditions and so is unlikely to have significant large-scale effects, but it could have an impact on regional weather near airports. A team of researchers from the National Center for Atmospheric Research (NCAR) in Boulder, Colo., the NASA Langley Research Center in Hampton, Va., and the University of Wyoming in Laramie report the new findings on the inadvertent aircraft cloud seeding in the July 1 issue of Science.