Turboprop and jet aircraft inadvertently seed mid-level clouds, leaving behind odd-shaped holes or channels in the clouds, said Andrew Heymsfield of the National Center for Atmospheric Research, which led the study.

The scientists said the key ingredient for developing the holes in clouds is water droplets at subfreezing temperatures, below about 5 degrees Fahrenheit. As air is cooled behind an aircraft's propellers or over a jet plane's wings, the water droplets freeze and fall toward Earth.

"Anytime aircraft fly through these specific conditions, they are altering the clouds in a way that can result in enhanced precipitation nearby," Heymsfield said. "Just by flying an airplane through these clouds, you could produce as much precipitation as with seeding materials along the same path in the cloud."

Precipitation from planes may be particularly common in regions such as the Pacific Northwest and western Europe because of the frequent occurrence of cloud layers with supercooled droplets, Heymsfield said.

The study that included scientists from Colorado State University and the University of Wyoming appears in the Bulletin of the American Meteorological Society.

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