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September-October 2010

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Weather Front

Hole Clouds Tell a Seedy Story

Cloud seeding by aircraft is no longer just for weather modification programs in drought-stricken agricultural zones. According to a new study in the Bulletin of the American Meteorological Society, it happens all the time. Under certain atmospheric conditions, turboprop and jet aircraft cool the air in mid-level clouds and cause narrow bands of snow or rain to develop and fall to the ground. The holes or channels left behind in the clouds, referred to as hole-punch or canal clouds, have long baffled skywatchers. According to the study, they are a telltale sign of this inadvertent cloud seeding.

Super-cooled water droplets (reaching temperatures below about 5°F) make up the key ingredient for developing these holes in the clouds. As air is cooled behind aircraft propellers or over jet wings, the water droplets freeze and drop toward Earth.

“Any time aircraft fly through these specific conditions, they are altering the clouds in a way that can result in enhanced precipitation nearby,” said Andrew Heymsfield, an NCAR scientist and lead author of the study. “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.”

KIMBRA CUTLIP is a freelance writer and former assistant editor for Weatherwise.

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