Mon Jun 14, 2010
Aircraft can accidentally punch holes in clouds, leaving a trail of snow or rain in their wake, a new study finds. Turboprop and jet aircraft that climb or descend under certain atmospheric conditions can inadvertently trigger what's known as cloud seeding. This technology is usually associated with scemes to control whether. However, cloud seeding can happen by accident as planes soaring through mid-level clouds leave behind odd-shaped holes or channels in the clouds and cause narrow bands of snow or rain to develop and fall to the ground.Holes punched in clouds are a phenomenon that has been recognized for many years and seen in photosfrom around the world.
A front-page feature on Yahoo! carried the headline "A Halo over Moscow" after photos emerged of just such a hole in October 2009. The secret behind these mysterious clouds has now been revealed: Supercooled water droplets that remain liquid even at subfreezing temperatures - below about 5 degrees Fahrenheit (minus 15 degrees Celsius). When an airplane cuts through clouds containing the supercooled water droplets, air is cooled behind aircraftpropellers or over jet wings, and these 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 study co-author Andrew Heymsfield of the National Center for Atmospheric Research (NCAR) in Boulder, Colo.Hole-punched clouds and accidental cloud seeding may be more common in regions such as the Pacific Northwest and western Europe where cloud layers with supercooled droplets are more common, Heymsfield said.
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