A Cloud Mystery Takes Flight

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You know those weird round holes that show up once in a while in cold mid-level clouds and look like they have been punched through by a flying saucer heading home?

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Well, the Colorado researchers who quashed this perfectly fine flying saucer theory by tracing the phenomena to airplane turbulence now are able to account for the stuff that used to be in the hole.

A study of several commercial airports around the world shows that aircraft taking off and landing can, under certain atmospheric conditions, pierce the mid-level clouds that contain supercooled water and the turbulence can force the air to expand and the droplets to form ice crystals that fall out as snow or rain.

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Researchers led by cloud specialist Andrew Heymsfield at the National Center for Atmospheric Research in Boulder report in the journal Science that this rainmaking effect is small but measurable.

The team used cloud measurements taken from NASA’s Calipso satellite to determine when cloud layers were present and the air was cold enough to produce the “punched hole” effect around six airports: London Heathrow, Frankfurt, Charles De Gaulle-Paris, Seattle-Tacoma, Chicago O’Hare, Yellowknife Northwest Territories, Canada, and Byrd Station, Antarctica. At the coldest airports, Byrd Station and Yellowknife, ideal conditions existed most often.

Depending on the type of aircraft, the researchers found that the right atmospheric conditions exist about 6 percent of the time.

In a statement issued by NCAR, Heymsfield said the rainmaking effect “is not necessarily enough precipitation to affect global climate, but it is noticeable around major airports in the midlatitudes.” More research is needed, he said, but the effect could be large enough to prompt more de-icing of planes when these atmospheric conditions prevail.

IMAGES: Top, photo of punch-hole cloud that formed over Texas January 29, 2007; bottom, a computer simulation of the snowfall pattern from the cloud. Courtesy, American Association for the Advancement of Science.