Look up in the sky near an airport and you might see some unusual cloud formations. The one on the left is called a "hole-punch," and meteorologists have been speculating on the cause. They suggested that the holes may have been the result of shock waves from jets or warming of the air by jets.
Researchers from the National Center for Atmospheric Research and elsewhere now say that the odd-shaped clouds can be caused by either turboprop or jet aircraft as they pass through a particular type of cloud layer. Their study appears in the Bulletin of the American Meteorological Society.
On average, about 7.8 percent of the Earth is covered by midlevel liquid-layer topped stratiform clouds (those are the ones that look like a flat layer of cloud). The liquid is super-cooled, at a temperature below freezing but still in liquid form. When a plane passes nearby, pressure changes from the spinning turboprop or air passing over wings can cool the liquid even further, turning it into ice. That ice becomes the "seed" for precipitation. More water droplets condense and freeze on these seeds, forming snow. If the air below is warm enough, if melts into rain. The same process is also responsible for canal clouds, which are just a long and thin versions of the hole-punch.
The cloud layer needed for this phenomenon is especially common in the Pacific Northwest and western Europe. I'm off to Seattle this weekend; I think I'll have to check out the skies. (HT: Greg Laden)
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