If you've ever been lucky enough to see a hole-punch cloud form in the afternoon sky (above), you'd be forgiven for thinking a UFO landed somewhere near by. But according to a new study, the clouds form when much more pedestrian flying objects -- turboprop and jet airplanes -- fly through and change water droplets into ice crystals.
Droplets in many clouds exist in a strange supercooled state; they can be as cold as -34 degrees C (-35 degrees F), well below freezing, and yet remain in liquid form. When airplanes come cruising through, they can cause a quick drop in temperature that freezes the droplets. Suddenly the cloud is populated by ice crystals. Droplets begin condensing around them in a chain reaction and then -- poof! -- a hole of blue sky appears where fluffy white cloud had been.
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Where does the water and ice go? It falls to Earth, either as snow or rain -- sometimes a fair amount of it.
In 2007 Andrew Heymsfield, of the National Center for Atmospheric Research (NCAR) in Colorado was a on a research flight west of Denver International Airport when he and his team flew right below a big hole-punch cloud. When they went back and looked at footage from a ground-looking camera on the plane, they found the area directly beneath the hole had been coated in two inches of fresh snow.
Of course, not all planes that fly through clouds seed them this way. Generally speaking, there's a "goldilocks" set of conditions that must be met -- clouds that are too high in the atmosphere are too cold, and may already be frozen. Low clouds are warmer, and a plane's disturbance won't produce ice.
Turboprop planes are better than jets for producing the effect, too, because they tend to fly at the right altitude to encounter clouds with supercooled droplets, and they generate thrust by pushing large amounts of air that cools in their wakes (as opposed to heated jet exhaust).
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In some cases, planes flying through a cloud for an extended period form "canal clouds" that can be tens of miles long, and visible from satellite. The circular hole punch form probably forms when planes ascend or descend through a cloud layer.
(The image below, from the study, was taken by NASA's Terra satellite in 2007, and shows a cloud bank riddled with canals and hole punches. State boundaries are shown in blue.)