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.Where does the water and ice go? It falls to Earth, either as snow or rain -- sometimes a fair amount of it.