Secrets of hole-punch clouds - Boing Boing

What causes the bizarre "hole-punch" clouds like the ones seen above? The National Center for Atmospheric Research (NCAR) has just published a scientific paper addressing this wonder of nature. During certain conditions, flying airplanes can sometimes "seed" clouds, resulting in brief and highly-localized snow or rain falls. According to the NCAR, air is rapidly cooled behind the propellers of prop planes or as the wings of jets cut through the clouds. Water droplets in the cloud freeze and then fall to Earth, leaving the telltale hole in the cloud.