Alan Sealls photo
What causes the bizarre "hole-punch" clouds like the one 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. From the National Science Foundation:
Precipitation from planes may be particularly common in regions such as the Pacific Northwest and western Europe because of the frequent occurrence of cloud layers with supercooled droplets, Heymsfield says…
Researchers have proposed a number of possible aviation-related causes, from acoustic shock waves produced by jets, to local warming of the air along a jet's path, to the formation of ice along jet contrails. Indeed, the earliest observations implicated jet aircraft, but not propeller aircraft, as producing the holes.
Researchers in the 1980s observed that propeller aircraft could transform supercooled droplets into ice crystals, and experiments were launched in the 1990s to characterize the phenomenon.
But scientists had not previously observed snow as it fell to the ground as a result of aircraft until Heymsfield and his colleagues happened to fly through some falling snow west of Denver International Airport with an array of instruments.