This unusual hole punched in a cloud may have been caused by the cloud-seeding effect of aircraft.
Various explanations have been proposed for such holes, from shock waves produced by jet engines to local warming of the air.
Andrew Heymsfield of the National Center for Atmospheric Research in Boulder, Colorado, and colleagues studied a hole in clouds that appeared above the state capital, Denver. Flight records revealed two aircraft had flown nearby just before it formed. They also measured a band of snowfall minutes after the aircraft passed by.
The team suggest that air was expanded as it passed the propellers and wings of the planes. This caused it to cool enough for cloud droplets to freeze and fall as snow, forming a hole in the cloud.
Journal reference: Bulletin of the American Meteorological Society, DOI: 10.1175/2009bams2905.1