Strange circular holes in clouds which have frequently led to speculation about UFOs are caused by nothing more mysterious than aircraft, a study has shown - and they can cause several inches of snow or rain too.
New research shows that as airplanes climb or descend under certain atmospheric conditions, they can inadvertently 'seed' mid-level clouds, causing narrow bands of rain to develop. As they do so, they leave 'hole-punch' gaps in the clouds.
As air is cooled to below five degrees Farenheit behind aircraft propellers or over jet wings, the water droplets freeze and drop toward Earth.
"Any time aircraft fly through these specific conditions, they are altering the clouds in a way that can result in enhanced precipitation nearby," says Andrew Heymsfield, a scientist with the National Center for Atmospheric Research (NCAR).
Sightings of blue-sky holes piercing a cloud layer have been explained as 'aerial crop circles' by some.
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.
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.
While they did not notice anything unusual at the time of their 2007 flight, a subsequent review of data from a ground-based radar in the area revealed an unusual echo, indicating that the band of precipitation had evolved quickly and was unusually shaped.
The team went back through data from their aircraft's forward-viewing camera and noticed a hole in an otherwise-solid deck of altocumulus clouds, as well as a burst of snow that extended to the ground.
Since the hole was oriented in the same direction as the standard flight tracks of commercial aircraft in the region, Heymsfield surmised that a plane flying through the cloud might have somehow caused ice particles to form and "snow out" along its path, leaving a canal-shaped, hole-punch cloud behind.
A subsequent review of flight track records from the Federal Aviation Administration revealed that turboprop planes operated by two airlines flew close to the hole-punch location.
Snow crystals began falling about five minutes after the second aircraft flew through the cloud. The snowfall, in a band about 20 miles long and 2.5 miles wide, continued for about 45 minutes, resulting in about two inches of snow on the ground.
The researchers also examined data from onboard spectrometers that profiled the snowflakes in the band of snow beneath the hole-punch.
They found that these flakes showed evidence of riming - accumulation of liquid water - whereas ice particles elsewhere in the cloud showed little or none.
"This tells us that the aircraft literally 'seeded' the cloud just by flying through it," Heymsfield says.