The truth is up there - and it isn't alien spaceships or covert government operations.
"Hole punch" clouds, so named for gaps in clouds that appear to be made by a giant hole punch, have fascinated and worried people around the world for decades, leading to speculation about UFOs and other sinister plots.
In October 2009, a hole-punch cloud caused a furor in Russia, sparking such headlines as "UFO cloud hovers over Moscow" in the China Daily newspaper.
Now a study led by scientist Andrew Heymsfield of the Boulder, Colo.-based National Center for Atmospheric Research found that airplanes taking off or landing are the likely cause of these cloud holes. The airplanes can also cause additional rain or snow to fall after creating the holes.
The research determined that precipitation is triggered by water droplets at extremely cold temperatures, below about 5 degrees. As the air is cooled behind the planes, the water droplets freeze and drop toward Earth.
"Anytime aircraft fly through these specific conditions, they are altering the clouds in a way that can result in enhanced precipitation nearby," Heymsfield says.
Through this inadvertent "cloud seeding" process, planes also leave holes or channels in the clouds.
"Any airplanes, whether jet or propeller, can make these things," Heymsfield says. "They often occur near the end of airport runways." Common locations are across the southern USA, he says.
Heymsfield's discovery was made after flying aboard a research aircraft near Denver in December 2007 when he and his colleagues observed a cloud hole, along with snow that fell after other airplanes passed through the clouds.
Because the hole was oriented in the same direction as the flight tracks of commercial airplanes in the area, Heymsfield surmised that an airplane flying through the cloud likely caused ice particles to form and "snow out" along its path, leaving a hole-punch cloud behind.
The snowfall, in a band about 20 miles long and 2.5 miles wide, continued for about 45 minutes, leaving about 2 inches of new snow on the ground directly under the hole. There was only very light snow elsewhere in the area, Heymsfield says, and none that accumulated.
"This tells us that the aircraft literally 'seeded' the cloud just by flying through it," he says.
Could airplanes inadvertently be adding to the USA's precipitation totals each year?
"Although this process might not generate much additional snow over the USA each year, where it could have an effect is at airports in the wintertime when there might be instances that de-icing operations are needed," he says.
As far back as the 1940s, scientists have wondered about the causes of these clouds.
"These holes did not exist before planes," Heymsfield says. "Before World War II, there were no documented sightings or reports of these clouds."
The study appeared in the Bulletin of the American Meteorological Society.
