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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.

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