Other phenomena, such as rocket launches, can set the stage for night-shining clouds. Above, the space shuttle Endeavour'
s exhaust interacts with the cold atmosphere above Florida'
s Kennedy Space Center to form the unusual clouds in an undated photograph.
In addition, more night-shining clouds tend to light up the skies during times when the sun is quiet, according to Daniel Marsh
of the National Center for Atmospheric Research in Boulder, Colorado.
(Related: "Sun Oddly Quiet -- Hints at Next 'Little Ice Age'?"
That's because when solar activity is most intense, ultraviolet radiation breaks up the air's water molecules and prevents the clouds from forming, Marsh said.
Photograph by Mike Theiss, National Geographic Stock