These night-shining clouds were spotted over Billund, Denmark on July 15, 2010. These rare clouds are technically called "noctilucent" or "polar mesospheric" clouds, and form at high altitudes, 80 to 85 kilometers (50 to 53 miles) high, where the mesosphere is located. The clouds' high position in the atmosphere allows them to reflect sunlight long after the sun has dropped below the horizon. They only form when the temperature drops below –130 degrees Celsius (-200 degrees Fahrenheit), whereupon the scant amount of water high in the atmosphere freezes into ice clouds. This happens most often in countries at high northern and southern latitudes (above 50 degrees) in the summer, when the mesosphere is coldest.
Studies suggest that night-shining clouds are becoming brighter and more common, which is linked to the mesosphere getting colder and more humid. These changes may be happening because of increased levels of heat-trapping greenhouse gases such as carbon dioxide and methane in the atmosphere. In the mesosphere, carbon dioxide radiates heat into space, causing cooling. More methane, on the other hand, puts more water vapor into the atmosphere, because sunlight breaks methane up into water molecules at high altitudes. Research is ongoing.