Image of the Day: Rarely Seen Night-Shining Clouds
You might think you know every type of cloud there is, from cumulus congestus to cirrostratus. Or perhaps you're someone who couldn't tell a cumulonimbus from a stratus. Regardless, chances are that you've never seen these clouds, which are only visible at twilight in late spring and early summer. These sinnewy and glowing clouds have several names. They're known as polar mesospheric clouds, and are also referred to as "noctilucent" ("night-shining") clouds. They occur high in the atmosphere (much higher than ordinary clouds), between 47 to 53 miles up where they catch light from the sun after sunset.
The exact cause of these clouds is unknown, but there is some speculation that by changing the gas composition of the atmosphere, global warming may be causing them to form more frequently, according to NASA. This particular photo was photographed by an astronaut aboard the International Space Station on June 13.