Shimmering noctilucent, or "night shining,"clouds (seen above in Finland in an undated photograph) have long graced high-latitude sunsets around the globe.
Scientists suspect that the increase in night-shining clouds may be due to climate change: Even as surface temperatures rise, the upper atmosphere is getting colder due to the buildup of carbon dioxide, creating perfect conditions for cloud formation, experts say.
Too thin and wispy to be seen during the day, night-shining clouds appear after sunset (above, the clouds brighten a Swedishsummer night in an undated photograph). Night-shining clouds are high enough in the atmosphere that the sun still hits them, even though it's dark on the ground.
"These clouds exist literally on the edge of space," AIM's Russell said, adding that the clouds form only in a very narrow band a little more than 50 miles (80 kilometers) above Earth's surface.
Once seen mostly in the Arctic, night-shining clouds are now appearing more frequently at lower latitudes, including the above clouds photographed in Hungary on June 15, 2007.
The wispy clouds form in cold, dry conditions: Temperatures high in the atmosphere are well below -200 degrees Fahrenheit (-129 degrees Celsius), and the air is a million times drier than the Sahara, AIM's Russell said.
Since 2007, scientists using the AIM satellite have been documenting night-shining clouds as seen from space, such as the above clouds (seen in white and blue) over the Arctic regions of Europe and North America on June 11, 2007--four days before the previous photograph was taken.
Based on five polar seasons of data, the satellite has revealed that the clouds' seasonal appearances turn on and off as abruptly as a "geophysical light bulb," according to the AIM Web site.
High-altitude night-shining clouds (such as those seen above in an astronaut's photo) are similar in structure to lower-level clouds--a fact that is "startling," said AIM deputy principal investigator Scott Bailey, of Virginia Polytechnic Institute and State University. That's because the two types of clouds form under such radically different conditions, Bailey said.
AIM's data on night-shining clouds have told scientists a lot about the upper atmosphere, he added: "The processes that control these clouds are very likely similar to the ones that control clouds down near the surface of Earth."
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 aboveFlorida's Kennedy Space Center to form the unusual clouds in an undated photograph.
Volcanoes also inject water vapor into the upper atmosphere, which can lead to night-shining clouds.
In 1883, the eruption of the Indonesian volcano Mount Krakatau (Krakatoa) caused widespread reverberations in climate, including night-shining clouds like those seen from the United Kingdom in this 1885 sketch by William Ascroft, an artist who recorded phenomena thought to have been linked to the eruption.
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