My previous post showed spectacular views of these rare clouds (see also this post). The following article presents 6 more images of those sighted this year.
Pictures: First Night-Shining Clouds of 2012
By Richard A. Lovett, National Geographic News, 12 July 2012.
By Richard A. Lovett, National Geographic News, 12 July 2012.
1. Rippling Over Riga
Each summer high-north residents are treated to noctilucent, or night-shining, clouds, such as these pictured above Riga, Latvia, earlier this month - among the first observed this year.
Too thin and wispy to be seen during the day, noctilucent clouds are high enough that the sun's steeply raked post-sunset rays hit the clouds even after the ground has gone dark.
Forming from ice crystals, the rarely seen clouds waft through the mesosphere, slightly more than 50 miles (80 kilometres) above Earth's surface - making them the world's highest known clouds.
2. Astronaut's-Eye View
Photographed June 13 by an astronaut aboard the International Space Station, night-shining clouds form an electric blue boundary against the blackness of space. (See another astronaut picture of noctilucent clouds.)
Noctilucent clouds also form in the Southern Hemisphere, but they're less often photographed there, because relatively few people live far enough south to see the atmospheric oddities.
3. Special Delivery
Night-shining clouds hover over Szubin, Poland - and a pair of nesting storks - on June 17. Where the luminous clouds form, the air is a hundred times less dense than ground-level air, and temperatures are well below -200°F (-130°C).
What's more, the water needed to form the clouds is scarce at these heights. "The air is a million times drier than the Sahara," James Russell III of Virginia's Hampton University told National Geographic News in 2009.
Too thin to be visible during the day, noctilucent clouds stand out against the darkening sky long after lower clouds (pictured above near the horizon) have gone dark.
4. Noble View
Pictured from the U.K.'s Kendal Castle, night-shining clouds ripple over the county of Cumbria on June 26.
Noctilucent clouds form most easily when the upper-level air is extremely cold. Ironically that happens in summer, said atmospheric chemist Dan Marsh of the U.S. National Centre for Atmospheric Research.
"The reason it is cold during summer is somewhat complex," he said by email, adding that it's related to the increased upwelling of polar air during the warmer months.
"Ascending air cools, and as it does, the water vapour in the air condenses to form ice clouds."
5. Latvian Sundown
Although people have been living in northern latitudes for thousands of years, the first recorded observation of night-shining clouds was in 1885.
6. Front Row Seat
In recent decades noctilucent clouds - such as these over Jurmala, Latvia, on June 21 - have become more common, leading to speculation that they might be linked to climate change (interactive).
Such speculation makes sense, scientists say. By causing the upper atmosphere to radiate more energy into space, rising carbon dioxide levels actually cool this layer of the sky.
Alternatively, the cause could be methane, a gas whose concentration in the atmosphere has more than doubled since 1750, according to the U.S. Environmental Protection Agency. The increase is likely due in part to industrialization and emissions triggered by the thawing of formerly frozen lands.
When methane reaches the stratosphere, just below the mesosphere, the greenhouse gas breaks down, Marsh said, "creating more water vapour that could form additional noctilucent clouds."
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