Pictures: First Night-Shining Clouds of 2012

Astronaut's-Eye View

Photograph courtesy NASA

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.

(Also see "Planes Create Weird Clouds—And Snow, Rain Fall Out.")

Published July 12, 2012

Special Delivery

Photograph courtesy Marek Nikodem

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.

(Related: "Secrets of Giant Cloud Holes Revealed.")

Published July 12, 2012

Noble View

Photograph courtesy Stuart Atkinson

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 Center 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 vapor in the air condenses to form ice clouds."

(Pictures: New Cloud Type Discovered?)

Published July 12, 2012

Latvian Sundown

Photograph by Sergejs Babikovs, My Shot

Noctilucent clouds float like flattened, sluggish auroras over Jurmala, Latvia, on June 18.

Although people have been living in northern latitudes for thousands of years, the first recorded observation of night-shining clouds was in 1885.

(Also see "UFO-Like Clouds Linked to Military Maneuvers?")

Published July 12, 2012

Front Row Seat

Photograph by Sergejs Babikovs, My Shot

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).

(See "Mysterious Clouds More Common Due to Climate Change?")

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 vapor that could form additional noctilucent clouds."

(Photos: NASA Rockets Make Weird Clouds Near Edge of Space.)

Published July 12, 2012

See More Noctilucent-Cloud Pictures

Photograph by Pekka Parviainen, TWAN

Published July 12, 2012