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.
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."
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."