Noctilucent clouds are extremely high-altitude atmospheric formations that glow at night. Furthermore, although these fascinating “clouds ride in the sky above 99.9 percent of the atmosphere and over 40 miles above the highest clouds . . . [and] skirt the lowest fringes of the aurora,”—so they are certainly suggestive of the auroras or northern and southern lights—they are an entirely different phenomenon. According to Wikipedia, noctilucent clouds

are tenuous cloud-like phenomena that are the “ragged-edge” of a much brighter and pervasive polar cloud layer called polar mesospheric clouds in the upper atmosphere, visible in a deep twilight. They are made of crystals of water ice. The name means roughly night shining in Latin. They are most commonly observed in the summer months at latitudes between 50° and 70° north and south of the equator.

They are the highest clouds in the Earth’s atmosphere, located in the mesosphere at altitudes of around 76 to 85 kilometers (47 to 53 mi). They are normally too faint to be seen, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in the Earth’s shadow. Noctilucent clouds are not fully understood and are a recently discovered meteorological phenomenon; there is no evidence that they were observed before 1885.

Noctilucent clouds can form only under very restrictive conditions; their occurrence can be used as a sensitive guide to changes in the upper atmosphere. Since their discovery the occurrence of noctilucent clouds has been increasing in frequency, brightness and extent. It is theorized that this increase is connected to climate change.

Noctilucent clouds are increasing and spreading into other latitudes. The observance of these clouds is also linked to solar activity, but it is thought that an increase in greenhouse gases may play a role too. More from New Scientist:

The clouds were first seen above polar regions in 1885, suggesting they may have been caused by the eruption of Krakatoa two years before. But in recent years the clouds have spread to latitudes as low as 40°, while also growing in number and getting brighter. The reason for the clouds’ spread is unclear, but some suspect it could be due to an increase in greenhouse gases. That’s because the gases actually cause Earth’s upper atmosphere to cool, and the clouds need cold temperatures to form.

Although the average number of noctilucent clouds has been increasing in recent decades, their abundance also seems to rise and fall with the sun’s 11-year cycle of activity. The clouds thrive when the sun is quiet and spews less ultraviolet radiation, which can destroy water needed to form the clouds and can keep temperatures too high for ice particles to form.

Because the sun has been abnormally quiet in recent years, noctilucent clouds could be especially bright and numerous this summer in the Northern hemisphere.

A recent study suggests “the Tunguska explosion of 1908 [was] caused by a comet hitting Earth . . . based on the behaviour of water vapour from the space shuttle’s exhaust” forming noctilucent clouds. From Xenophilia:

The research, accepted for publication (June 24, 2009) by the journal Geophysical Research Letters, published by the American Geophysical Union, connects the two events by what followed each about a day later: brilliant, night-visible clouds, or noctilucent clouds, that are made up of ice particles and only form at very high altitudes and in extremely cold temperatures.

“It’s almost like putting together a 100-year-old murder mystery,” said Michael Kelley, the James A. Friend Family Distinguished Professor of Engineering at Cornell who led the research team. “The evidence is pretty strong that the Earth was hit by a comet in 1908.” Previous speculation had ranged from comets to meteors.

The researchers contend that the massive amount of water vapor spewed into the atmosphere by the comet’s icy nucleus was caught up in swirling eddies with tremendous energy by a process called two-dimensional turbulence, which explains why the noctilucent clouds formed a day later many thousands of miles away.

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The space shuttle exhaust plume, the researchers say, resembled the comet’s action.

A single space shuttle flight injects 300 metric tons of water vapor into the Earth’s thermosphere, and the water particles have been found to travel to the Arctic and Antarctic regions, where they form the clouds after settling into the mesosphere.

Kelley and collaborators saw the noctilucent cloud phenomenon days after the space shuttle Endeavour (STS-118) launched on Aug. 8, 2007. Similar cloud formations had been observed following launches in 1997 and 2003.

Following the 1908 explosion, known as the Tunguska Event, the night skies shone brightly for several days across Europe, particularly Great Britain — more than 3,000 miles away.

Noctilucent cloud video:

Video showing noctilucent clouds from space and a lightening storm in the lower atmosphere:

Tunguska explosion of 1908:

Here are some amazing images from Flickr showing noctilucent clouds viewed from the cockpit of an airplane.

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