Noctilucent clouds, also known as polar mesospheric clouds are bright cloud-like atmospheric phenomena visible in a deep twilight. The name means roughly "night shining" in the Latin language. They are most commonly observed in the summer months at latitudes between 50° and 60° north and south of the equator.
They are the highest clouds in the Earth's atmosphere, located in the mesosphere at altitudes of around 85 kilometres, and are visible only when illuminated by sunlight from below the horizon while the ground and lower layers of the atmosphere are in the Earth's shadow; otherwise they are too faint to be seen.
Noctilucent clouds are not fully understood, as under most understood meteorological concepts, clouds generally are not able to reach such high altitudes, especially under such thin air pressures.
As an explanation, it was once proposed that they were composed of volcanic or meteoric dust, but they are now known to be primarily composed of water ice (confirmed by UARS). They appear to be a relatively recent phenomenon - they were first reported in 1885, shortly after the eruption of Krakatoa - and it has been suggested that they may be related to climate change.
At least one researcher, Dr. Michael Stevens of the United States Naval Research Laboratory, believes space shuttle exhaust may contribute to the formation of noctilucent clouds.
Noctilucent clouds can be studied from the ground, from space, and in situ by sounding rockets; they are too high to be reached by weather balloons.
The AIM satellite mission, scheduled for launch in 2006, is dedicated to research into noctilucent clouds.
Photos: "Night Shining" Clouds Getting Brighter National Geographic - December 24, 2009
Secrets of Mysterious 'Night-Shining' Clouds Unlocked by NASA's AIM Satellite and Models Science Daily - December 16, 2009
Mysterious, Glowing Clouds Appear Across America’s Night Skies Wired - July 21, 2009
Noctilucent Cloud Storm Panorama NASA - July 11, 2009
Noctilucent Clouds Over Germany NASA - June 24, 2009
Mysterious Clouds More Common Due to Climate Change? National Geographic - December 12, 2007
Spacecraft chases highest clouds BBC - December 11, 2007
Mission to target highest clouds BBC - May 26, 2006 - June 13, 2006
The crew of the ISS have been observing electric-blue "noctilucent" clouds hovering on the edge of space NASA - February 19, 2003
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