Noctilucent cloud
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Noctilucent clouds (also known as polar mesospheric clouds) are rare bright cloudlike atmospheric phenomena visible in a deep twilight (the name means roughly "night shining"). They are most commonly observed in the summer months at latitudes between 50° and 60° (north and south).
They are the highest clouds in the Earth's atmosphere, is located in the mesosphere at altitudes of around 85 kilometers, 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. Recent evidence also suggests that most noctilucent clouds today are created by water exhaust from manned spaceflight, such as the Space Shuttle.
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.