Noctilucent clouds, also known as polar mesospheric clouds, are
bright cloudlike atmospheric phenomena visible in a deep
twilight. 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 85
km, 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 a not fully understood
meteorological phenomenon. Clouds generally are not able to reach such high
altitudes, especially under such thin air pressures.
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