Monday, June 23, 2014

Mysterious 'Noctilucent' Clouds Have Returned

Mysterious 'Noctilucent Clouds Have Returned

By Joe Rao
Space.com
6-22-14

      Earth's polar skies have shined with eerie blue-white glowing clouds slowly twisting and undulating in the twilight sky every summer since the late 19th century.

These mystifying clouds are referred to as night-shining or noctilucent clouds. Such clouds form in an upper layer of the Earth’s atmosphere called the mesosphere during the summer and can be seen from the high latitudes on Earth.

If skywatchers want to possibly see noctilucent clouds, four criteria must be met: The sky must be free of tropospheric ("ordinary") clouds. The region of the atmosphere where they form must be sunlit. This means that the sun must be no more than 16 degrees below the horizon. The background sky must be adequately dark enough for the clouds to stand out. This final requirement means that the sun must be at least 6 degrees below the horizon, what astronomers refer to as the end of civil twilight. Your viewing location should be at a latitude north of 45 degrees, although as you will soon see the clouds have been sighted at more southerly latitudes in recent years.



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