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Noctilucent Clouds / Nattlysende Skyer
Noctilucent Clouds (NLC) are known as shining clouds. They are the uppermost clouds on our planet. In fact they borderline space at an altitude ranging up to 85 km, in a layer called the Mesosphere. The Mesosphere is very cold (-130 C) and dry. Still, NLC is a condensation phenomena. The condensated ice is made of water.
It is still a mystery how these ice crystals form in the dry and cold environment of the Mesosphere. The Mesosphere is known to be millions of times drier than the Sahara desert.
Many theories have been launched to explain the formation of NLC. One of the most plausible theories is that the condensation base comes from space, the Earth receives tons of meteroids from comets and asteroids on an every day basis. These small debris are the right size to work as a condensation base and hence form the NLCs.
If you would like to observe noctilucent clouds you should be on alert from mid-May to mid-August. That is, if you live on latitudes between 50 - 65 degrees on both hemispheres. They are primarily visible on these latitudes at this time(s) of the year, because the Sun will have to be between 6 to 16 degrees below horizon to illuminate the clouds against the summer sky.
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