Noctilucent Clouds
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Noctilucent clouds were only fairly recently discovered. They are extremely high thin clouds. About 50 miles (85 km) high. They are so high that not even weather balloons can reach them.
Noctilucent clouds were only fairly recently discovered. They are extremely high thin clouds. About 50 miles (85 km) high. They are so high that not even weather balloons can reach them.
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Scientists used to think that water based clouds 
could not exist at such a high altitude where the atmospheric pressue is so low. But now they know they do. Noctilucent clouds are made of water ice crystals.
These clouds are so thin that they are not visible in daylight. The only time one can normally see noctilucent clouds is when the sun is well below the horizon, i.e. deep twilight.
LINK: http://en.wikipedia.org/wiki/Noctilucent_cloud
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could not exist at such a high altitude where the atmospheric pressue is so low. But now they know they do. Noctilucent clouds are made of water ice crystals.These clouds are so thin that they are not visible in daylight. The only time one can normally see noctilucent clouds is when the sun is well below the horizon, i.e. deep twilight.
LINK: http://en.wikipedia.org/wiki/Noctilucent_cloud
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