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Nacreous clouds, sometimes called mother-of-pearl clouds, are rare but once seen are never forgotten. They are mostly visible within two hours after sunset or before dawn when they blaze unbelievably bright with vivid and slowly shifting iridescent colours. They are filmy sheets slowly curling and uncurling, stretching and contracting in the semi-dark sky. Compared with dark scudding low altitude clouds that might be present, nacreous clouds stand majestically in almost the same place - an indicator of their great height.
They need the very frigid regions of the lower stratosphere some 15 - 25 km (9 -16 mile) high and well above tropospheric clouds. They are so bright after sunset and before dawn because at those heights they are still sunlit.
They are seen mostly during winter at high latitudes like Scandinavia, Iceland, Alaska and Northern Canada. Sometimes, however, they occur as far south as England. They can be less rare downwind of mountain ranges. Elsewhere their appearance is often associated with severe tropospheric winds and storms.
Nacreous clouds far outshine and have much more vivid colours than ordinary iridescent clouds which are very much poor relations and seen frequently all over the world.
Caption: View of a spectacular nocti-lucent cloud after sunset. This cloud was formed from the exhaust of a missile launched from a distant firing range. Noctilucent clouds are those which reflect the Sun's light after it has set, which requires that they be at high altitude.
The uppermost parts of this cloud are iridescent (nacreous), having the appearance of mother-of- pearl. Natural nacreous clouds occur at altitudes of 20-25 kilometres. The lower parts of this cloud are redder due to the scattering of blue light by the large amount of dust and water in the lower atmosphere. Its convoluted shape is due to the differing wind speeds at different altitudes. Source