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.   Theycan be less rare downwind of mountain ranges. Elsewhere their appearance is oftenassociated with severe tropospheric winds and storms.
Nacreous clouds far outshine and have much morevivid colours than ordinary iridescent cloudswhich are very much poor relations and seen frequently all over the world.

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