Nacreous clouds.

Nacreous clouds or mother of pearl clouds.

Nacreous clouds occur in the low stratosphere at an altitude of 15-30 km. They look like thin cirrus clouds often with strong irisation. The vivid colours are an effect of the light's diffraction by the small particles of similar size which the clouds are composed of. They can remain visible for as long as two hours after sunset or before sunrise because at their great altitude they are in sunlight. Nacreous clouds usually develop behind bigger mountain chains where the air is forced upwards by the wind into the stratosphere. They are normally seen during winter between latitudes 45 - 70° north and south

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