High Clouds

The highest clouds of all are rare and occur in the mesosphere at about 70-80 km elevation (Section 1.8). They are mainly seen in summer at latitudes above 50°, and are called noctilucent because they can be briefly observed with the naked eye only in the evening, when the Sun has just set at ground level but still shines on these high thin veils. They consist of extraterrestrial dust coated with ice.

Almost equally unusual and tenuous are the nacreous (or mother-of-pearl) clouds which form in the lower stratosphere, up to 30 km from the ground. The clouds are due to occasional insertions of ice crystals and aerosols from the troposphere below, which can happen through either volcanic eruptions or extraordinarily strong thunderstorms overshooting the tropopause.

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