Noctilucent clouds are accumulations of ice crystals above the mesosphere in the mesopause. There the absolute minimum temperature of the Earth’s atmosphere is reached. They appear at a height of 81 to 85 km – in contrast to the cloud forms of the troposphere, which reach a maximum height of 13 km. They are rarely observed, but mostly in the months around the summer solstice.
Pictures: Klaus Vollmann, member of the STScI. Text: Wikipedia.
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