Polar Mesospheric Clouds
Polar mesospheric clouds (PMCs)—also called noctilucent clouds—are very thin ice clouds that form around 83 km near the polar summer mesopause. The polar summer mesopause is the coldest region of the atmosphere because of cooling that results from the gravity wave-driven circulation in the mesosphere. This circulation is characterized by ascent at the summer pole, summer-to-winter meridional flow, and descent at the winter pole. The cold temperatures enable ice crystals to form even in the very dry mesosphere.
Polar mesospheric clouds in the Northern Hemisphere on 3 July 2014.
