Polar mesospheric clouds

Polar mesospheric clouds (PMCs) are a diffuse scattering layer of water ice crystals near the summer polar mesopause. Polar mesospheric clouds (PMCs) are a diffuse scattering layer of water ice crystals near the summer polar mesopause. Observed from the ground, this phenomenon is known as noctilucent clouds. From satellites, PMCs are most frequently observed above 70°–75° in latitude and have a season of 60 to 80 days duration centered about a peak which occurs about 20 days after the summer solstice. This holds true for both hemispheres. Great variability in scattering is observed from day-to-day and year-to- year, but averaging over large time and space scales reveals a basic underlying symmetry and pattern. The long- term behaviour of polar mesospheric cloud frequency has been found to vary inversely with solar activity. PMC's have four major types based on physical structure and appearance. Type I veils are very tenuous and lack well-defined structure, somewhat like cirrostratus or poorly defined cirrus. Type II bands are long streaks that often occur in groups arranged roughly parallel to each other. They are usually more widely spaced than the bands or elements seen with cirrocumulus clouds. Type III billows are arrangements of closely spaced, roughly parallel short streaks that mostly resemble cirrus. Type IV whirls are partial or, more rarely, complete rings of cloud with dark centres.