Most clouds, including all of the standard clouds, occur in the troposphere, the atmospheric layer closest to the ground. This is where almost all of the water vapor which produces these clouds is found. Clouds visible in the upper atmosphere (above the troposhere) include Polar stratospheric clouds, which form in the stratosphere between about 15 km (50 000 ft) and 30 km (150 000 ft) at high latitudes of both hemispheres during winter, and noctilucent clouds, which form in the mesosphere at altitudes of about 80–85 km (280 000–300 000 ft), at or near the mesopause in summer. Noctilucent clouds are also known as polar mesospheric clouds.
Polar stratospheric clouds only occur at very low temperatures, which typically occur in the polar stratospheric polar vortices that form in winter. They are more common in the Antarctic where the southern hemisphere polar vortex is geographically linked to the circumpolar land mass, thus allowing maximum winter cooling with little mixing in of warmer air from lower latitudes. They are less common in the Arctic because of a less stable and more mobile polar vortex. Polar stratospheric clouds are the only atmospheric cloud that exists both above and below the ice frost point.
Polar stratospheric clouds that form above the ice frost point are created through the co-condensation of nitric acid and water. Clouds that form below the ice point are composed primarily of ice because of the abundance of water vapour compared to nitric acid. Owing to their optical signature, these ice polar stratospheric clouds are also known as nacreous (mother-of-pearl) clouds. The two types of clouds were first detected by remote-sensing instruments and were called type 1 (nitric acid and water) and type 2 (ice) clouds. This nomenclature is no longer used in the light of better understanding of the particles in the different types of polar stratospheric clouds.