nomenclature in the manner of tropospheric clouds, but rather descriptive names of several general forms using common English. Supercooled nitric acid and water PSC's, sometimes known as type 1, typically have a stratiform appearance resembling cirrostratus or haze, but because they are not frozen into crystals, do not show the pastel colours of the nacreous types. This type of PSC has been identified as a cause of ozone depletion in the stratosphere. The frozen nacreous types are typically very thin with mother-of-pearl colorations and an undulating cirriform or lenticular (stratocumuliform) appearance. These are sometimes known as type 2. Mesospheric classification and distribution Noctilucent clouds are the highest in the atmosphere and are found near the top of the mesosphere at about or roughly ten times the altitude of tropospheric high clouds. They are given this Latin derived name because of their illumination well after sunset and before sunrise. They typically have a bluish or silvery white coloration that can resemble brightly illuminated cirrus. Noctilucent clouds may occasionally take on more of a red or orange hue. They are not common or widespread enough to have a significant effect on climate. However, an increasing frequency of occurrence of noctilucent clouds since the 19th century may be the result of climate change. Ongoing research indicates that convective lift in the mesosphere is strong enough during the polar summer to cause adiabatic cooling of small amount of water vapour to the point of saturation. This tends to produce the coldest temperatures in the entire atmosphere just below the mesopause. There is evidence that smoke particles from burnt-up meteors provide much of the condensation nuclei required for the formation of noctilucent cloud. Noctilucent clouds have four major types based on physical structure and appearance. Type I veils are very tenuous and lack well-defined structure, somewhat like cirrostratus fibratus 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. Distribution in the mesosphere is similar to the stratosphere except at much higher altitudes. Because of the need for maximum cooling of the water vapor to produce noctilucent clouds,