four species that indicate vertical size which can affect the altitude levels. Mesospheric cloud identification and classification Clouds that form in the mesosphere come in a variety of forms such as veils, bands, and billows, but are not given Latin names based on these characteristics. These clouds are the highest in the atmosphere and are given the Latin-derived name noctilucent which refers to their illumination during deep twilight rather than their physical forms. They are sub-classified alpha-numerically and with common terms according to specific details of their physical structures. Extreme-level stratiform, stratocumuliform, and cirriform Noctilucent clouds are thin clouds that come in a variety of forms based from about and occasionally seen in deep twilight after sunset and before sunrise. Type 1 Veils, very tenuous stratiform; resembles cirrostratus or poorly defined cirrus. Type 2 Long stratocumuliform bands, often in parallel groups or interwoven at small angles. More widely spaced than cirrocumulus bands. 2A Bands with diffuse, blurred edges. 2B Bands with sharply defined edges. Type 3 Billows. Clearly spaced, fibrous cirriform, roughly parallel short streaks. 3A Short, straight, narrow streaks. 3B Wave-like structures with undulations. Type 4 Whirls. Partial (or, more rarely, complete) cirriform rings with dark centers. 4A Whirls possessing a small angular radius of curvature, sometimes resembling light ripples on a water surface. 4B Simple curve of medium angular radius with one or more streaks. 4C Whirls with large-scale ring structures. Stratospheric cloud identification and classification Polar stratospheric clouds form at very high altitudes in polar regions of the stratosphere. Those that show mother-of-pearl colors are given the name nacreous. Very high-level stratiform Nitric acid and water polar stratospheric Sometimes known as type 1, a thin sheet-like cloud resembling cirrostratus or haze. Contains supercooled nitric acid and water droplets; sometimes also contains supercooled sulfuric acid in ternary solution. Very high-level cirriform and stratocumuliform Nacreous polar stratospheric cloud (mother of pearl) Sometimes known as type 2, a thin usually cirriform or lenticular (stratocumuliform) looking cloud based from about and seen most often between sunset and sunrise. Consists of ice crystals only. Troposphere cloud identification and classification Tropospheric clouds are divided into physical forms defined by structure, and levels defined by altitude range. These divisions are cross-classified to produce ten basic genus-types. They have Latin names as authorized by the World Meteorological Organization (WMO) that indicate physical structure, altitude or étage, and process of formation. High-level cirriform, stratocumuliform, and stratiform