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Clouds are visible aerosols composed of tiny water droplets or ice crystals, formed when air cools to its dew point or gains moisture. The science of clouds, called nephology, uses a classification system pioneered by Luke Howard in 1802, which organizes tropospheric clouds into ten basic genera based on five physical forms (stratiform, cumuliform, stratocumuliform, cumulonimbiform, and cirriform) and altitude levels—with prefixes like alto- and cirro- distinguishing mid- and high-level variants. Beyond the troposphere, clouds also appear in the stratosphere and mesosphere (mostly in polar regions), and have been observed on other planets where they may consist of methane, ammonia, or sulfuric acid. Tropospheric clouds play a critical role in climate change by either reflecting incoming solar radiation (cooling) or trapping outgoing longwave radiation (warming), making them the main uncertainty in climate sensitivity models. Interestingly, the English word "cloud" derives from the Old English clud or clod (meaning a hill or mass of stone), and began being used as a metaphor for rain clouds around the 13th century, eventually replacing the older term weolcan.