The word cloud originally meant a lump of rock
In Old English, clud meant a hill or a mass of stone. Around the 13th century people began applying it to heaped rain clouds that looked like boulders, and the metaphor eventually pushed out weolcan, the older word. The Latin names forecasters use today came from one methodical English amateur in 1802.
A cloud is an aerosol: a visible mass of tiny water droplets, ice crystals or other particles hanging in the air. On Earth one forms when air becomes saturated, either because it cools to its dew point or because it picks up enough moisture to raise the dew point to the surrounding temperature. Cooling usually comes from lifting. Daytime heating sends air rising in convection and builds heaped cumulus; fronts and low-pressure systems force stable air upward; and wind pushed over mountains creates orographic lift, which can produce smooth lens-shaped cap clouds.
Luke Howard, a careful observer with good Latin, classified the lower atmosphere's clouds in 1802. The French naturalist Jean-Baptiste Lamarck produced a rival scheme the same year with twelve categories and informal French names such as broom-like clouds, but it failed to catch on even at home. An expanded version of Howard's system was formally adopted in 1891. It now rests on five physical forms, from sheets and heaps to towering cumulonimbus and wispy cirrus, crossed with altitude levels to give ten basic genera, with prefixes like alto- for middle heights and cirro- for high ones.
Clouds also appear higher up, in the stratosphere and mesosphere, mostly over the poles, where they take common names and never form heaps or towers. Other worlds have them too, often made of methane, ammonia or sulphuric acid as well as water. Aristotle's Meteorologica, around 340 BC, called clouds and precipitation meteors, from a Greek word for high in the sky, which gave us meteorology.
Low clouds matter for climate. Depending on their height, shape and thickness, they can reflect sunlight and cool the surface or trap infrared heat rising from the ground and warm it. Clouds above the troposphere are too thin and rare to count. Clouds remain the main source of uncertainty in estimates of climate sensitivity.
Source: Cloud
