Clouds - An Overview

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What is a cloud?

When most people think of a cloud they know what it is, a white puffy thing in the sky. But when pushed to be precise it is more difficult. Formally speaking a cloud is an aerosol, that is a suspension of one phase of matter in another. The glossary of the American Meteorological Society defines an aerosol as follows:

A colloidal system in which the dispersed phase is composed of either solid or liquid particles, and in which the dispersion medium is some gas, usually air.

A characteristic of an aerosol is that it is disperse. It is not one thing, but many things, or many repetitions of the same thing, dispersed in space. This makes the boundary of the aerosol, or a cloud, somewhat difficult to define objectively and precisely. But clouds are a special type of aerosol, so special in fact that we rarely speak of a cloud as being an aerosol. Clouds are a type of aerosol that comes into being when the atmosphere becomes supersaturated with respect to water, and cloud particles grow rapidly and become visibly apparent in a way that the aerosol in a sub-saturated environment rarely is. This difference encourages the tendency to associate the atmospheric aerosol with only the smallest particles, traditionally those under 1 µ m, and the growing particles that are found in water saturated environments as clouds. This distinction encourages one to speak of clouds as singular, compact, entities in contrast to the disperse atmospheric aerosol. However a cloud’s origin as a component of the atmospheric aerosol lingers in attempts to define “a cloud” objectively.