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Clouds occur in the Earth’s atmosphere in both liquid water and ice phases. John Aitken, a Scottish physicist who did research on atmospheric dust and the formation of dew, cyclones, and evaporation, first reported in 1880 that when water vapor condenses in the atmosphere, it always does so on some solid nucleus; that the dust particles in the air form the nuclei on which it condenses; and if there was no dust in the air there would be no fogs, no clouds, no mists, and probably no rain. Atmospheric dust is today referred to as cloud condensation nuclei, and they are ever present in the Earth’s atmosphere as a result of breaking waves, dust storms, atmospheric chemical transformation in urban and industrial areas, and smoke from natural and manmade fires. But what kind of clouds does the Earth have today?

The MODIS instrument on the Terra satellite reveals that during July 2006, there were high concentrations of liquid water clouds off Namibia and Angola, Peru and Ecuador, and southern California. These low, boundary layer clouds, are known as marine stratocumulus clouds and are prevalent in the subtropical eastern ocean basins. During this month there were also a large number of liquid water clouds in the Gulf of Alaska and north Pacific, as well as in the Norwegian Sea and Barents Sea. Ice clouds, in addition to being present as anvils in the Intertropical Convergence Zone, are especially prevalent in the Bay of Bengal and Philippines of the western tropical Pacific. (Submitted by M. D. King and S. Platnick.)