minute particles slowly settling or suspended by slight currents and existing in varying amounts in all air. There is least dust at high levels over the ocean and most at low levels over cities; dust from smoke is a serious urban problem (see air pollution). Sources of atmospheric dust are winds blowing over dry earth (plowed fields, deserts, and roads), the various products of combustion, volcanic eruptions, salt spray from the oceans, pollen and other material from plants, and meteoric particles. The detonation of nuclear devices in the atmosphere creates radioactive dust ( fallout), a serious hazard to all forms of life.

Dust sometimes settles quickly on surfaces, but vast quantities are carried to the upper layers of the air and suspended there for long periods of time. The effects of a volcanic eruption such as that of Krakatoa in Indonesia have been observed three years after its occurrence. Large seasonal dust storms occur in the Sahara and neighboring W Africa and in the Taklimakan and Gobi deserts and neighboring NE Asia. Dust from dust storms in Africa often travels as far as the S United States and the Caribbean, where it can affect air quality, and dust from the Gobi Desert in Asia has been carried as far east as Minnesota. Such dust storms can have negative health and economic effects.

Hygroscopic dust particles (those to which water adheres) are the nuclei of condensation in free air; the nucleus of each droplet in a fog or cloud and of each raindrop and snowflake is one of these invisible particles of inorganic or organic dust. John Aitken, a Scottish physicist who in 1880 invented a device for counting particles in air, first correlated dust particles and condensation. Dust is also chiefly responsible, through its scattering effect upon light (diffusion), for one type of haze and for sunrise and sunset colors.

See also Dust Bowl.

The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved.

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