dust, atmospheric

dust, atmospheric,

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 pollutionair pollution,
contamination of the air by noxious gases and minute particles of solid and liquid matter (particulates) in concentrations that endanger health. The major sources of air pollution are transportation engines, power and heat generation, industrial processes, and the
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). 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 (falloutfallout,
minute particles of radioactive material produced by nuclear explosions (see atomic bomb; hydrogen bomb; Chernobyl) or by discharge from nuclear-power or atomic installations and scattered throughout the earth's atmosphere by winds and convection currents.
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), 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 KrakatoaKrakatoa
or Krakatau
, volcanic island, c.5 sq mi (13 sq km), W Indonesia, in Sunda Strait between Java and Sumatra; rising to 2,667 ft (813 m). A momentous volcanic explosion on Aug.
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 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; Beijing is annually affected by such storms. Dust from large 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, which are aggravated by desertification, can have negative health and economic effects; in addition to potentially harmful mineral particles, the dust may include bacteria, fungi, viruses, and various pollutants.

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 hazehaze,
suspension in the atmosphere of minute dust or salt particles that are not individually seen but that nevertheless reduce visibility. So-called damp haze and dry haze produce different optical effects because the particles of each are of different sizes, with the dry haze
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 and for sunrise and sunset colors.

See also Dust BowlDust Bowl,
the name given to areas of the U.S. prairie states that suffered ecological devastation in the 1930s and then to a lesser extent in the mid-1950s. The problem began during World War I, when the high price of wheat and the needs of Allied troops encouraged farmers to
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