Dust, fine particles of earth or other matter capable of being carried by currents of air. Dust can range in size from microscopic particles to grains of sand.
Dust may consist of one or more of the following:
Tiny particles of dry soil picked up by the wind.
Very small particles of ash, carbon, and other substances produced by manufacturing, refining, and mining operations. These substances are forms of particulate matter.
Desert sand lifted into the air during windstorms.
Particles ejected into the atmosphere by volcanic eruptions.
Material from space, such as meteor dust.
Certain other substances are sometimes also considered to be dust. These include the following:
Particles of sea salt.
Radioactive particles produced by nuclear explosions. These particles are known as fallout.
Pollen from many kinds of plants.
Dust picked up by the wind is usually spread out over so great an area that it is not noticeable and does no harm. Indoors, however, it gathers on flat surfaces and its removal is one of the main chores of housekeeping. A heavy concentration of particulate matter in the air above urban areas is a form of air pollution, and constitutes a health hazard.
The effects of dust are not all bad. Dust causes rain and snow, filters sunlight, and helps make the sky blue. The formation of rain and snow depends upon the presence of certain types of dust particles, called condensation nuclei, which readily retain moisture. Most condensation nuclei are tiny particles of sea salt. When the atmosphere cools, water vapor condenses around condensation nuclei in the form of rain droplets or snow crystals. When the atmosphere is too warm or too dry for condensation to occur, dust and water remain separate and form haze.
Dust particles in the atmosphere reflect some of the sun's light away from the earth. Scattering of light waves by dust in the upper atmosphere is primarily responsible for the sky's blue color. Dust also causes the varied colors sometimes seen in sunsets.
Strong winds blowing over a desert or dry countryside pick up large quantities of dust. This dust is carried away by the wind, sometimes for vast distances. When the wind slackens, most of the dust falls to the ground. Flying dust has filled valleys, built vast plains, hills, and dunes, changed coastlines, and buried entire cities.
Dust storms occur when winds carry many dust particles at high speeds close to the surface of the earth. They are common in deserts, and in areas suffering from a temporary lack of rainfall.