Innumerable dust particles are held in suspension in the lower layers of the atmosphere. It may be pointed out that the term Must particles’ includes all the solid particles present in air excepting the gases and water vapour.
There is a great variation in the amount of dust over the earth’s surface. Even over the oceans, the air contains hundreds of dust particles per cubic centimeter. Many particles are invisible to the naked eye and are microscopic.
They originate from different sources, both natural and man-made. They include sea salts from breaking sea waves, pollen and various organisms lifted by the wind, smoke and soot from fires, tiny sand particles raised from active volcanoes.
These dust particles are held in air by the movements of the atmosphere. However, certain particles are too large to remain for very long in the air.
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At times, the ascending air currents carry the dust particles to great heights in the atmosphere. The upper atmosphere also receives a very small amount of dust from the disintegration of innumerable meteors passing through it.
From a meteorological viewpoint, these tiny solid particles may be very significant. They absorb a part of the incoming short- wave solar energy. Also part of the insolation, especially the short wave lengths of blue light, is scattered by the molecules of air and fine dust.
A certain percentage of the solar radiant energy is reflected back by these solid particles. Dust particles, by the process of scattering, contribute to the varied colours of red and orange at sunrise and sunset.
The blue colour of the sky is also due to selective scattering by dust particles. Some of the dust particles are hygroscopic in character and, therefore, act as nuclei of condensation.
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Thus, dust particles are a major contributory factor in the formation of clouds and fogs. The duration of dawn and twilight as well as their intensity are all controlled by the presence of these solid particles in the air.