1.3. Dust Particles

The atmosphere also carries in suspension variable amounts of solid material in the lower layers of atmosphere. Convectional air currents may transport them to great heights. The higher concentration of dust particles is found in subtropical and temperate regions due to dry winds in comparison to equatorial and polar regions. The term ‘dust particles’ includes all the solid particles present in the air except the gases and water vapour. It includes sea salts, fine soil, smoke-soot, ash, pollen, dust and disintegrated particles of meteors and originates from different sources.

Dust particles provide the necessary nuclei on which water vapour can condense to form clouds and eventually precipitation. Condensation on these fine particles near the surface causes formation of fog. Large amount of dust tend to make the atmosphere hazy, and in extreme cases, where pollution is involved, dust particles can be positively harmful to health. By the process of scattering, dust particles 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. The duration of twilight is also affected by the presence of these dust particles in the air.