There is always some dust in the atmosphere. At high levels over the ocean there is least dust. The most dust is found low over cities and is caused by the smoke from coal and oil fires used in heating and manufacturing. Volcanoes shoot tons of fine ash and dust into the air whenever they erupt or explode. This dust may float in the air at very high levels for years at a time. The wind blowing over fields where the earth has been turned over by plows will often lift fine bits of earth, sand, and tiny seeds high into the air.
Some of this dust will settle down to earth very quickly but much of it will float in the air for as long as three years. Water vapour in the air will condense or turn to liquid when it touches certain kinds of dust. Each tiny particle or bit of dust becomes the centre of a droplet of water. When there are enough droplets they form fog, clouds, rain, or snow. The brilliant colours we sometimes see at sunrise and sunset are caused by dust. The dust scatters the rays of sunlight, changing them into bright colours. The sky's blue colour is caused by vapour and dust. All the colours that we see are caused by light rays that have different wave lengths. Blue has very short waves. The dust in the atmosphere holds back the longer waves of light from the sun but the short ones, which look blue to us. are able to get through.
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