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Heavy Cloud

  • 14/08/2002

Water moves into the atmosphere on evaporation from oceans, rivers and other waterbodies. When the air containing water vapour cools below a critical temperature, it condenses into droplets. But it needs a surface to condense on and particulates provide such a surface.

If these water droplets are very small, they remain suspended in the atmosphere; millions of such droplets hanging in the air form clouds in the sky. In fact, if the air were entirely devoid of dust and pollution, there would be no clouds at all.

Small droplets then combine to form larger droplets, which fall on the Earth as rain. However, if there are too many particulates available in the air for water vapour to condense on, droplets may never become big enough in size to fall as rain or snow. Up to four times more droplets can be formed in polluted air than in clean air. This will directly affect replenishing of the world's major stores of freshwater including lakes, groundwater supplies, glaciers and high altitude snow.

Furthermore, particulates can partially block the amount of sunlight reaching the surface owing to their absorbing or scattering nature. This reduces evaporation from waterbodies, consequently affecting formation of clouds.

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