Composition and Structure of the Atmosphere

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The atmosphere is the envelope of air surrounding the Earth. It is a relatively thin layer of gases, dust, and vapour that extends to about 10,000 km above the Earth's surface.

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Air is colourless, tasteless, odourless, mobile, elastic, and compressible. Its presence is usually felt only when it moves horizontally as wind.

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Atmospheric pressure is the pressure exerted by air on the Earth's surface. Air has weight, so it presses down on everything on Earth.

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Nitrogen makes up 78% of clean dry air and oxygen makes up 21%. Together they form about 99% of the clean dry air.

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Nitrogen is the main atmospheric gas and is chemically inert, so it acts like a neutral filling substance. A small amount is absorbed by the soil and used by plants.

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Oxygen is highly chemically active. It takes part in oxidation, such as combustion of fuels and weathering of rocks.

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Carbon dioxide forms only 0.033% of the atmosphere, but it is important because it absorbs heat, helps warm the lower atmosphere, and is used by green plants in photosynthesis.

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Argon is present in a very small percentage. It is inert and not useful in natural processes, but it is extracted for commercial and industrial uses such as light bulbs and welding.