Composition of the Atmosphere

The atmosphere is not a single, uniform gas but a layered mixture of three broad components: gases, water vapour, and dust particles. What matters for understanding weather and climate is not just what these components are, but how their proportions change with height and from place to place. The composition is far from constant — some gases are fixed in proportion, while others vary dramatically.

Gases

The major gases of the atmosphere are nitrogen and oxygen, but the proportion of gases changes as we go higher. Oxygen becomes almost negligible in quantity at a height of 120 km. Similarly, carbon dioxide and water vapour are found only up to 90 km from the surface of the earth. Above these heights, the air is essentially made of lighter gases.

Carbon dioxide is meteorologically a very important gas. Its significance lies in its behaviour towards radiation: it is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation. In plain terms, sunlight passes through it easily, but the heat radiated back from the earth's surface does not. It absorbs a part of this terrestrial radiation and reflects some of it back towards the earth's surface. This property makes carbon dioxide largely responsible for the greenhouse effect.

The volume of most other gases in the atmosphere is constant, but the volume of carbon dioxide has been rising over the past few decades, mainly because of the burning of fossil fuels. This increase has also raised the temperature of the air.

Ozone is another important component of the atmosphere. It is found between 10 and 50 km above the earth's surface. Ozone acts as a filter — it absorbs the ultra-violet rays radiating from the sun and prevents them from reaching the surface of the earth. Without this layer, harmful UV radiation would reach the ground.

Water Vapour

Water vapour is a variable gas in the atmosphere, and it decreases with altitude. Its amount varies greatly with place and climate:

• In the warm and wet tropics, it may account for four per cent of the air by volume.
• In the dry and cold areas of deserts and polar regions, it may be less than one per cent of the air.

Water vapour also decreases from the equator towards the poles. Its role in the heat balance is crucial. It absorbs parts of the insolation coming from the sun and preserves the earth's radiated heat. In this way, it acts like a blanket, allowing the earth to become neither too cold nor too hot. Water vapour also contributes to the stability and instability of the air — it is central to the formation of weather systems.

Dust Particles

The atmosphere has a sufficient capacity to keep small solid particles suspended in it. These particles originate from different sources and include:

• sea salts
• fine soil
• smoke-soot
• ash
• pollen
• dust
• disintegrated particles of meteors …