Geography: Class 11 (Part 2) — Chapter 7: Composition and Structure of Atmosphere

The atmosphere is composed of various gases with changing proportions in higher layers:

• Carbon dioxide: Meteorologically crucial as it is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation. It is largely responsible for the green house effect.
• Ozone: Found between 10 and 50 km above the earth's surface. It acts as a filter, absorbing harmful ultra-violet rays from the sun.
• Oxygen becomes negligible at a height of 120 km, while carbon dioxide and water vapour are found only up to 90 km.

A variable gas that decreases with altitude and from the equator towards the poles.

• In warm, wet tropics, it may account for 4% of the air by volume, whereas in dry/cold deserts and polar regions, it is less than 1%.
• It absorbs parts of insolation and preserves terrestrial radiation, acting like a blanket.

Concentrated in the lower layers, originating from sea salts, fine soil, smoke-soot, ash, and pollen.

• They act as hygroscopic nuclei around which water vapour condenses to produce clouds.

The troposphere is the most important layer for biological activity. The temperature decreases at a specific rate:

The boundary separating the troposphere and stratosphere is the tropopause, where air temperature is about minus 80°C over the equator and minus 45°C over the poles.

• The stratopause lies at the top of the stratosphere.
• The mesopause is the upper limit of the mesosphere at 80 km.