Chapter 7 — Composition and Structure of Atmosphere

Chapter 7 of the Class 11 Geography NCERT textbook (Fundamentals of Physical Geography), "Composition and Structure of Atmosphere", explains the gases, water vapour, and dust particles that make up the atmosphere, and describes its five vertical layers — troposphere, stratosphere, mesosphere, ionosphere, and exosphere — each with distinct temperature and density characteristics.

• Composition of the atmosphere — The air is a mixture of gases dominated by nitrogen and oxygen, plus variable water vapour and dust. Nearly all of its mass is packed within just 32 km of the surface.
• Meteorologically important components — Though small in amount, carbon dioxide traps outgoing terrestrial radiation and drives the greenhouse effect, while ozone between 10 and 50 km shields life by absorbing harmful ultraviolet rays.
• Water vapour and dust — Water vapour, richest in warm humid tropics, supplies the moisture for weather, while dust particles act as hygroscopic nuclei on which vapour condenses to form clouds.
• Vertical layers of the atmosphere — The atmosphere is stacked into five layers defined by temperature and density — the weather-bearing troposphere, the ozone-rich stratosphere, the cold mesosphere, the radio-reflecting ionosphere, and the exosphere.

1. 01The atmosphere is composed of gases, water vapour, and dust particles; 99% of its mass lies within 32 km of the surface.
2. 02Nitrogen constitutes the major portion of the atmosphere; oxygen becomes negligible at 120 km altitude.
3. 03Carbon dioxide is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation and is largely responsible for the greenhouse effect.
4. 04Ozone is found between 10 and 50 km and acts as a filter, absorbing ultraviolet rays from the sun.
5. 05Water vapour is a variable gas, reaching up to 4% by volume in warm wet tropics and less than 1% in dry polar regions; it decreases from equator towards the poles.
6. 06Dust particles act as hygroscopic nuclei around which water vapour condenses to form clouds.
7. 07The troposphere (average height 13 km) is the most important layer for all biological activity; all weather and climate changes occur here, and temperature decreases at 1°C per 165 m of height.
8. 08Above the troposphere lie the stratosphere (contains ozone layer, up to 50 km), mesosphere (up to 80 km, temperature drops to −100°C), ionosphere (80–400 km, reflects radio waves), and exosphere (highest, merges with outer space).