Summary
NCERT Class 11 Geography Chapter 7 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.
The atmosphere is a mixture of gases, water vapour, and dust particles that envelops the earth. Ninety-nine per cent of its total mass is confined within 32 km of the surface. Its major gaseous components include nitrogen and oxygen, while carbon dioxide — though small in volume — is meteorologically vital because it is transparent to incoming solar radiation yet opaque to outgoing terrestrial radiation, making it chiefly responsible for the greenhouse effect. Ozone, found between 10 and 50 km, absorbs harmful ultraviolet rays. Water vapour and dust particles (sea salts, fine soil, smoke-soot, pollen, ash) complete the composition. Structurally, the atmosphere is divided into five layers: troposphere, stratosphere, mesosphere, ionosphere, and exosphere, each defined by its temperature profile and density.
Key points & formulas
- 01The atmosphere is composed of gases, water vapour, and dust particles; 99% of its mass lies within 32 km of the surface.
- 02Nitrogen constitutes the major portion of the atmosphere; oxygen becomes negligible at 120 km altitude.
- 03Carbon dioxide is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation and is largely responsible for the greenhouse effect.
- 04Ozone is found between 10 and 50 km and acts as a filter, absorbing ultraviolet rays from the sun.
- 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.
- 06Dust particles act as hygroscopic nuclei around which water vapour condenses to form clouds.
- 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.
- 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).
Frequently asked questions
01What is the composition of the atmosphere according to NCERT Class 11 Geography Chapter 7?
The atmosphere is composed of gases, water vapour, and dust particles. The proportion of gases changes at higher altitudes — oxygen becomes negligible at 120 km, and carbon dioxide and water vapour are found only up to 90 km from the surface.
02Which gas constitutes the major portion of the atmosphere?
Nitrogen constitutes the major portion of the atmosphere.
03Why is carbon dioxide considered meteorologically important?
Carbon dioxide is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation. It absorbs part of terrestrial radiation and reflects some back towards the earth's surface, making it largely responsible for the greenhouse effect.
04Where is the ozone layer found and what does it do?
The ozone layer is found between 10 and 50 km above the earth's surface in the stratosphere. It acts as a filter, absorbing ultraviolet rays radiating from the sun and preventing them from reaching the earth's surface.
05What are dust particles in the atmosphere made of?
Dust particles in the atmosphere include sea salts, fine soil, smoke-soot, ash, pollen, dust, and disintegrated particles of meteors. They act as hygroscopic nuclei around which water vapour condenses to produce clouds.
06What are the five layers of the atmosphere?
The five layers of the atmosphere are the troposphere, stratosphere, mesosphere, ionosphere (thermosphere), and exosphere, each defined by distinct temperature conditions and density.
07Why is the troposphere the most important layer of the atmosphere?
The troposphere is the most important layer because all changes in climate and weather take place in it and it is the most important layer for all biological activity. Its average height is 13 km — about 8 km near the poles and 18 km at the equator.
08What is the tropopause?
The tropopause is the zone separating the troposphere from the stratosphere. The air temperature at the tropopause is about −80°C over the equator and about −45°C over the poles, and it is nearly constant.
09What is the ionosphere and why is it significant?
The ionosphere is located between 80 and 400 km above the mesopause. It contains electrically charged particles called ions. Radio waves transmitted from the earth are reflected back to the earth by this layer.
10How does water vapour vary in the atmosphere?
Water vapour is a variable gas that decreases with altitude. In warm and wet tropical regions it may account for four per cent of the air by volume, while in dry and cold desert and polar regions it may be less than one per cent. It also decreases from the equator towards the poles.
11What are the elements of weather and climate mentioned in Chapter 7?
The main elements of atmosphere that influence human life are temperature, pressure, winds, humidity, clouds, and precipitation.
12Is the NCERT Class 11 Geography Chapter 7 PDF free to download?
Yes, it is free to download with no sign-up.
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