1. The envelope of air which is surrounded completely over the earth is known as atmosphere.
2. The atmosphere extends to about 1000 km from the surface of the earth.
3. But 99% of the total mass of the atmosphere is found within 32 km.
4. This is because the atmosphere is held by the gravitational pull of the earth.

 Composition of the Atmosphere

78% of the air comprises of nitrogen and 21% of oxygen. The rest 1% of the dry air is mixture of a number of gases.The composition is as follows :

•  Nitrogen- 78%
• Oxygen-28%
• Argon-0.93%
• Carbondioxide-0.03%
• Neon-0.0018%
• Helium-0.0005%
• Ozone-0.0006%
• Hydrogen-0.00005%

1. Carbon dioxide is present in small quantity in the atmosphere.
2. It is an important constituent of air because it has the ability to absorb heat and thus keep the atmosphere warm, thereby, balancing the heat of the earth.
3. Water vapour is the most significant component of the atmosphere as far as it effect on weather is concerned although its quantity varies considerably from practically none (0) to up to about 4% by volume.
4. Water vapour is the source of all clouds and precipitation (rain, hail, storm etc.)
5. Water vapour like carbon dioxide, has the ability to absorb heat energy.
6. Water vapour also regulates the hydrological cycle.
7. Dust intercepts and reflect incoming insolation.

     8.  The pollute particles present in the air not only absorb larger amount of insolation but also greatly absorb the terrestrial radiation.

     9.  Dust in the atmosphere contributes to the red and orange colour of sunrise and sunset.

Layers of the Atmosphere

There are five distinct layers of the atmosphere:

• Troposphere
• Stratosphere
• Mesosphere
• Thermosphere
• Exosphere

  Troposphere:

1. This is the first layer of the atmosphere.
2. It extends to a height of 18 km at the equator and 8 km at the poles.
3. In this layer temperature decreases with height. This is due to the fact that the density of air decreases with height and so the heat absorbed is less.
4. It contains more than 90% of gases in the atmosphere.
5. Since most of the water vapour form clouds in this layer, all weather changes occur in the troposphere.
6. The height at which the temperature decreases is called tropopause. Here the temperature may be as low as -58°C.

 Stratosphere:

1. This the second layer of the atmosphere.
2. It extends from the tropopause to about 50 km above the surface of the Earth.
3. Temperature increases due to the absorption of the ultraviolet radiation of the sun by ozone present in this layer. The temperature slowly increases to 4°C.
4. This layer does not include clouds and associated weather phenomena. Hence, it provides ideal flying conditions for large jet planes.
5. At about 50 km the temperature begins to fall again. This marks the end of the stratosphere. The end of the stratosphere is called the stratopause.

 Mesosphere:

1. Above the stratosphere lies the mesosphere.
2. The mesosphere extends to a height of 80 km.
3. Here the temperature decreases again, falling as low as -90°C.
4. The end of this layer is known as the mesopause.

 Thermosphere:

1. The thermosphere is present above the mesosphere.
2. This layer extends to a height of 90 km.
3. In this layer temperature rises dramatically, reaching upto 1480°C, this layer absorb the X-rays and the short wave ultraviolet radiation of the sun.
4. This results in the break-up of the gas molecules into positively and negatively charged particles or ions. Thus, this layer also known as the ionosphere.
5. The electrically charged gas molecules of the thermosphere reflect radio waves from the Earth back into space. Thus, this layer also helps in long distance communications.

 Exosphere:

1. This layer lies above the thermosphere.
2. The exosphere extends beyond the thermosphere upto 960 km.
3. It gradually merges with interplanetary space.
4. The temperatures in this layer range from about 300°C to 1650°C.
5. This layer contains only traces of gases like oxygen, nitrogen argon and helium because the lack of gravity allows the gas molecules to escape easily into space.