Class 7 Social Science Air Exam Notes Set A

10. The atmosphere also has large amount of fine dust particles, salt particles, pollens from plants, ash from volcanic eruptions, smoke from fire, etc. They are found in the lower layers only. These particles act as nuclei for water vapour to form raindrops around them. They also scatter the sun's radiation during sunrise and sunset.

 ♦ STRUCTURE OF THE ATMOSPHERE

1. The atmosphere has a layered structure.

2. Each layer has different characteristics.

3. The upper layer presses down the lower layers.

4. Thus, the upper layers are less dense than the lower layers.
5. It is estimated that about 90 per cent of the total mass of the atmosphere lies within a height of about 20 km from the earth's surface.
6. The total extent of atmosphere, on the basis of temperature and atmospheric pressure, can be divided into five layers starting from the earth's surface. These are Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere.

 ♦ TROPOSPHERE

1. This is the most important layer of the atmosphere. It is the lower most and densest layer.

2. The height of the troposphere varies from about 8 km at the Poles to about 18 km near the Equator.

3. About three-fourth of the total mass of atmosphere is found in this layer.

4. The air we breath is available in this layer only.

5. In this layer, the temperature of the air decreases with increase in height.

6. Almost all types of weather phenomena like rainfall, fog, clouds, hailstorm, etc. occur in this layer.

7. The water vapour and dust particles are found only in this layer.

8. This layer is heated more from below than from above.

9. The upper limit of troposphere is known as tropopause.

  ♦ STRATOSPHERE

1. This layer lies above the tropopause and extends up to a height of about 50 km from the sea level.

2. Temperature in the lower part of this layer is mostly constant.

3. The air is generally thin, cold and dry.

4. Strong winds, known as jet stream, blow horizontally from west to east in this layer.
5. The stratosphere is almost free from various weather phenomena and the lower part is completely free from water vapour and dust particles.
6. Thus, the lower part of stratosphere is ideal for flying jet aircrafts.

7. The upper limit of stratosphere is known as stratopause.
8. An important feature of stratosphere is the presence of ozone in its lower part. The ozone can absorb the incoming ultraviolet rays of the solar radiations. Thus, this ozone layer is essential for sustainin life on the earth. The scientists have observed that there is a decline in the total global ozone. This can cause global warming.

  ♦ MESOSPHERE

1. This layer lies above the stratosphere and extends up to a height of about 90 km from the sea level.

2. The temperature in this layer decreases with height and reaches up to –110ºC near the upper limit.

3. It is the coldest layer in the atmosphere.

4. The meteors burn up in this layer on entering from space.
5. The upper limit of mesosphere is known as the mesopause. From mesopause onwards, the temperature increases.

  ♦ THERMOSPHERE

1. This layer lies above the mesosphere and extends up to a height of abut 400 km from the sea level.

2. In this layer, the temperature increases rapidly with increase in height.
3. Ionosphere is a part of this layer, which contains electrically charged particles, called ions. The ions reflect radio waves back to the earth's surface. This enables us to have wireless communication.

  ♦ EXOSPHERE

1. This is the uppermost layer of the atmosphere and lies between 400 km to 1500 km above the earth.

2. Here the air is very thin as the main gases present are hydrogen and helium.

3. The temperature increases with height, may reach up to 5000ºC.

4. This layer merges gradually with the interplanetary space.

The atmosphere is extremely significant for the growth and development of the life on the earth.

1. The atmosphere protects us from harmful unltraviolet rays coming from the sun.

2. It controls the extremes of temperature during the daytime and the night time through greenhouse effect.

3. It protects us from the meteors coming towards the earth from the outer space.
4. The air has weight and exerts pressure. Being pressed down, it always remains in contact with land and water.
5. The change in the weather conditions is exclusively due to the presence of atmosphere.

6. The gases, like oxygen and carbon dioxide, have made life possible on the earth.

7. The ionosphere helps us in wireless and long distance communication.

8. Sound waves can only travel through the air.

9. The energy of the sun has made the atmosphere dynamic.

10. Nitrogen is required to maintain the fertility of the soil and is vital for plants.

11. Oxygen is the gas that animals breathe in.

12. Carbon dioxide helps plants to produce food in the presence of sunlight. It also keeps the earth warm by absorbing the heat radiated from the surface. This is called the greenhouse effect.
13. The water vapour in the atmosphere causes precipitation.

14. The dust particles form the nucleus for condensation which results in the formation of clouds that cause rainfall and snowfall.