NCERT Class 7 Geography Chapter 4 - Air Notes

Chapter 4 - Air Notes

1. All living beings depend on the atmosphere, use the air for breath and protect us from the harmful effects of the sun’s rays.

2. The air which we breathe is mixture of many gases. The constituents mainly consists of Nitrogen, Oxygen, Argon, Carbon dioxide.

3. Also, tiny dust particles are also present in the air.

4. Nitrogen is the most plentiful gas (78%) in the air. Oxygen, Argon and Carbon dioxide are 21%, 93% and 0.03% respectively.

5. Plants need nitrogen for their survival. They cannot take nitrogen directly from the air. Bacteria that live in the soil and roots of some plant take nitrogen from the air and change its form so that plants can use it.

6. Oxygen that is taken by human and animals as breathe is produced by plants during photosynthesis. In this way oxygen content in the air remains constant. If we cut trees then this balance gets disturbed.

7. Green plants use carbon dioxide to make their food and release oxygen. The amount of carbon dioxide released by humans or animals seems to be equal to the amount used by the plants which make a perfect balance. However, the balance is upset by burning of fuels.

8. Our atmosphere is divided into five layers starting from the earth’s surface.
a) Troposphere
b) Stratosphere
c) Mesosphere
d) Thermosphere
e) Exosphere

9. The first layer is the Troposphere whose average height is 13 km.

10. The second layer is the Stratosphere which extends up to the height of 50 km.

11. The third layer is Mesosphere which extends up to the height 80 km.

12. The fourth layer is Thermosphere which extends up to height of 80 to 400km.

13. The fifth layer is Exosphere. This layer has very thin air. Light gases like helium and hydrogen float into the space from here.

14. Weather is hour-to-hour, day to day condition of the atmosphere.

15. The average weather condition of a place for a longer period of time represents the climate of a place.

16. The degree of hotness and coldness of the air is known as temperature. The temperature of the atmosphere changes between day and night and also season to season. Summers are hotter than winters.

17. An important factor that influences the distribution of temperature is insolation. The amount of insolation decreases from the equator towards the poles. Therefore, the temperature decreases in the same manner.

18. Air pressure is defined as the pressure exerted by the weight of air on the earth’s surface.

19. Horizontally, the distribution of air pressure is influenced by temperature of air at a given place. Wherever, pressure of air is low, the air is cold. Likewise, if pressure is high, the air becomes warmer.

20. The air always moves from high-pressure areas to low-pressure areas.

21. Winds can be broadly divided into three types:
a) Permanent winds
b) Seasonal winds
c) Local winds.

22. Permanent winds blow constantly throughout the year in a particular direction.

23. Seasonal winds change their direction in different seasons.

24. Local winds blow only during a particular period of the day or year in a small area.

25. When water evaporates from land and different water bodies, it becomes water vapour. Moisture at any time is known as humidity.

26. When the water is full of water vapour we call it a humid day. On a humid day, clothes take longer to dry and sweat from our body does not evaporate easily.

27. Carbon dioxide released in the atmosphere creates a greenhouse effect by trapping the heat radiated from the Earth. It is therefore called a greenhouse gas and without it the Earth would have been too cold to live in. However, when its level in the atmosphere increases due to factory smoke or car fumes, the heat retained increases the temperature of the Earth. This is called global warming.

28. When air is heated, it expands, becomes lighter and goes up. Cold air is denser and heavy. That is why it tends to sink down. When hot air rises, cold air from surrounding area rushes there to fill in the gap. That is how air circulation takes place.

29. The Earth receives only 1 in 2,000,000,000 parts of the sun’s energy.

30. The standard unit of measuring temperature is Degree Celsius. It was invented by Anders Celsius. On the Celsius scale the water freezes at 0°Cand boils at 100°C.

31. On the moon there is no air and hence no air pressure. Astronauts have to wear special protective space suits filled with air when they go to the moon. If they did not wear these space suits, the counter pressure exerted by the body of the astronauts would make the blood vessels burst. The astronauts would bleed.

32. A wind is named after the direction from which it blows, Example: The wind blowing from the west is called westerly.

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