EXTERNAL STRUCTURE OF THE EARTH  

1. a) The Atmosphere

• Layer of gases surrounding the earth.
• The earth revolves with it because it’s held onto it by gravity
• It’s about 330km thick.

Composition of the Atmosphere

1. a) Gases-exist as a mixture
2. b) Smoke particles
3. c) Dust particles
4. d) Water vapour

The structure of the Atmosphere

It’s divided into 4 layers/zones namely:

1. Troposphere
2. Lowest layer of the atmosphere
3. Contains 90% of water vapour
4. Rainfall is got from it
5. Temperature decreases with increase in altitude (lapse rate)
6. Air is turbulent due to mixing of air
7. Contains dust particles There is a zone of transition between troposphere and stratosphere called tropopause.
8. Stratosphere/ozonosphere
9. Layer lying next to troposphere
10. Has layers
11. Lower isothermal layer in which temperature is constant
12. Upper layer of temperature inversion in which temperature increases with increasing altitude
13. Has ozone layer which absorbs harmful ultraviolet radiation.
14. Air is calm so it’s used by passenger jets
15. Limited amounts of water vapour
16. There is a zone of transition between stratosphere and mesosphere called stratopause.
17. Mesosphere
18. Middle layer of the atmosphere.
19. Temperature decreases with increasing altitude.
20. There is a zone of transition between mesosphere and thermosphere called mesopause which is an inversion layer.
21. Thermosphere/ionosphere

• High radiation is present.
• The pressure is very low.
• Gases and molecules in this layer exist as ions due to high radiation.
• Has no definite top but merges gradually into the outermost part of the atmosphere called exosphere.
• Exosphere consists of rare gases like hydrogen and helium.
• Beyond the atmosphere there is the outer space.
• Outer space is the universe beyond the atmosphere in which other planets and stars exist.

Significance of Atmosphere

1. a) Animals and plants breathe in from it oxygen for respiration.
2. b) Plants use carbon dioxide from it for photosynthesis.
3. c) Water vapour in the atmosphere condenses to form clouds which give us rain.
4. d) Ozone layer in the stratosphere shields us from ultraviolet radiation which may cause cancers.
5. e) Carbon dioxide and methane in the atmosphere cause global warming through the green house effect.
6. b) The Hydrosphere

• Part of the earth’s surface covered by water masses e.g. oceans, seas, lakes, rivers and even underground water.
• It comprises 73% of the earth’s surface area.
• The atmosphere and hydrosphere are related in that atmospheric gases penetrate to the ocean depth in solution form.

• The lower atmosphere, hydrosphere and the upper part of the earths crust are called biosphere meaning the sphere of the earth in which organic life exists

Ajayi