Composition and Structure of the Atmosphere
ICSE Class 9 · Geography · Chapter 12
What this chapter covers
- The atmosphere and why it matters
- Composition of the atmosphere
- Structure of the atmosphere
- Troposphere
- Stratosphere
- Ionosphere
- Exosphere
- Ozone in the stratosphere
- Depletion of the ozone layer
- Global warming
- Impact of global warming
Key definitions
Nitrogen is the most plentiful gas of the atmosphere, forming about 78 per cent of dry air by volume.
Oxygen is the gas, forming about 21 per cent of dry air by volume, that living things use for respiration and that is needed for anything to burn.
Argon is an inert gas that forms a little under 1 per cent of the air and takes no part in weather or life.
Carbon dioxide is a gas present in very small quantity, only about 0.04 per cent of the air, yet it is one of the most important gases for life and climate.
Water vapour is water in the form of an invisible gas mixed with the air.
Dust particles are tiny solid particles suspended in the atmosphere, such as fine soil, sea salt, smoke, soot, volcanic ash, pollen and the remains of burnt-up meteors.
Normal lapse rate is the average rate at which temperature falls with increasing height in the troposphere, about 6.5 °C per kilometre.
Ionisation is the process in which an atom or molecule loses or gains electrons and becomes an electrically charged particle called an ion.
Aurora is a natural display of shimmering green, red or violet light in the night sky of high latitudes, produced when charged particles from the Sun strike the gases of the upper atmosphere.
Ozone is a form of oxygen in which each molecule is made of three oxygen atoms, written O3, and it is a pale blue gas with a sharp smell.
Ozone depletion is the thinning of the ozone layer in the stratosphere, caused mainly by chlorine and bromine released from man-made chemicals, which destroy ozone faster than sunlight can make it.
Chlorofluorocarbons are man-made compounds of chlorine, fluorine and carbon, known as CFCs, that were widely used in refrigerators, air conditioners, aerosol spray cans and the making of foam packaging.
Key points
- The atmosphere is a mixture of gases held to the Earth by gravity; about 99 per cent of its mass lies within about 30 km of the surface.
- Dry air contains about 78 % nitrogen, 21 % oxygen, 0.93 % argon and about 0.04 % carbon dioxide; water vapour varies from almost 0 to 4 %.
- Dust particles help clouds form and scatter sunlight; water vapour gives all forms of condensation and precipitation.
- The four layers named in the syllabus are the troposphere, stratosphere, ionosphere and exosphere.
- The troposphere is about 8 km thick at the poles and 18 km at the equator; it holds all weather, and temperature falls by about 6.5 °C per km.
- The stratosphere extends to about 50 km; it is calm, contains the ozone layer, and its temperature rises with height.
- The ionosphere, from about 80 km to 400 km, contains ionised gases that reflect radio waves; auroras form here and meteors burn up near its base.
- The exosphere, above about 400 km, is made of very thin hydrogen and helium and merges into space; many satellites orbit there.
- Ozone, O3, is concentrated between 15 km and 35 km and absorbs harmful ultraviolet rays.
- CFCs, halons and similar chemicals release chlorine and bromine that destroy ozone; the ozone hole forms over Antarctica every spring.
- The Montreal Protocol of 1987 has cut ozone-depleting substances, and the ozone layer is slowly recovering.
- The greenhouse effect keeps the Earth warm; global warming is the extra rise in temperature caused by greenhouse gases from human activities.
- Global warming melts glaciers, raises sea level, brings extreme weather, harms crops, corals and health, and can be slowed by cutting greenhouse gas emissions.
Questions answered
Name the two most abundant gases of the atmosphere and give their approximate shares by volume.
Give two reasons why dust particles in the atmosphere are important.
Why is water vapour called the most variable part of the atmosphere?
Name the four layers of the atmosphere in order from the Earth's surface upwards.
Why is the troposphere thicker at the equator than at the poles?
The temperature at the foot of a hill at sea level is 30 °C. Using a lapse rate of 6.5 °C per km, find the temperature at the top of the hill, which is 1,600 m high.
Why do jet aircraft prefer to fly in the lower stratosphere?
Explain why the temperature rises with height in the stratosphere.
Past board exam questions
ICSE 2019 sample paper · 3 marks
Draw a neat labelled diagram of the structure of the Earth’s atmosphere.