Composition and Structure of the Atmosphere
ICSE Class 7 · Geography · Chapter 3
What this chapter covers
- Atmosphere: an introduction
- Composition of the atmosphere
- Structure of the atmosphere
- Troposphere
- Stratosphere and the ozone layer
- Mesosphere, thermosphere and exosphere
- Importance of the atmosphere
Key points
- The atmosphere is the layer of air around the Earth, held in place by gravity, and it thins out with height.
- Dry air is about 78% nitrogen, 21% oxygen, 0.93% argon and about 0.04% carbon dioxide, with tiny amounts of other gases.
- Air also contains a changing amount of water vapour (up to about 4%) and particles of dust, smoke, salt and pollen.
- The five layers from the ground upwards are the troposphere, stratosphere, mesosphere, thermosphere and exosphere.
- The troposphere is the lowest layer, about 8 km thick at the poles and 18 km at the equator; it holds most of the air and all the weather, and temperature falls with height by about 6.5 °C per km.
- The stratosphere reaches about 50 km, gets warmer with height and contains the ozone layer, which absorbs harmful ultraviolet rays.
- The mesosphere, up to about 80 km, is the coldest layer, and meteors burn up in it.
- The thermosphere, up to about 600 km, gets very hot with height and contains the ionosphere, which reflects radio waves; auroras occur here.
- The exosphere is the outermost layer, made mainly of hydrogen and helium, and fades into space.
- The atmosphere supplies the gases of life, keeps the Earth warm, shields it from harmful rays and meteors, carries water for rain and sound for hearing, and must be kept clean by everyone.
Questions answered
What is the atmosphere, and what holds it close to the Earth?
The atmosphere is the layer of air that surrounds the Earth on all sides. It is held close to the planet by the pull of gravity.
Name the two most plentiful gases in dry air and give the approximate share of each.
Nitrogen, about 78% by volume, and oxygen, about 21% by volume.
Why is nitrogen in the air useful even though people do not use it for breathing?
Nitrogen dilutes the oxygen so that fires do not spread out of control, and bacteria in the soil and root nodules change it into a form that plants use to grow.
Carbon dioxide makes up only about 0.04% of the air. Give two reasons why it is still important.
Green plants use it to make their food, and it traps some of the heat given off by the Earth, keeping the planet warm.
Why do clouds and rain depend on dust particles?
Water vapour turns into droplets around tiny specks of dust, salt and smoke. Without these particles, far fewer cloud droplets would form.
Name the five layers of the atmosphere in order, starting from the ground.
Troposphere, stratosphere, mesosphere, thermosphere, exosphere.
Why is the troposphere thicker over the equator than over the poles?
The strong heat at the equator makes the air expand and rise higher, while the cold, heavy air over the poles stays lower.
The temperature at the foot of a hill is 28 °C. Using a fall of 6.5 °C for every kilometre, find the temperature at a point 2 km higher.
Fall = 6.5 × 2 = 13 °C, so the temperature is 28 - 13 = 15 °C.