Atmosphere Structure
Notes
The atmosphere is the envelope of gases held by gravity around the Earth. About 50% of its mass lies below 5.6 km and 99% within about 32 km.
Composition (dry air)
- Nitrogen 78.08%, oxygen 20.95%, argon 0.93%, carbon dioxide about 0.04%, with traces of neon, helium, methane, ozone and hydrogen.
- Water vapour (0-4%) varies with place and is concentrated in the lower atmosphere; dust and aerosols act as condensation nuclei.
Layers (by temperature)
Layer | Height | Temperature | Features |
|---|---|---|---|
Troposphere | 0-8 km at the poles, up to 18 km at the equator (average about 13 km) | Falls with height (normal lapse rate 6.5°C per km) | Contains about 75% of the mass and nearly all the water vapour, clouds and weather; top is the tropopause |
Stratosphere | up to about 50 km | Rises with height (ozone absorbs UV) | Dry, stable, no weather; the ozone layer at about 15-35 km; jet aircraft fly here; top is the stratopause |
Mesosphere | 50-80 km | Falls (the coldest, about -90°C) | Meteors burn up; top is the mesopause |
Thermosphere | 80-400 km (to about 600 km) | Rises sharply (above 1,000°C) | Contains the ionosphere (80-400 km), which reflects radio waves; auroras; satellites orbit here |
Exosphere | above about 400 km | Very high | Mainly hydrogen and helium; gases escape into space |
Other points
- Ozone layer: absorbs harmful ultraviolet radiation; the Antarctic ozone hole was caused by CFCs; the Montreal Protocol (1987) phases them out.
- The ionosphere is made of electrically charged particles in the D, E and F layers.
- Homosphere (up to about 80 km, uniform composition) and heterosphere (above, layers of different gases).
- The Karman line (about 100 km) is the conventional boundary of space.
Key Points
- Composition: nitrogen 78%, oxygen 21%, argon 0.93%, CO₂ about 0.04%.
- Troposphere: weather, temperature falls (6.5°C per km), thicker at the equator.
- Stratosphere: ozone layer, temperature rises.
- Mesosphere: coldest; thermosphere: ionosphere, auroras.
- Exosphere: hydrogen and helium; merges into space.
MCQs
Q1. The ozone layer is found in the:
(a) Troposphere
(b) Stratosphere
(c) Mesosphere
(d) Thermosphere
Ans: (b).
Q2. Weather phenomena occur in the:
(a) Troposphere
(b) Stratosphere
(c) Mesosphere
(d) Exosphere
Ans:
(a).
Q3. The ionosphere, which reflects radio waves, lies in the:
(a) Troposphere
(b) Stratosphere
(c) Thermosphere
(d) Mesosphere
Ans: (c).