Pariksha SaathiLesson 5 of 10 · Physical and World Geography
The Atmosphere: Composition, Layers, Insolation, Heat Budget and Temperature Inversion
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Composition
Dry air is about 78 per cent nitrogen, 21 per cent oxygen, 0.93 per cent argon and about 0.04 per cent carbon dioxide, with traces of neon, helium, ozone, hydrogen and methane. Water vapour varies from almost nil in cold, dry air to about 4 per cent in the humid tropics. Small as their shares are, carbon dioxide and water vapour matter most for climate, because they let sunlight through but absorb the heat the Earth radiates back. Dust and salt particles act as nuclei on which water vapour condenses into clouds and fog. About 99 per cent of the atmosphere's mass lies within 32 km of the surface.
Troposphere and stratosphere
The troposphere, the lowest layer, is (in NCERT figures) about 8 km thick at the poles and about 18 km at the Equator, where strong convection carries heat higher. It holds almost all the water vapour and dust, so all weather occurs here. Temperature falls with height at the normal lapse rate of about 6.5 degrees Celsius per km, reaching about minus 80 degrees Celsius over the Equator and minus 45 degrees over the poles at the tropopause. The stratosphere above extends to about 50 km. It holds the ozone layer, which absorbs harmful ultraviolet rays, so here temperature rises with height; its calm, cloud-free lower part suits jet aircraft.
Upper layers
The mesosphere, from about 50 to 80 km, is where temperature falls again, to about minus 100 degrees Celsius at the mesopause, the coldest part of the atmosphere; most meteors burn up here. The ionosphere, from about 80 to 400 km and part of the thermosphere, contains electrically charged particles (ions) produced by solar radiation. It reflects radio waves back to the Earth, making long-distance radio communication possible, and auroras glow in it; its layers include the Kennelly-Heaviside (E) layer and the Appleton (F) layer. The exosphere is the outermost, extremely thin layer that merges into space.
The ozone layer
Ozone (O3) is a molecule of three oxygen atoms concentrated in the stratosphere, measured in Dobson units. In 1985 scientists reported a seasonal ozone hole over Antarctica, caused mainly by chlorofluorocarbons (CFCs) from refrigerators, air conditioners and aerosol sprays. The Vienna Convention (1985) set up the framework for protecting the ozone layer, the Montreal Protocol (1987) under it phased these chemicals out, and 16 September, the day the protocol was signed, is the International Day for the Preservation of the Ozone Layer.
Insolation and the heat budget
Energy received from the Sun is insolation (incoming solar radiation). It arrives as short waves, about 1.94 calories per square centimetre per minute at the top of the atmosphere (the NCERT figure), a value called the solar constant. The amount reaching a place depends on the angle of the Sun's rays, the length of the day and the clearness of the air, so the highest insolation falls on the cloudless subtropical deserts, not on the cloudier Equator. In the NCERT heat budget, out of 100 units of insolation about 35 units are reflected back to space by clouds, ice and the air; this reflected share is the Earth's albedo. The atmosphere absorbs 14 units and the surface 51 units. The surface sends its 51 units back as long-wave terrestrial radiation: 17 units go straight to space and 34 are absorbed by the atmosphere, which radiates 48 units (14 plus 34) to space. As 35, 17 and 48 add up to 100, the Earth as a whole neither heats up nor cools down.
Heating of the air and temperature inversion
The atmosphere is heated mostly from below, by terrestrial radiation, conduction and convection, rather than directly by sunlight, which is why temperature normally falls with height. Horizontal transfer of heat by wind is called advection; the hot Loo of north India is an example. Sometimes temperature increases with height instead: this is temperature inversion. It forms on long winter nights with clear skies and calm, dry air, when the ground loses heat fast and chills the air touching it. Inversions make the air stable, stop convection and trap fog, smoke and pollution near the ground, as in winter smog over north Indian cities. In hills, cold, dense air drains down slopes at night into the valleys (air drainage), so frost forms in valley bottoms while the slopes stay warmer, and orchards and plantations are placed on the slopes.
The greenhouse effect
Like the glass of a greenhouse, some gases let sunlight in but keep heat from escaping. Carbon dioxide, water vapour, methane, nitrous oxide, ozone and CFCs absorb long-wave terrestrial radiation and send part of it back to the surface. This natural greenhouse effect keeps the Earth's average surface temperature at about 15 degrees Celsius instead of about minus 18 degrees. Burning fossil fuels and clearing forests have strengthened it, causing global warming, melting glaciers and rising seas. The Kyoto Protocol (1997) set binding cuts for developed countries, and the Paris Agreement (2015) aims to keep warming well below 2 degrees Celsius and to try to limit it to 1.5 degrees.
Key facts
- Dry air by volume: nitrogen 78%, oxygen 21%, argon 0.93%, carbon dioxide about 0.04%; water vapour varies from nil to about 4%.
- Troposphere: the weather layer, about 8 km thick at the poles and 18 km at the Equator (NCERT); temperature falls about 6.5 degrees C per km.
- Stratosphere (up to about 50 km): ozone layer, temperature rises with height, calm air used by jet aircraft.
- Mesosphere (50-80 km): coldest layer, about minus 100 degrees C at the mesopause; meteors burn up here.
- Ionosphere (80-400 km): ions reflect radio waves back to the Earth; auroras appear here; E layer = Kennelly-Heaviside, F layer = Appleton.
- Ozone hole over Antarctica reported in 1985, caused by CFCs; Montreal Protocol 1987; Ozone Day 16 September.
- Heat budget (NCERT): of 100 units, 35 reflected (albedo), 14 absorbed by the atmosphere, 51 by the surface; 17 + 48 units radiated back to space.
- Maximum insolation falls on the subtropical deserts (clear skies), not the cloudy Equator; solar constant about 1.94 calories per sq cm per minute (NCERT).
- Temperature inversion: long, clear, calm winter nights; brings fog, frost and smog; frost collects in valleys through air drainage.
- Greenhouse effect keeps the Earth at about 15 degrees C instead of about minus 18 degrees C; Kyoto Protocol 1997, Paris Agreement 2015.
Quick revision
Nitrogen 78%, oxygen 21%, argon 0.93%, CO2 about 0.04%; 99% of mass within 32 km; troposphere 8 km at poles, 18 km at Equator, lapse rate 6.5 C per km; stratosphere to 50 km with ozone; mesosphere 50-80 km, coldest, meteors burn; ionosphere 80-400 km reflects radio waves; ozone hole 1985, Montreal Protocol 1987; insolation highest over subtropical deserts; albedo 35 units; inversion on clear calm winter nights; greenhouse effect keeps Earth at 15 C.