The atmosphere and its gases

Detail

What the atmosphere is

• Atmosphere is the layer of air around the Earth. It is a mix of gases, water vapour and dust.
• Gravity holds it in place. Without gravity the gases would drift off into space.
• Air has no colour and no smell. We cannot see it. We feel it only when it moves as wind.
• Most of the air sits close to the ground. 99 per cent of the atmosphere's mass lies within 32 km of the Earth's surface.
• The air gets thinner the higher you go.
• Climbers on high mountains therefore struggle to breathe.

Why the atmosphere matters

• It supports life.
• It gives oxygen to animals and humans for breathing.

• It gives carbon dioxide to plants for photosynthesis (the way plants make food from sunlight, water and CO2).

• It shields life. It blocks most of the Sun's harmful ultraviolet (UV) rays. The ozone layer does most of this work (see "Ozone" below).

• It keeps the Earth warm.
• Some gases trap part of the Sun's energy.
• Without them, nights would be freezing and the gap between day and night temperatures would be very large.

Composition of the atmosphere

• Composition of the atmosphere means which gases make up the air, and in what share.
• The shares below are for dry air (air with the water vapour taken out), by volume.
• Permanent gases stay in a nearly fixed ratio near the surface. Variable gases, like water vapour, change from place to place and from time to time.

Gas	Share of dry air (by volume)	Why it matters
Nitrogen (N2)	about 78.08%	Largest share; a fairly inactive gas that thins out the oxygen
Oxygen (O2)	20.95%	Breathing; burning (combustion)
Argon (Ar)	0.93%	A noble gas; third largest share
Carbon dioxide (CO2)	about 0.04%	Tiny share, but drives the greenhouse effect
Neon, helium, krypton, xenon, hydrogen, ozone	traces	Ozone filters UV rays

• Order to remember (largest to smallest): nitrogen > oxygen > argon > carbon dioxide.
• Trap: argon, not CO2, is the third most common gas in dry air.

The mix changes with height

• The gas ratio is not the same all the way up. Heavier gases stay lower.
• Oxygen is almost nil by 120 km.
• Carbon dioxide and water vapour are found only up to 90 km.
• So weather, clouds and the greenhouse effect all happen in the lower atmosphere.

Carbon dioxide — "meteorologically very important"

• Carbon dioxide (CO2) is a gas made of one carbon atom and two oxygen atoms. Breathing, burning and decay release it. Plants and oceans take it in.
• How it warms the Earth (the greenhouse effect):
• Incoming sunlight is short-wave radiation. CO2 is transparent to it, so sunlight passes through and heats the ground.
• The warm ground gives off heat as long-wave radiation, also called terrestrial radiation (heat sent out by the Earth).
• CO2 is opaque to this long-wave heat. It absorbs the heat and sends part of it back down.

• Result: the lower air stays warmer. This is the greenhouse effect (the trapping of the Earth's outgoing heat by gases in the air, like glass traps heat in a greenhouse).

• Why CO2 keeps rising: burning fossil fuels (coal, oil, natural gas) adds CO2 faster than plants and oceans can absorb it.

• Measurement units: CO2 is measured in ppm (parts per million), the number of CO2 molecules in every one million molecules of air.
• Rise over time:
• About 280 ppm before the Industrial Revolution (before about 1750).
• 315.98 ppm annual mean at Mauna Loa in 1959 [3].
• 401.01 ppm in 2015, the first full year above 400 ppm [3].
• 424.61 ppm in 2024 and 427.35 ppm in 2025 (Mauna Loa annual means) [3].

• Monthly mean of 427.55 ppm in August 2026, up from 425.48 ppm in August 2025 [2].

• Mauna Loa record: C. David Keeling of the Scripps Institution of Oceanography began direct CO2 readings at Mauna Loa (Hawaii, USA) in March 1958. NOAA began its own parallel readings in May 1974 [2]. This is the world's longest unbroken record of CO2 measured directly in the air.

• Global picture (WMO Greenhouse Gas Bulletin No. 21, October 2025):
• The global average surface CO2 was 423.9 ppm in 2024, a record high [4].
• CO2 rose by 3.5 ppm from 2023 to 2024, the largest yearly jump since modern measurements began in 1957 [4].
• Causes of the jump: continued human emissions, more wildfires, and weaker carbon sinks (land and oceans that absorb CO2). The 2024 El Niño and record heat reduced how much CO2 land and oceans could take up [4].
• CO2 is about 52% above pre-industrial levels. Methane (1,942 ppb) is 166% above, and nitrous oxide (338.0 ppb) is 25% above (2024) [4].

• CO2 stays in the air for hundreds of years, so today's emissions will affect the climate for centuries [4].

• Mauna Loa (one site) and WMO (global average) give slightly different numbers. Always state the source and year in an answer.

Ozone — a trace gas with a big job

• Ozone (O3) is a form of oxygen with three atoms.
• Most of it lies between 10 and 50 km above the surface.
• It filters ultraviolet rays and stops most of them from reaching the ground. (The ozone layer and its depletion are covered in detail in the stratosphere section.)

Water vapour — the variable gas

• Water vapour is water in the form of an invisible gas. It enters the air through evaporation and transpiration (water released by plants).
• It is a variable gas: its amount changes with place, height and season.
• How it is spread out:
• It decreases with height.
• It decreases from the equator towards the poles, because warm air can hold more moisture than cold air.

Region	Water vapour (by volume)	Reason
Warm, wet tropics (e.g. equatorial belt, Kerala, Northeast India)	up to 4%	High heat → high evaporation → warm air holds more vapour
Deserts (e.g. Thar, Sahara)	under 1%	Little water to evaporate
Polar regions	under 1%	Cold air holds very little vapour

• (NCERT Class 9: "0.1 to 0.4 per cent".) This is a typical average, not the full range. The 4% figure is the tropical maximum.
• What water vapour does:
• It absorbs part of the insolation. Insolation (incoming solar radiation) is the Sun's energy that reaches the Earth.
• It keeps in the Earth's heat like a blanket. That is why cloudy, humid nights stay warm and dry desert nights get very cold.

• It controls the stability of air. Stable air resists rising, so the sky stays clear. Unstable air rises easily. As moist air rises it cools and its vapour condenses into clouds, which can lead to thunderstorms (see Atmospheric moisture and precipitation).

• Water vapour and global warming:

• Water vapour is the Earth's most abundant greenhouse gas. It causes about half of the greenhouse effect [5].
• For every 1 °C rise in air temperature, the air can hold about 7% more water vapour [5].
• This is a positive feedback (a change that pushes itself further):
  • CO2 warms the air.
  • Warmer air holds more water vapour.
  • More water vapour traps more heat.
  • Result: this more than doubles the warming that CO2 alone would cause [5].
• The IPCC Sixth Assessment Report finds total water vapour in the air is rising 1–2% per decade [5].

Prelims Hooks

• Gases of dry air from largest to smallest share: nitrogen (78.08%) > oxygen (20.95%) > argon (0.93%) > CO2 (~0.04%).
• 99% of the atmosphere's mass lies within 32 km of the surface.
• Oxygen is almost nil by 120 km. CO2 and water vapour are found only up to 90 km.
• CO2 is transparent to incoming short-wave solar radiation and opaque to outgoing long-wave terrestrial radiation, which causes the greenhouse effect.
• Water vapour: up to 4% in the humid tropics and under 1% in deserts and polar regions. It decreases with height and from the equator to the poles.
• Most abundant greenhouse gas = water vapour, not CO2. It causes about half of the greenhouse effect [5].
• The Mauna Loa CO2 record began in 1958 (Keeling). Its annual mean first went above 400 ppm in 2015 and reached 427.35 ppm in 2025 [2][3].
• WMO: global CO2 was 423.9 ppm in 2024, and the 3.5 ppm rise from 2023 to 2024 was the largest on record [4].
• Ozone layer: 10–50 km. It filters UV rays.
• Trap: the third most common gas in dry air is argon, not CO2.

Mains Points

• Small gases, big effects:
• CO2 (~0.04%) and water vapour (0.1–4%) are only a tiny share of the air.
• Yet they control the Earth's heat balance.

• The water vapour feedback more than doubles the warming from CO2 [5]. This is why small changes in CO2 matter so much (GS-I climatology; GS-III climate change).

• Rising CO2 and its causes:

• CO2 has risen from about 280 ppm before industry to 427.35 ppm in 2025 (Mauna Loa) [3].
• Causes: burning fossil fuels, wildfires, and weaker land and ocean sinks during El Niño years [4].

• Because CO2 stays in the air for centuries [4], this supports deep cuts in emissions under India's net-zero target for 2070.

• Spread of water vapour and weather in India:

• There is much more water vapour over the humid tropics than over deserts.
• This explains heavy convectional rain over Kerala and Northeast India, compared with large day–night temperature swings and dry air in the Thar.

• More water vapour in a warming climate (1–2% per decade [5]) means more extreme rainfall events, which are relevant to disaster management (GS-III).

• The atmosphere as a life-support system: it supplies oxygen and CO2, filters UV through ozone and keeps the planet warm. This is why air pollution, ozone depletion and greenhouse emissions are linked environmental problems.