Composition of the atmosphere

Topic: Atmosphere: Composition, Structure and Elements of Weather · NCERT: Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 7 "Composition and Structure of Atmosphere"

Meaning

Composition of the atmosphere means which gases make up the air and how much of each there is. In dry air (air with the water vapour taken out), measured by volume, there is about 78.08% nitrogen, 20.95% oxygen, 0.93% argon and about 0.04% carbon dioxide (CO2). Tiny amounts of other gases make up the rest.

It matters because the main gases support life through breathing, burning and photosynthesis. A few gases that are present only in tiny amounts, such as CO2, water vapour and ozone, control the Earth's heat balance and block harmful ultraviolet (UV) rays.

Explanation

What the air is made of

• The atmosphere is the layer of air around the Earth. It is a mix of gases, water vapour and dust, and gravity holds it in place.
• Most of the air sits close to the ground. 99% of the atmosphere's mass lies within 32 km of the Earth's surface.
• The shares of gases in dry air, by volume:

Gas	Share of dry air	Its job
Nitrogen (N2)	about 78.08%	Largest share. It does not react easily, and it thins out the oxygen
Oxygen (O2)	20.95%	Breathing and burning (combustion)
Argon (Ar)	0.93%	A noble gas (a gas that hardly reacts with anything). Third largest share
Carbon dioxide (CO2)	about 0.04%	Tiny share, but it drives the greenhouse effect
Neon, helium, krypton, xenon, hydrogen, ozone	traces	Ozone filters UV rays

• Permanent gases and variable gases:
• Permanent gases, such as nitrogen, oxygen and argon, stay in a nearly fixed ratio near the surface.
• Variable gases, such as water vapour, change from place to place and from time to time.

The mix changes with height

• Heavier gases stay lower. The ratio of gases is not the same all the way up.
• Oxygen is almost nil by 120 km.
• CO2 and water vapour are found only up to 90 km.

• Result: weather, clouds and the greenhouse effect all happen in the lower atmosphere.

• Ozone (O3) is a form of oxygen with three atoms. Most of it lies between 10 and 50 km above the surface, where it stops most UV rays from reaching the ground.

Carbon dioxide: a small share with a big effect

• How CO2 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 sends out heat as long-wave radiation, also called terrestrial radiation (heat given off by the Earth).
• CO2 is opaque to this heat, so it absorbs the heat and sends part of it back down.

• Result: the lower air stays warm. This trapping of the Earth's outgoing heat is the greenhouse effect.

• CO2 is measured in ppm (parts per million). This is the number of CO2 molecules in every one million molecules of air.

• How CO2 has risen:
• About 280 ppm before the Industrial Revolution (before about 1750).
• 315.98 ppm in 1959 (Mauna Loa annual mean) [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].

• The monthly mean was 427.55 ppm in August 2026, up from 425.48 ppm in August 2025 [2].

• The global average (WMO Greenhouse Gas Bulletin No. 21, October 2025):

• 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].

• 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].

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

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 decreases with height. It also decreases from the equator towards the poles, because warm air can hold more moisture than cold air.
• How much there is:
• Up to 4% in the warm, wet tropics.
• Under 1% in deserts and polar regions.

• NCERT Class 9 gives "0.1 to 0.4 per cent". This is a typical average, not the full range.

• What water vapour does:

• It absorbs part of the insolation (the Sun's energy that reaches the Earth).
• It holds in the Earth's heat like a blanket. That is why humid nights stay warm and desert nights get very cold.

• It controls the stability of air. Stable air resists rising. Unstable air rises easily, cools and forms clouds.

• The water vapour feedback (a change that pushes itself further):

• Water vapour is the most abundant greenhouse gas and causes about half of the greenhouse effect [5].
• For every 1 °C of warming, the air can hold about 7% more water vapour [5].
• CO2 warms the air. Warmer air holds more vapour, and more vapour traps more heat. Result: this more than doubles the warming that CO2 alone would cause [5].

Where it occurs

• Mauna Loa (Hawaii, USA): C. David Keeling of the Scripps Institution of Oceanography began direct CO2 readings here 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.
• Warm, wet tropics: the equatorial belt, Kerala and Northeast India have up to 4% water vapour. This supports heavy convectional rain (rain from warm, moist air that rises on its own).
• Deserts: the Thar (India) and the Sahara (Africa) have under 1% water vapour, dry air and large day–night temperature swings.
• Polar regions: under 1% water vapour, because cold air holds very little moisture.
• By height: 99% of the atmosphere's mass lies within 32 km, the ozone layer is at 10–50 km, CO2 and water vapour reach only up to 90 km, and oxygen is almost nil by 120 km.

Don't confuse with

• Most abundant gas vs most abundant greenhouse gas: nitrogen is the most abundant gas in dry air. Water vapour is the most abundant greenhouse gas, and it is not CO2 [5].
• Argon vs carbon dioxide: argon (0.93%) is the third most common gas in dry air. CO2 (~0.04%) comes fourth.
• Permanent gases vs variable gases: permanent gases (N2, O2, Ar) stay in a nearly fixed ratio near the surface. Variable gases such as water vapour change with place, height and season.
• Mauna Loa reading vs WMO global average: Mauna Loa is one site (427.35 ppm in 2025) [3]. WMO gives a global average (423.9 ppm in 2024) [4]. Always state the source and year.

Prelims Hooks

• Dry air from largest to smallest share: nitrogen (78.08%) > oxygen (20.95%) > argon (0.93%) > CO2 (~0.04%). The third most common gas is argon, not CO2.
• 99% of the atmosphere's mass lies within 32 km. Oxygen is almost nil by 120 km. CO2 and water vapour reach only up to 90 km.
• CO2 is transparent to incoming short-wave solar radiation and opaque to outgoing long-wave terrestrial radiation. This causes the greenhouse effect.
• Water vapour is up to 4% in the humid tropics and under 1% in deserts and polar regions. It decreases with height and from the equator towards the poles.
• 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. The 3.5 ppm rise from 2023 to 2024 was the largest on record [4].

Mains Points

• Small gases, big effects (GS-I climatology; GS-III climate change):
• 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], so even small changes in CO2 matter a lot.

• Rising CO2: causes and policy (GS-III):

• CO2 has risen from about 280 ppm before industry to 427.35 ppm in 2025 (Mauna Loa) [3].
• The causes are burning fossil fuels, 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 stays in the air for hundreds of years [4]. This makes the case for deep emission cuts under India's net-zero target for 2070.

• Water vapour, India's weather and disasters (GS-I, GS-III):

• There is much more water vapour over Kerala and Northeast India than over the Thar. This explains heavy convectional rain in the first and dry air with large day–night temperature swings in the second.
• Total water vapour is rising 1–2% per decade (IPCC Sixth Assessment Report) [5]. This means more extreme rainfall events, which matters for disaster management.

Related concepts

• Atmosphere
• Carbon dioxide
• Water vapour

Read more

• The atmosphere and its gases