Atmosphere: Composition & Structure – NDA Geography Notes

The Earth is surrounded by a thin layer of gases: the atmosphere. Without it, life on Earth would be impossible. The atmosphere gives us the air we breathe, protects us from the Sun’s harmful radiation, keeps the planet warm enough to sustain life, and is the medium through which all weather occurs.

NDA tests this chapter very heavily, through questions on the layers of the atmosphere, temperature trends in each layer, the ozone layer, radio wave reflection, and temperature inversion. If you understand the structure of the atmosphere clearly once, these questions become very easy.

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Composition of the Atmosphere

The atmosphere is a mixture of gases. By volume, the approximate composition near the surface is:

Gas	Percentage
Nitrogen (N₂)	78.09%
Oxygen (O₂)	20.95%
Argon (Ar)	0.93%
Carbon dioxide (CO₂)	0.04%
Water vapour	Variable (0–4%)
Other trace gases	Very small amounts

Nitrogen is the most abundant gas, about 78%. It is relatively inert and acts as a diluting agent for oxygen.

Oxygen is second at about 21%, essential for life and combustion. The gas placed second in respect of abundance in the Earth’s atmosphere is oxygen. [NDA 2017-II]

Water vapour varies greatly with location, from almost zero in deserts to about 4% in humid tropical areas. Despite its small percentage, it is the source of all precipitation.

Carbon dioxide is present in only about 0.04% but plays a crucial role in the greenhouse effect: trapping heat and keeping the Earth warm.

Air close to the Earth’s surface is heavier (denser). It also contains a larger quantity of water vapour and dust particles because both are heavier than air and stay close to the surface.

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Gases and Altitude

As you go higher, some gases decrease faster than others. The correct order in which gases disappear as you move upward from the surface:

Carbon dioxide → Oxygen → Nitrogen [NDA 2009-II]

Carbon dioxide is heaviest and stays closest to the surface. Oxygen extends higher. Nitrogen is the lightest of these three and extends highest before becoming negligible.

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Layers of the Atmosphere

The atmosphere is divided into layers based on how temperature changes with altitude. Temperature does not decrease uniformly. It decreases in some layers and increases in others.

From the surface upward: Troposphere → Stratosphere → Mesosphere → Thermosphere → Exosphere [NDA 2013-II]

The alternating temperature pattern: down, up, down, up: is the single most important framework in this chapter.

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The Troposphere

The troposphere is the lowest layer of the atmosphere, extending from the Earth’s surface up to about 12 km on average, higher over the equator (about 16 km) and lower over the poles (about 8 km).

The thickness of the troposphere is greatest at the equator. [NDA 2025-II]

Temperature in the Troposphere

Temperature decreases with altitude in the troposphere. The average rate of decrease is about 6.4–6.5°C per kilometre: called the normal lapse rate or environmental lapse rate. [NDA 2018-II | NDA 2024-I]

Why does temperature decrease upward in the troposphere? The troposphere is heated from below: by the Earth’s surface. The Sun’s radiation heats the ground, which radiates heat upward as long-wave (terrestrial) radiation. Moving up and away from this heat source, temperature drops. [NDA 2014-I]

Air temperature is measured at a standard height of 1.2 m (4 feet) above the ground surface. [NDA 2024-I]

Temperature in the troposphere decreases at approximately 1°F for every 165 m of height. [NDA 2025-II]

All weather happens in the troposphere. The troposphere contains 75–80% of the total mass of the atmosphere and almost all the water vapour. All clouds, rain, snow, storms, and other weather phenomena occur here. [NDA 2023-II]

Cirrus clouds form at the top layers of the troposphere.

The Tropopause

The top of the troposphere is called the tropopause. The normal lapse rate of temperature drops to 0°C at the upper boundary of the tropopause. [NDA 2019-II]

The air temperature of the tropopause is NOT highest above the poles. It is actually coldest above the tropics and somewhat warmer above the poles. [NDA 2025-II]

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The Stratosphere and the Ozone Layer

The stratosphere extends from the tropopause (about 12 km) up to about 50 km altitude.

Temperature in the Stratosphere

Temperature increases with altitude in the stratosphere, the opposite of the troposphere. The reason is the ozone layer: ozone absorbs ultraviolet radiation from the Sun and converts it to heat, warming the upper stratosphere. [NDA 2016-II]

Because warm air sits above cool air, the stratosphere is very stable: no turbulence from weather. This is why jet aircraft fly in the lower stratosphere.

The statement that temperature is constant at different heights in the stratosphere is incorrect: temperature increases with altitude. [NDA 2018-II]

The Ozone Layer

★ IMPORTANT The ozone layer: one of the most frequently tested topics across NDA and CDS papers: is located in the stratosphere at approximately 15 to 35 km altitude. [NDA 2007-II | NDA 2013-II | NDA 2016-II]

What is ozone? Ozone is a molecule of three oxygen atoms (O₃): a tri-atomic molecule of oxygen. Regular oxygen we breathe has two atoms (O₂). [NDA 2016-II]

What does the ozone layer do? The ozone layer absorbs most of the Sun’s ultraviolet (UV) radiation before it reaches the Earth’s surface. UV radiation causes skin cancer, eye damage, and genetic mutations.

Injurious ultraviolet radiations are mostly absorbed by the stratosphere. [NDA 2007-II]

The atmospheric gas that filters out most of the ultraviolet radiation of the Sun is ozone. [NDA 2013-II]

The Ozone Hole: the ozone layer has been thinning, most severely over Antarctica. Ozone holes are more pronounced at the poles.

Cause: Chlorofluorocarbons (CFCs): chemicals used in refrigerators, air conditioners, and aerosol sprays: rise into the stratosphere. UV radiation breaks them apart, releasing chlorine atoms that destroy ozone molecules in a chain reaction.

Montreal Protocol (1987): international agreement to phase out CFCs.

World Ozone Day = 16 September (the date the Montreal Protocol was signed). The statement that 16th November is World Ozone Day is FALSE.

Most ozone (about 90%) is located in the stratosphere.

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The Mesosphere

The mesosphere extends from the stratosphere (about 50 km) up to about 80–90 km altitude.

Temperature decreases with altitude in the mesosphere, just like the troposphere. The top of the mesosphere (the mesopause) is the coldest place in the entire atmosphere. Temperatures can drop below –90°C.

Meteors burn up in the mesosphere due to friction with the air.

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The Thermosphere and Ionosphere

The thermosphere extends from the mesosphere (about 80–90 km) upward to about 600 km altitude.

Temperature increases sharply with altitude: the thermosphere absorbs high-energy X-ray and UV radiation from the Sun.

The Ionosphere: the thermosphere is also called the ionosphere because solar radiation ionises the gas molecules here, creating electrically charged particles (ions). The ionosphere reflects radio waves back to Earth, allowing long-distance radio communication. Without the ionosphere, radio waves would escape into space.

Aurora (Northern and Southern Lights): the aurora borealis (Northern Lights) and aurora australis (Southern Lights) occur in the thermosphere. Charged particles from the Sun (solar wind) hit gas molecules near the poles, causing them to glow. Different gases produce different colours. Oxygen glows green and red; nitrogen glows blue and purple. [NDA 2024-I]

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The Exosphere

The exosphere is the outermost layer: extending from about 600 km outward, gradually merging with outer space. The atmosphere has no definite upper limits.

The exosphere contains extremely thin gas: mostly hydrogen (the lightest element). [NDA 2015-II] Helium is also present. Gas molecules here can escape Earth’s gravity entirely.

Hydrogen is found in the highest quantity in the exosphere. [NDA 2015-II]

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Temperature Inversion

Normally, temperature decreases as you go up in the troposphere. But sometimes a layer of warm air sits above cooler air near the surface. This is called temperature inversion: the normal pattern is inverted.

Causes

On calm, clear nights, the Earth’s surface radiates heat away rapidly. The surface cools quickly. The air just above it also cools. But air a little higher up remains warmer. So temperature increases with altitude near the surface: the opposite of normal.

A long winter night with clear skies is an ideal situation for temperature inversion. It is a short-term phenomenon common worldwide.

Effects

Temperature inversion traps pollutants near the surface. The warm air layer above acts like a lid, preventing the cool, polluted air below from rising and dispersing. This creates smog. The most important factor for smog formation is the creation of an inversion lid.

Surface inversion causes stability in the lower layers: not instability. It lasts for a few hours until the Sun comes up and heats the surface, breaking down the inversion.

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