Atmosphere: Composition & Structure – NDA Geography Notes

Introduction

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.

Compared to the Earth itself, the atmosphere is extraordinarily thin. If you shrink the Earth to the size of an apple, the atmosphere would be thinner than the apple’s skin. Yet this thin layer is what makes Earth habitable and distinguishes it from every other planet we know.

NDA tests this chapter heavily — through questions on the layers of the atmosphere and their sequence, temperature trends in each layer, the ozone layer, radio wave reflection, and temperature inversion. The ozone layer alone has appeared in over a dozen NDA and CDS papers. If you understand the structure of the atmosphere clearly once, these questions become very easy.

Understand the Topic

Composition of the Atmosphere

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

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

Oxygen is second at about 21%. It is 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, temperature, and altitude — from almost zero in deserts to about 4% in humid tropical areas. Despite its small percentage, water vapour 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:
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. [NDA tested via CDS]

Decrease of Gases with 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 is:

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

Carbon dioxide is heaviest and stays closest to the surface. Oxygen is lighter and extends higher. Nitrogen is lightest among 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 throughout the atmosphere — it decreases in some layers and increases in others. This creates distinct, identifiable layers.

From the surface upward, the layers are:

1. Troposphere
2. Stratosphere
3. Mesosphere
4. Thermosphere (Ionosphere)
5. Exosphere

The correct sequence from the surface upward is Troposphere → Stratosphere → Mesosphere → Thermosphere. [NDA 2013-II | NDA tested via CDS]

1. The Troposphere

The troposphere is the lowest layer of the atmosphere. It extends 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. This is 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 then radiates heat upward as long-wave (terrestrial) radiation. As you move upward away from this heat source, temperature drops. This is the fundamental reason for the lapse rate. [NDA 2014-I]

Air temperature is measured at a standard height of 1.2 m (4 feet) above the ground surface. The average rate of temperature decrease with height is the environmental temperature lapse rate. [NDA 2024-I]

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 in the troposphere. [NDA 2023-II]

However, all storms and cloudiness are NOT restricted to the stratosphere — they occur in the TROPOSPHERE. [NDA tested via CDS]

Cirrus clouds form at the top layers of the troposphere. [NDA tested via CDS]

The Tropopause:
The top of the troposphere is called the tropopause. This is where temperature stops decreasing and becomes roughly constant before starting to increase in the next layer.

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]

Temperature in the troposphere decreases at approximately 1°F for every 165 m of height (which corresponds to the 6.4°C per km metric rate). [NDA 2025-II]

2. The Stratosphere

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. This is the opposite of the troposphere. The reason is the ozone layer — ozone in the stratosphere absorbs ultraviolet radiation from the Sun and converts it to heat. This warming increases with altitude up to the stratopause. [NDA 2016-II | NDA tested via CDS]

Because warm air sits above cool air in the stratosphere, it is very stable — warm above cold does not convect. This is why jet aircraft fly in the lower stratosphere — smooth, stable air with no turbulence from weather.

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

The Ozone Layer

The stratosphere contains the ozone layer — a band of relatively concentrated ozone gas (O₃) at an altitude of approximately 15 to 35 km (most concentrated between 20–35 km).

What is ozone?
Ozone is a molecule of three oxygen atoms (O₃). Regular oxygen we breathe has two atoms (O₂). Ozone is a tri-atomic molecule of oxygen. [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 is harmful to living organisms — it 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]

Ozone layer facts:

1. Ozone is mostly found in the stratosphere — TRUE
2. Ozone layer lies 15–35 km above the surface — TRUE (sometimes stated as 25–30 km for the densest band)
3. Ozone absorbs UV radiation — TRUE [NDA tested via CDS]
4. Ozone is a tri-atomic molecule of oxygen — TRUE [NDA 2016-II]

Why is the ozone layer important?
The ozone layer prevents entry of UV rays — protecting all life on Earth. [NDA tested via CDS]

The Ozone Hole:
The ozone layer has been thinning — particularly over the poles. The thinning is most severe over Antarctica — this is called the ozone hole. Ozone holes are more pronounced at the poles. [NDA tested via CDS]

Cause: Chlorofluorocarbons (CFCs) — chemicals used in refrigerators, air conditioners, and aerosol sprays — rise into the stratosphere. There, UV radiation breaks them apart and releases chlorine atoms that destroy ozone molecules in a chain reaction. [NDA tested via CDS]

The ozone hole is a region over the Antarctic caused mainly by CFC gases. [NDA tested via CDS]

Montreal Protocol (1987):
International agreement to phase out CFCs and other ozone-depleting substances.

World Ozone Day = 16 September — this is the date the Montreal Protocol was signed.
The statement that 16th November is World Ozone Day is FALSE — it is 16th September. [NDA tested via CDS]

Most ozone is located in the stratosphere — about 90% of all atmospheric ozone is there. [NDA tested via CDS]

3. The Mesosphere

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

Temperature in the mesosphere:
Temperature decreases with altitude — just like the troposphere. The top of the mesosphere (mesopause) is the coldest place in the atmosphere — temperatures can drop below –90°C. [NDA tested via CDS]

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

The percentage of Helium gas is maximum in the exosphere — but Helium becomes relatively more concentrated in the upper thermosphere. [NDA 2013-II]

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4. The Thermosphere (Ionosphere)

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

Temperature in the thermosphere:
Temperature increases sharply with altitude. This is because 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).

Radio wave reflection:
The ionosphere reflects radio waves back to Earth — allowing long-distance radio communication. Without the ionosphere, radio waves would escape into space. [NDA tested via CDS]

Aurora (Northern and Southern Lights):
The aurora borealis (Northern Lights) and aurora australis (Southern Lights) occur in the thermosphere. They are caused by charged particles from the Sun (solar wind) hitting gas molecules near the poles and causing them to glow. [NDA 2024-I]

Three correct statements about aurora: [NDA 2024-I]

1. Solar wind directed toward magnetic poles causes colourful lights — TRUE
2. Different gases glow with different colours — TRUE (oxygen = green and red; nitrogen = blue and purple)
3. In April 2023, aurora was captured in India at Hanle, Ladakh — TRUE

5. The Exosphere

The exosphere is the outermost layer — extending from about 600 km outward. It gradually merges with outer space — there is no sharp upper boundary.

The exosphere contains extremely thin gas — mostly hydrogen and helium, the lightest elements. Gas molecules here move so fast that some escape Earth’s gravity entirely.

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

The atmosphere has no definite upper limits but gradually thins out. [NDA tested via CDS]

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Summary Table — Atmospheric Layers

Layer	Altitude	Temperature Trend	Key Feature
Troposphere	0–12 km	Decreases (~6.4°C/km)	All weather; thickest at equator [NDA 2025-II]
Stratosphere	12–50 km	Increases	Ozone layer (15–35 km); jet aircraft
Mesosphere	50–80 km	Decreases	Coldest layer; meteors burn up
Thermosphere/Ionosphere	80–600 km	Increases	Radio waves reflected; aurora
Exosphere	600+ km	—	Mostly hydrogen; merges with space

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.

What causes it?
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.

Temperature increases with increasing altitude during inversion. A long winter night with clear skies is an ideal situation for temperature inversion. It is a short-term phenomenon common all over the globe. [NDA tested via CDS]

Temperature inversion and smog:
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 the formation of smog is the formation of an inversion lid. [NDA tested via CDS]

Surface inversion commonly lasts for a few hours until the Sun comes up and heats the surface — breaking down the inversion. Surface inversion causes stability in the lower layers, not instability. [NDA tested via CDS]