UPSC NCERT Notes - Atmosphere Composition and Structure

The study of the Earth’s atmosphere is crucial in understanding the dynamics of our planet’s climate, weather patterns, and the delicate balance of life-sustaining gases. Comprising a diverse array of gases, particles, and layers, the atmosphere acts as a protective blanket, shielding Earth from harmful radiation and regulating temperature. To comprehend its intricacies, we delve into the composition and structure of the atmosphere, a foundational aspect of geography. In this exploration, we uncover the fundamental principles governing the distribution and behavior of atmospheric components, laying the groundwork for a comprehensive understanding of Earth’s atmospheric systems. These insights, indispensable for aspiring civil servants preparing for the UPSC examinations, form the cornerstone of NCERT notes on Geography, facilitating a holistic comprehension of atmospheric phenomena.

Table of Contents

Introduction to the Atmosphere:

The Earth’s atmosphere, tethered to the surface by gravitational force, is a dense gaseous envelope. It comprises layers with distinct characteristics, including composition, temperature, and pressure.
The atmosphere exhibits varying densities and temperatures across its layers. Density is highest near the Earth’s surface and decreases with altitude.
Functioning as a colossal greenhouse, the atmosphere permits shortwave insolation to penetrate while becoming opaque to outgoing longwave terrestrial radiation. This greenhouse effect helps maintain an average surface temperature of 15°C.

Atmosphere Composition:

Gases in the atmosphere display dynamic ratios in the upper layers. For instance, carbon dioxide and water vapor are present only up to approximately 90 km above the Earth’s surface.
Beyond 120 km, oxygen content nearly diminishes.
Water vapor, dust particles, and other particulates are crucial for maintaining global climatic conditions. 90% of dust particles and water pour are only present up to a height of 10 kr. whereas 99% of various gases are found up to a height of 32 km.

Gases in the Atmosphere

Air is a mixture of different gases in varying proportions. The two important gases in the atmosphere are Nitrogen and Oxygen.
Nitrogen is composed of 78% of the total gases in the atmosphere while oxygen comprises 21%.
The remaining 1% includes argon, carbon dioxide, ozone, water vapor, and many other gases.

Nitrogen

Nitrogen (N₂) constitutes the majority of the air and is vital for plant survival. However, plants cannot directly assimilate atmospheric nitrogen.
Bacteria residing in soil and plant roots convert atmospheric nitrogen into a usable form for plants.
Nitrogen plays a significant role in extinguishing fires by depleting the oxygen supply.
Bacteria, such as Pseudomonas, facilitate the fixation of atmospheric nitrogen, supporting the growth of algae and leguminous plants.
The nitrogen cycle, a biogeochemical process, transforms inert atmospheric nitrogen into a more accessible form.
The nitrogen cycle is crucial for plants, as nitrogen is essential for chlorophyll production.

Oxygen

Oxygen (O) is the second most abundant gas in the atmosphere.
Humans and animals inhale oxygen during respiration, while green plants release oxygen through photosynthesis. This maintains a constant oxygen content in the air.
Oxygen is a prerequisite for combustion, requiring fuel, an oxygen source, and a heat source.
The oxygen cycle involves the circular flow of oxygen through the lithosphere, biosphere, and atmosphere. Humans and other living organisms utilize oxygen for metabolism and digestion.

Carbon Dioxide

Green plants utilize carbon dioxide (CO₂) in photosynthesis to produce their own food and release oxygen. Humans and animals release carbon dioxide during respiration.
Carbon dioxide is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation.
Responsible for the greenhouse effect, carbon dioxide constitutes approximately 0.036% of the atmosphere.
Global agreements like the Kyoto Protocol (1997) and the Paris Climate Conference (2015) aim to regulate atmospheric carbon dioxide levels.

Ozone

Ozone (O₃) is present between 10 and 50 km above the Earth’s surface.
Acting as a filter, ozone absorbs ultraviolet rays from the Sun, preventing them from reaching the Earth’s surface.
Harmful substances like nitrous oxide from jet planes, chlorofluorocarbons (CFCs), and hydrochlorofluorocarbons (HFCs) released by appliances such as air conditioners pose a threat to the ozone layer.
A molecule called ozone has three oxygen atoms in it. It is blue and smells strong. Two French scientists, Charles Fabry and Henri Buisson, discovered the ozone layer in 1913.
Ozone occurs naturally in small (trace) amounts in the upper atmosphere (the stratosphere). The upper stratosphere has abundant ozone has been found between 30 and 50 km in height.

Ozone Hole and Related Protocols

Joe Farman, Brian Gardiner, and Jonathan Shanklin identified the hole in stratospheric ozone above Antarctica in 1985. An estimate shows that if a group of 500 supersonic jet planes takes off every day, they will deplete the ozone layer by 12%.
The Montreal Protocol is an international agreement established in 1987 to safeguard the stratospheric ozone layer by gradually banning the manufacturing and use of chemicals that deplete the ozone layer (ODS).
The United Nations General Assembly declared 16th September to be the International Day for the Preservation of the Ozone Layer in 1994. The Kigali Agreement is an amendment to the Montreal Protocol, signed in 2016.

Inert Gases in Atmosphere

Of the inert gases, the most important is Argon, forming about 0.93% of the atmosphere by volume.
Other inert gases including Neon, Helium, Krypton, and Xenon, have no significance in the Earth’s weather processes.

Principle Gases of Dry Air

Constituent	Percent by Volume	Concentration in Parts Per Million (PPM)
Nitrogen (N)	78.084	780840.0
Oxygen (O₂)	20.946	209460.0
Argon (Ar)	0.934	9340.0
Carbon dioxide (CO₂)	0.036	360.0
Neon (Ne)	0.002	18.2
Helium (He)	0.000524	5.24
Methane (CH₄)	0.00015	1.5
Krypton (Kr)	0.000114	1.14
Hydrogen (H₂)	0.00005	0.5
Xenon (Xe)	0.00009	0.9

Water Vapour

Water vapor is a variable gas in the atmosphere, constituting about 4% of the air by volume in warm and wet tropics but less than 1% in dry and cold areas of deserts and polar regions.
The amount of water vapor decreases with altitude. Water vapor absorbs parts of the insolation and is the immediate cause of condensation and precipitation.
Earth’s most prevalent greenhouse gas is water vapor, causing nearly half of the greenhouse effect on Earth.
It functions as a blanket over the planet’s atmosphere and stabilises the temperature. Water vapour is the cause of all meteorological events, including dew, fog, clouds, rainbows, and halos.

Aerosols

Aerosols are minute particles suspended in the atmosphere, originating from various sources such as sea salts, fine soil, smoke-soot, ash, pollen, dust, storms, and disintegrated meteor particles.
These particles facilitate water vapour condensation, leading to cloud formation and precipitation. Aerosols act as hygroscopic nuclei around which water vapour condenses to form clouds. When aerosols come in contact with gases like sulphur dioxide, smog is formed.

Structure of the atmosphere

The structure of the atmosphere is intricate, comprising various layers with distinct densities and temperatures. While the atmosphere has been extensively studied up to a height of 16 to 29,000 km, the uppermost 800 km above the ocean’s surface are particularly significant.

1. based on Uniformity:

Homosphere: This zone exhibits uniform mixing and composition of gases and other constituents. Major gases like nitrogen, oxygen, argon, carbon dioxide, water vapor, and particulates are uniformly distributed.
Extending up to 88 km from the Earth’s surface, it includes the troposphere, stratosphere, and mesosphere.
Heterosphere: Encompassing the region from 88 km to about 10,000 km, this layer includes both the ionosphere and exosphere.
The air is no longer uniform, and its chemical composition varies, with regions dominated by a few atomic or molecular species. Due to the absence of convective heating, the material in the heterosphere is layered by mass.

2. based on Temperature:

Troposphere: As the lowermost and warmest layer, the troposphere has an average altitude of 13 km, extending roughly up to 8 km near the poles and about 18 km at the equator.
It contains 75% of the total gaseous mass, along with dust particles and water vapors.
The troposphere experiences a gradual decrease in temperature with an increase at a constant rate of 1°C for every 165 m, known as the normal lapse rate.
All meteorological phenomena occur in this layer.

Stratosphere:

Extending above the tropopause to about 50 km, the stratosphere lacks atmospheric turbulence, making it suitable for jet airways.
The air temperature increases steadily with altitude due to the absorption of UV rays by the ozone layer, but it never exceeds the melting point of water (0°C).
The lower stratosphere may have cirrus clouds, and the stratosphere is separated from the mesosphere by the stratopause, exhibiting stable thermal conditions.
The lower portion of the stratosphere with the maximum concentration of ozone is referred to as the ozoneosphere.
In the lower levels of the stratosphere, the temperature remains the same. There is a tetrease in temperature due to the absorption of ultraviolet rays by the ozone layer.

Mesosphere

The mesosphere is the frigid atmospheric layer with the coldest atmospheric conditions extending up to 80 km from stratopause, Temperature dee up to here with altitude and reaches upto -100°C.
Meteors burn up in mesosphere as shooting stars. It is separated from the thermosphere by mesopause. The upper layer of mesosphere is marked by mesopause.

Thermosphere:

Extending from mesopause to about 640 km in altitude, the thermosphere’s lower layer is termed the ionosphere due to the presence of free radicals or ionic particles.
Despite a rapid increase in temperature with altitude, the molecules are so widely dispersed that the atmospheric heat isn’t perceptible.

Ionosphere:

The ionosphere spans from 80 km to 640 km above the mesopause. Radiowaves transmitted from Earth are reflected by this layer, and temperature increases with altitude.
Abundant ionic particles in the ionosphere serve as protection against celestial bodies like meteors. It consists of various thermal layers, such as the D-layer, E-layer, etc., separated by gradients known as pauses.

Exosphere:

Representing the uppermost layer beyond 640 km, the exosphere harbors the most rarified gases and an abundance of electrically charged particles.
Hydrogen and helium gases are present in the exosphere, with N₂, O₂, He, and H₂ in different layers.
Molecules in the exosphere possess enough kinetic energy to escape gravity and venture into space, leading to the disappearance of helium.

Prelims Facts

What is the thin layer of gases around the Earth is called? Atmosphere BPSC pre 2000
Among the noble gases like helium, argon, radon and neon which gas is not present in air? – Radon /UPPSC (Pre) 2005)
The reason for the diffusion of light in the atmosphere is – Dust particles (UPPSC (Pre) 2021)
The thickness of the atmosphere is maximum over the Equator /LAS (Pre) 2004)
The height of the ozone layer above the surface of the Earth is – 15-20 km /BPSC (Pre) 1996)
In which atmospheric layer are communication satellites located? – Erosphere (UKPSC (Pre) 2005]
Which atmosphere layer that occupies 85% of the atmosphere? – Troposphere IMPSC (Pre) 2020%
Lightening and thundering are the characteristics of – Troposphere APSC (Pre) 2010
Most weather activity o- Troposwhich atmospheric layer?- Troposphere [RAS/RTS (Pre) 1999)
How much has the global average temperature risen in last century? – 1.8° F (RAS/RTS (Pre)2021

UPSC NCERT Practice Questions

1. The atmosphere is a mixture of several gases. Near the Earth surface it contains mainly

(a) nitrogen and oxygen

(d) ethane and oxygen

(b) nitrogen and carbon dioxide

2. The correct sequence of different layers of the atmosphere from the surface of the Earth upwards is UPPSC (Pre) 2005, 2021

(a) Troposphere, Stratosphere, lonosphere, Mesosphere

(b) Stratosphere, Troposphere, Ionosphere, Mesosphere

(d) Stratosphere, Troposphere, Mesosphere, lonosphere

3. The stratosphere is said to be ideal for flying jet aircraft. This is because JPSC (Pre) 2010, 2011

(a) this layer is rich in ozone which reduces fuel consumption.

(b) the temperature is constant and ideal for aircraft engine efficiency

(d) the absence of clouds and other weather phenomena.

4. Which of the following statements is not true?UPPSC (Pre) 2002

(a) Presence of water vapour is highly variable in the lower atmosphere.

(b) The zone of maximum temperature is located along the Equator.

(d) Jet streams are high altitude winds affecting the surface weather conditions.

5. Arrange the following gases of the atmosphere in increasing order by volume.

1. Nitrogen

2. Oxygen

3. Carbon dioxide

4. Argon

Codes

(a) 1, 2, 3 and 4

(b) 1, 2, 4 and 3

(d) 4, 3, 2 and 1

6. At how many kilometers above from the surface of the Earth ozone can be found?

(a) 30 to 50 km

(b) 1 to 5 km

(d) 3 to 7 km

7. Which is the warmest layer of the atmosphere?

(a) Thermosphere

(b) Troposphere

(d) Mesosphere

8. Which of the following layers of the Earth’s atmosphere provide ideal flying conditions for Jet aircraft?

(a) Mesosphere

(b) Thermosphere

(d) Troposphere

Know Right Answer

1 (a)

2 (c)

3 (d)

4 (b)

5 (c)

6 (a)

7 (a)

8 (c)

Frequently Asked Questions (FAQs)

Q1: What is the composition of the Earth’s atmosphere?

A1: The Earth’s atmosphere is primarily composed of nitrogen (about 78%), oxygen (around 21%), and traces of other gases such as carbon dioxide, argon, and water vapor. These gases play a crucial role in maintaining the delicate balance necessary for supporting life on Earth.

Q2: How does the composition of the atmosphere vary with altitude?

A2: The composition of the atmosphere varies with altitude. In the troposphere, which is the lowest layer, the concentration of gases decreases with height. The stratosphere, above the troposphere, contains the ozone layer, which absorbs and scatters ultraviolet solar radiation. The mesosphere and thermosphere follow, with diminishing concentrations of gases and increasing temperatures as you move upward.

Q3: What role does the atmosphere play in regulating Earth’s temperature?

A3: The atmosphere acts as a natural insulator, regulating Earth’s temperature through the greenhouse effect. Gases like carbon dioxide and water vapor trap some of the outgoing heat radiated by the Earth’s surface, preventing it from escaping directly into space. This process maintains a relatively stable temperature range, allowing life to thrive. However, human activities, such as the burning of fossil fuels, contribute to an enhanced greenhouse effect, leading to global warming and climate change.

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UPSC NCERT Notes – Geography – Atmosphere Composition and Structure