CHAPTER 8
COMPOSITION & STRUCTURE OF ATMOSPHERE

 

1. Atmosphere

The atmosphere is the vast envelope of air that surrounds the Earth. It is held around the Earth by gravitational force and is an essential part of the Earth system.

The atmosphere is made up of different gases, water vapour and dust particles. Its composition, density and temperature change with height.

Broad Structure of the Atmosphere

On the basis of chemical composition, the atmosphere is broadly divided into:

  1. Homo sphere – lower region where gases are relatively well mixed.
  2. Hetero sphere – upper region where gases are arranged in layers according to their molecular weight.

 

2. Layers of the Atmosphere

The atmosphere consists of several layers arranged one above another. They differ mainly in temperature, density and composition.

Simple Diagram of Atmospheric Layers

 

3. Troposphere

The troposphere is the lowest layer of the atmosphere and is the most important layer for human beings.

Important Features

  • Its average height is about 16 km.
  • It extends to about 10 km at the poles and up to about 18 km at the equator.
  • It is thickest at the equator because of strong convection currents.
  • Almost all major weather phenomena such as clouds, rainfall, storms and winds occur in this layer.
  • Temperature generally decreases with increasing altitude.
  • The normal lapse rate is about 6.5°C per 1,000 m of ascent.

Normal Lapse Rate

Normal lapse rate means the average decrease in temperature with increasing height in the troposphere.

Approx.: 1°C decrease for every 165 m of ascent
or 6.5°C decrease per 1,000 m.

 

4. Stratosphere

The stratosphere lies above the troposphere and extends up to about 50 km.

Important Features

  • It is relatively stable compared with the troposphere.
  • The lower part has nearly constant temperature up to about 20 km.
  • Temperature gradually increases with height in the upper part.
  • This increase is mainly associated with the ozone layer, which absorbs harmful ultraviolet radiation from the Sun.
  • The layer is generally more suitable for aircraft flight because of relatively stable air and fewer weather disturbances.

 

5. Mesosphere

The mesosphere lies above the stratosphere and extends approximately from 50 km to 80 km.

Important Features

  • Temperature again decreases with increasing height.
  • It is one of the coldest regions of the atmosphere.
  • The upper boundary of the mesosphere is called the mesopause.
  • Many meteoroids burn up due to friction with atmospheric gases in this region.

 

6. Ionosphere

The ionosphere is a region containing a large number of electrically charged particles called ions.

According to the supplied material, it extends approximately from 100 km to 400 km.

Importance

  • It contains ionized particles produced mainly by solar radiation.
  • It plays an important role in radio communication.
  • Certain radio waves can be reflected or refracted by ionized regions of the upper atmosphere.
  • It is also associated with phenomena such as auroras.

 

7. Heterosphere

The heterosphere is the upper region of the atmosphere where gases are not uniformly mixed. Instead, gases become arranged in layers according to their molecular mass.

Main Gases

The major gases found in different parts of the heterosphere include:

  • Nitrogen
  • Oxygen
  • Helium
  • Hydrogen

The heterosphere forms part of the very high atmosphere and extends upward toward outer space.

 

8. Exosphere

The exosphere is the outermost layer of the atmosphere.

Important Features

  • It lies above the ionosphere/upper atmosphere.
  • It begins at roughly 400 km according to the supplied classification.
  • The air is extremely thin.
  • Gas particles are widely separated.
  • It gradually merges with outer space.
  • Hydrogen and helium are among the major gases present at these extreme heights.

 

9. Temperature Pattern of Atmospheric Layers

The temperature does not change uniformly with height. It alternately increases and decreases in different atmospheric layers.

10. Composition of the Atmosphere

The atmosphere is composed mainly of:

  1. Gases
  2. Water vapour
  3. Dust particles

The two most abundant gases are nitrogen and oxygen, which together make up more than 99% of the atmosphere by volume.

Major Atmospheric Gases

GasApprox. Percentage
Nitrogen (N₂)78.08%
Oxygen (O₂)20.95%
Argon (Ar)0.93%
Carbon dioxide (CO₂)0.03%
Neon (Ne)0.002%
Helium (He)0.0005%
Ozone (O₃)0.00006%
Hydrogen (H₂)0.00005%

11. Nitrogen

Nitrogen (N₂) is the most abundant gas in the atmosphere, making up about 78% of its volume.

Importance

  • Essential for plant growth.
  • Forms an important part of proteins and other biological compounds.
  • Enters ecosystems mainly through the nitrogen cycle.
  • Helps maintain the overall balance of atmospheric gases.

 

12. Oxygen

Oxygen (O₂) constitutes about 21% of the atmosphere.

Importance

  • Essential for respiration in humans and most animals.
  • Supports combustion.
  • Required for many biological and chemical processes.
  • It is one of the most important gases for life.

 

13. Carbon Dioxide

Carbon dioxide (CO₂) is present in a very small proportion compared with nitrogen and oxygen, but it is extremely important for the Earth system.

Importance

  • Plants use CO₂ during photosynthesis.
  • It absorbs outgoing terrestrial radiation.
  • It is an important greenhouse gas.
  • Changes in atmospheric CO₂ influence Earth’s climate.

Greenhouse Effect

Greenhouse gases allow much of the incoming solar energy to reach Earth but absorb part of the outgoing terrestrial radiation, contributing to warming of the lower atmosphere.

 

14. Ozone

Ozone (O₃) is a form of oxygen containing three oxygen atoms.

It is concentrated mainly in the stratosphere, where it forms the ozone layer.

Importance

  • Absorbs much of the Sun’s harmful ultraviolet (UV) radiation.
  • Protects living organisms from excessive UV exposure.
  • Plays an important role in maintaining conditions suitable for life on Earth.

 

15. Water Vapour

Water vapour is the gaseous form of water present in the atmosphere.

It is an important component of the atmosphere and varies greatly from place to place.

Distribution

  • It may reach around 4% in warm and humid tropical regions.
  • It may be less than 1% in very cold and dry regions.
  • Even very dry air contains some water vapour.

Importance

Water vapour is essential for:

  • Formation of clouds
  • Rainfall and other precipitation
  • Humidity
  • Weather processes
  • Regulation of atmospheric temperature
  • The water cycle

 

SHORT QUESTION & ANSWER

 

Q1) What do you mean by Atmosphere?
Ans: Atmosphere is the vast expanse of air which envelopes the earth all around. The air of the atmosphere has become an integral part of the earth due to the earth’s gravitational force.
Structure of the Atmosphere: The atmosphere consists of almost concentric layers of air from the bottom to its highest limit. These layers differ from one another with respect to density and temperature. On this basis, the atmosphere is divided into two broad layers: Homosphere and Heterosphere.
 
Layers of the Atmosphere
1) Troposphere: This is the lowermost layer of the atmosphere. Its average height is 16 km, although it is only 10 km at the poles and extends up to 18 km at the equator. The thickness of the troposphere is the greatest at the equator due to the upward transportation of heat by strong convection currents. Temperature decreases with height in this layer roughly at the rate of 1°C for every 165 meters (or 6.5°C per 1,000 meters) of ascent. This is known as the normal lapse rate.
2) Stratosphere: It extends up to a height of 50 km. It is thicker at the poles than at the equator. The temperature remains almost constant in its lower portion up to a height of 20 km. Consequently, it is known as the stratosphere. It gradually increases up to a height of 50 km due to the presence of the ozone layer/gas which absorbs the Sun’s ultraviolet rays.
3) Mesosphere: The mesosphere lies above the stratosphere and extends up to a height of 80 km. Temperature starts decreasing with the increase in height up to 80 km. The upper limit of the mesosphere is known as the mesopause.
4) Ionosphere: It extends from 100 km to 400 km and lies above the mesosphere. It contains electrically charged particles which are known as ions. Hence, this layer is called the Ionosphere. It is used to reflect radio waves.
5) Heterosphere: It extends above the homosphere, i.e., above the level of the mesosphere. It has a heterogeneous chemical composition with a layered structure of nitrogen, oxygen, helium, and hydrogen. It is also part of the thermosphere which continues to the edge of space, about 60,000 km above the earth’s surface.
6) Exosphere: It is the uppermost layer of the atmosphere extending beyond the ionosphere. The region above a height of about 400 km is known as the exosphere.
 
Q2) Describe the composition of the atmosphere?
Ans: The atmosphere is composed of a number of gases, water vapour, and dust particles. Nitrogen and Oxygen are the two main gases which form over 99% of the total volume of the atmosphere. The other gases are argon, carbon dioxide, neon, helium, ozone, hydrogen, methane, etc.
Percentage of Major Gases in the Atmosphere:
  • Nitrogen (\(N_{2}\)): 78.08%
  • Oxygen (\(O_{2}\)): 20.95%
  • Argon (\(Ar\)): 0.93%
  • Carbon dioxide (\(CO_{2}\)): 0.03%
  • Neon (\(Ne\)): 0.002%
  • Helium (\(He\)): 0.0005%
  • Ozone (\(O_{3}\)): 0.00006%
  • Hydrogen (\(H_{2}\)): 0.00005%
  • Methane (\(CH_{4}\)): Trace
  • Krypton (\(Kr\)): Trace
  • Xenon (\(Xe\)): Trace

Water Vapour: This is the most important element of the atmosphere. The average amount of water vapour is nearly 2% of the atmosphere by volume. It may vary from 4% in the warm and wet tropics to less than 1% in the dry and cold areas of deserts and polar regions. Even the driest air has some proportion of water vapour.
Dust Particles: These comprise the third major component of the atmosphere. They are derived from different sources and include sea salts, fine soil, ash, smoke, pollen, and disintegrated particles of meteors.
 
 

Multiple Choice Questions (M.C.Q.)

Q1) Which one of the following layers of the atmosphere is most important for human beings?
  • a) Troposphere
  • b) Stratosphere
  • c) Mesosphere
  • d) Exosphere
  • Ans: a) Troposphere

Q2) Which one of the following gases constitutes the major portion of the atmosphere?
  • a) Oxygen
  • b) Nitrogen
  • c) Argon
  • d) Carbon dioxide
  • Ans: b) Nitrogen

Q3) Which one of the following gases is most important for life?
  • a) Oxygen
  • b) Carbon dioxide

  • c) Helium
  • d) Hydrogen
  • Ans: a) Oxygen

Q4) Which one of the following gases is transparent to incoming solar radiation and opaque to outgoing terrestrial radiation and causes the greenhouse effect?
  • a) Oxygen
  • b) Nitrogen
  • c) Helium
  • d) Carbon dioxide
  • Ans: d) Carbon dioxide

Q5) Which one of the following gases saves us from the harmful ultraviolet radiations coming from the Sun?
  • a) Oxygen
  • b) Nitrogen
  • c) Ozone
  • d) Carbon dioxide
  • Ans: c) Ozone

Q6) Oxygen gas is negligible at what height of the atmosphere?
  • a) 98 km
  • b) 120 km
  • c) 100 km
  • d) 150 km
  • Ans: b) 120 km

Q7) Which one of the following pairs of gases make up over 99% volume of the atmosphere?
  • a) Nitrogen and oxygen
  • b) Hydrogen and oxygen
  • c) Oxygen and ozone
  • d) Nitrogen and hydrogen
  • Ans: a) Nitrogen and oxygen

Q8) Sea salt, pollen, ash, smoke, fine soil—these are associated with?
  • a) Gases
  • b) Dust particles
  • c) Water vapour
  • d) Meteors
  • Ans: b) Dust particles
    PreviousNext