Composition and Structure of Atmosphere — NCERT Solutions

Answer:

Given/Concept: Elements of weather and climate are the atmospheric variables that change over time and influence human life.

The main elements of weather and climate are:

1. Temperature — degree of hotness or coldness of the air
2. Atmospheric Pressure — weight of the air column above a point
3. Winds — horizontal movement of air
4. Humidity — amount of water vapour in the air
5. Clouds — condensed water vapour in the atmosphere
6. Precipitation — moisture falling from clouds (rain, snow, hail)

These elements are subject to change and directly influence human life on Earth.

Answer:

Given/Concept: The atmosphere is composed of gases, dust particles, and water vapour.

The atmosphere is a mixture of several components:

1. Gases: Nitrogen (78.08%) and Oxygen (20.95%) together make up about 99% of the atmosphere. Other gases include Argon (0.93%), Carbon dioxide (0.03%), and traces of Neon, Helium, Ozone, Hydrogen, etc.

2. Water Vapour: It varies from 0 to 4% of the atmosphere. It absorbs solar radiation and acts as a blanket, preventing rapid cooling of the Earth.

3. Dust Particles: These include sea salt, pollen, ash, soot, and fine soil. They act as hygroscopic nuclei for condensation and affect visibility and weather.

Answer:

Given/Concept: The troposphere is the lowest and densest layer of the atmosphere.

The troposphere is the most important layer because:

1. It contains air essential for breathing and sustaining life.
2. All weather phenomena — clouds, rainfall, cyclones, fog — occur in this layer.
3. It contains about 75% of the total mass of the atmosphere and almost all the water vapour.
4. Temperature decreases with altitude at a normal lapse rate of 6.5°C per 1,000 m, influencing climate and vegetation.

Since it directly supports life and governs weather, it is the most significant layer for human beings.

Answer:

Given/Concept: The atmosphere is a mixture of gases, water vapour, and dust particles.

The atmosphere is composed of three major components:

1. Gases:

The atmosphere is a mixture of many gases. The major gases and their approximate percentage by volume are:

• Nitrogen is the most abundant gas. It dilutes oxygen and prevents rapid oxidation. It is also important for plant growth.
• Oxygen is essential for respiration and combustion. It is found in significant quantities up to a height of about 120 km.
• Carbon dioxide, though present in small amounts, is very significant as it is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation, thus acting as a greenhouse gas and maintaining the Earth's temperature.
• Ozone is found in the stratosphere and absorbs harmful ultraviolet (UV) radiation from the Sun, protecting life on Earth.

2. Water Vapour:

Water vapour is the gaseous form of water present in the atmosphere. Its proportion varies from 0 to 4% depending on location and season — it is more over tropical oceans and less over deserts and polar regions. Water vapour:

• Absorbs part of the incoming solar radiation and most of the outgoing terrestrial radiation.
• Acts as a blanket preventing rapid cooling of the Earth at night.
• Is the source of all forms of precipitation (rain, snow, hail).

3. Dust Particles:

The atmosphere also contains tiny solid particles such as sea salt, pollen, ash, smoke soot, and fine soil. These are called dust particles or aerosols. They are more concentrated in the lower layers of the atmosphere and over industrial and desert areas. Dust particles:

• Act as hygroscopic nuclei around which water vapour condenses to form clouds and fog.
• Affect the colour of the sky and visibility.
• Influence the heat budget of the atmosphere by scattering and absorbing solar radiation.

Conclusion: The composition of the atmosphere is thus a carefully balanced mixture that sustains life, regulates climate, and drives weather processes on Earth.

Answer:

Given/Concept: The atmosphere is divided into distinct layers based on temperature variation with altitude.

Diagram of the Structure of the Atmosphere:

(Note: Draw a vertical diagram with the Earth's surface at the bottom and the following layers marked from bottom to top with their approximate heights.)

↑ Height
|
1000 km ─────────────────── EXOSPHERE (merges with outer space)
|
|
 80 km ─────────────────── THERMOSPHERE / IONOSPHERE
 (Temperature rises sharply)
|
 50 km ─────────────────── MESOPAUSE
|
 50 km ─────────────────── MESOSPHERE
 (Temperature decreases with height)
|
 50 km ─────────────────── STRATOPAUSE
|
 50 km ─────────────────── STRATOSPHERE
 (Contains Ozone Layer ~15–35 km)
|
 12 km ─────────────────── TROPOPAUSE
|
 0–12 km ─────────────────── TROPOSPHERE
 (Weather occurs here)
|
─────── EARTH'S SURFACE ───────

Description of Each Layer:

1. Troposphere (0 to ~12 km):

• This is the lowermost and densest layer of the atmosphere.
• It extends up to about 8 km at the poles and 18 km at the equator, with an average of 12 km.
• Temperature decreases with altitude at a normal lapse rate of $6.5°C$ per 1,000 m.
• It contains about 75% of the total mass of the atmosphere and almost all the water vapour.
• All weather phenomena (clouds, rain, storms, cyclones) occur in this layer.
• The upper boundary of the troposphere is called the tropopause.

2. Stratosphere (~12 km to ~50 km):

• This layer lies above the tropopause.
• It is almost free from clouds and weather disturbances, making it ideal for flying aircraft.
• It contains the ozone layer (between 15–35 km), which absorbs harmful ultraviolet radiation from the Sun.
• Due to absorption of UV radiation by ozone, temperature increases with altitude in this layer.
• The upper boundary is called the stratopause.

3. Mesosphere (~50 km to ~80 km):

• This layer lies above the stratopause.
• Temperature again decreases with altitude, reaching as low as $-100°C$ at the top.
• Meteors burn up in this layer due to friction with air particles.
• The upper boundary is called the mesopause.

4. Thermosphere / Ionosphere (~80 km to ~400 km):

• Temperature increases rapidly with altitude due to absorption of solar radiation by oxygen molecules.
• This layer contains electrically charged particles (ions), which is why it is also called the ionosphere.
• It helps in radio wave transmission by reflecting radio waves back to the Earth's surface.
• The Northern and Southern Lights (Aurora Borealis and Aurora Australis) occur in this layer.

5. Exosphere (above ~400 km):

• This is the outermost and highest layer of the atmosphere.
• Very little is known about it.
• The gases are extremely rarefied (very thin) and gradually merge with outer space.
• It is dominated by hydrogen and helium.

Conclusion: The atmosphere is thus structured into distinct layers, each with unique characteristics of temperature, composition, and significance. The troposphere and stratosphere are the most important layers for life on Earth.