Composition and Structure of Atmosphere Class 11 Question Answer Guide

What Is the Atmosphere? Definition and Importance

The atmosphere is the layer of gases surrounding the Earth, held by gravity. It protects life by filtering harmful solar radiation, regulating temperature, and enabling weather systems.

Key points:

• Composed of gases, water vapour, and suspended particles
• Essential for sustaining life
• Acts as a shield against meteoroids

Understanding the atmosphere is fundamental in Class 11 Geography to explain weather, climate, and environmental processes.

Composition of Atmosphere: Gases and Their Proportions

The Earth's atmosphere is a mixture of several gases. The major components and their approximate volume percentages are:

Water vapour varies from 0 to 4% depending on location and weather.

Role of gases:

• Nitrogen: Inert, supports life indirectly
• Oxygen: Essential for respiration
• Carbon dioxide: Important for photosynthesis and greenhouse effect

This composition is stable but changes slightly due to natural and human activities.

Structure of Atmosphere: Layers and Characteristics

The atmosphere is divided into five main layers based on temperature gradients:

1. Troposphere (0-12 km): Weather occurs here; temperature decreases with altitude. 2. Stratosphere (12-50 km): Contains ozone layer; temperature increases with altitude. 3. Mesosphere (50-80 km): Temperature decreases; meteors burn here. 4. Thermosphere (80-700 km): Temperature rises sharply; auroras occur. 5. Exosphere (above 700 km): Outermost layer, merges into space.

Understanding these layers helps explain atmospheric phenomena.

Atmospheric Pressure and Temperature Variation

Atmospheric pressure is the weight of air above a surface. It decreases exponentially with altitude because air density reduces.

Formula for pressure change with height:

$$ P = P_0 \times e^{-\frac{h}{H}} $$

Where:

• $P$ = pressure at height $h$
• $P_0$ = pressure at sea level
• $H$ = scale height (~8.5 km)

Temperature variations depend on solar radiation absorption and atmospheric composition. For example, temperature decreases in the troposphere but increases in the stratosphere due to ozone absorption of UV rays.

This knowledge is vital for understanding weather patterns and climate.

Role of Greenhouse Gases and Atmospheric Pollution

Greenhouse gases like carbon dioxide, methane, and water vapour trap heat in the atmosphere, maintaining Earth's temperature suitable for life.

However, excessive greenhouse gases cause global warming.

Common pollutants include:

• Sulphur dioxide (SO₂)
• Nitrogen oxides (NOₓ)
• Particulate matter (PM)
• Carbon monoxide (CO)

Pollution affects air quality and human health, making it an important topic in Class 11 Geography.

Understanding the balance of gases helps students appreciate environmental challenges.

Solved Example: Calculating Atmospheric Pressure at a Height

Example: Calculate the atmospheric pressure at 5 km altitude if sea level pressure is 1013 hPa. Use scale height $H = 8.5$ km.

Solution:

Using formula:

$$ P = P_0 \times e^{-\frac{h}{H}} = 1013 \times e^{-\frac{5}{8.5}} $$

Calculate exponent:

$$ -\frac{5}{8.5} = -0.588 $$

Calculate $e^{-0.588} \approx 0.555$ (using calculator)

Therefore,

$$ P = 1013 \times 0.555 = 562.3 \text{ hPa} $$

Atmospheric pressure at 5 km altitude is approximately 562 hPa.

This example helps understand pressure variation with altitude practically.