**GEOGRAPHY – CLASS 11 (Course A)

1. INTRODUCTION TO CLIMATE

The Earth’s atmosphere is a dynamic system influenced by solar radiation, land–water distribution, altitude, and circulation patterns.
Weather and climate are two related but distinct concepts.
Climate studies help us understand long-term atmospheric behaviour, seasonal patterns, agricultural planning, and disaster mitigation.
India’s climate and the global climatic system are shaped by atmospheric composition, structure, and energy distribution.
Understanding climate is essential for geography because all natural processes—soil formation, vegetation distribution, hydrological cycle, and living conditions—are influenced by climatic conditions.

2. WEATHER VS CLIMATE

2.1 Weather

Short-term atmospheric conditions.
Describes daily variations in temperature, rainfall, humidity, cloudiness, wind, pressure, etc.
Highly variable and unpredictable over longer periods.
Measured hourly or daily.

2.2 Climate

Long-term average of weather conditions.
Calculated over a minimum period of 30 years (as per WMO).
More stable and predictable than weather.
Determines vegetation type, agricultural practices, and lifestyle of people.

3. COMPOSITION OF THE ATMOSPHERE

The atmosphere is made up of gases, water vapour, dust particles, and aerosols, each influencing climatic conditions in different ways.

3.1 Importance of Atmospheric Composition

Maintains Earth’s temperature through greenhouse effect.
Filters harmful solar radiation.
Helps in cloud formation and precipitation.
Plays a key role in atmospheric pressure and wind patterns.
Necessary for biological functions such as respiration and photosynthesis.

4. GASES OF THE ATMOSPHERE

Air is not a single gas—it is a mixture of several gases in definite proportions.
Composition is fairly uniform in the lower atmosphere (up to 80 km).

4.1 Major Gases

(Percentages represent volume at sea level)

(i) Nitrogen (N₂) – 78%

Most abundant gas.
Essential for plant growth (absorbed indirectly through nitrogen cycle).
Chemically inert; controls biological and chemical reactions in the atmosphere.

(ii) Oxygen (O₂) – 21%

Essential for respiration and combustion.
Produced mainly by photosynthesis.
Moderately active gas that supports life on Earth.

(iii) Argon (Ar) – 0.93%

Chemically inactive.
No major role in climatic processes.

(iv) Carbon Dioxide (CO₂) – 0.04%

Highly variable depending on location and human activity.
Important greenhouse gas—absorbs long-wave radiation.
Essential for photosynthesis in plants.
Increasing concentration due to industrialisation and deforestation.

(v) Ozone (O₃) – Trace Amount

Concentrated in the ozone layer (20–35 km altitude).
Absorbs harmful ultraviolet radiation.
Plays a major role in maintaining upper atmospheric temperature.

5. OTHER COMPONENTS OF THE ATMOSPHERE

5.1 Water Vapour

Varies from 0–4% by volume.
Major absorber of heat—responsible for humidity and precipitation.
Influences temperature variations:
- High humidity reduces diurnal temperature range.
- Dry regions show large temperature variations.
Plays a major role in cloud formation and condensation process.

5.2 Dust Particles

Include pollen, ash, soil particles, sea salt, microbes, and industrial pollutants.
Act as condensation nuclei for cloud droplets.
Influence visibility and scattering of sunlight.
Cause colourful sunrise and sunset.

5.3 Aerosols

Very fine solid or liquid particles suspended in air.
Sources: volcanic eruptions, fossil fuel combustion, sea spray.
Important in cloud formation and global temperature regulation.
Can contribute to atmospheric cooling by reflecting sunlight.

6. STRUCTURE OF THE ATMOSPHERE

Atmosphere is divided into layers based on temperature, composition, and density.

6.1 Troposphere (0–12 km)

Lowest and most important layer.
Contains 75% of total atmospheric mass.
All weather phenomena occur here.
Temperature decreases with height (lapse rate: 6.5°C per km).
Upper boundary: Tropopause (acts as a lid preventing vertical mixing).

6.2 Stratosphere (12–50 km)

Contains the ozone layer (20–35 km).
Temperature increases with height due to ozone absorption of UV radiation.
Ideal for aircraft flying because it is stable and cloudless.

6.3 Mesosphere (50–80 km)

Temperature decreases with height, making it the coldest layer.
Meteors burn up due to friction.
Upper boundary: Mesopause.

6.4 Thermosphere (80–400 km)

Temperature increases drastically with height.
Contains ionized gases—Ionosphere—helpful in radio communication.
Auroras occur in this layer.

6.5 Exosphere (above 400 km)

Outermost layer merging with outer space.
Very thin air consisting of hydrogen and helium.
Satellites orbit in this region.

7. ELEMENTS OF WEATHER AND CLIMATE

Weather and climate are described using several measurable elements.

7.1 Temperature

Measure of atmospheric heat.
Influenced by latitude, altitude, land-water contrast, ocean currents, winds, and cloud cover.
Determines seasons and climatic zones.

7.2 Atmospheric Pressure

Force exerted by air above a point.
Decreases with altitude.
Creates pressure belts—equatorial low, subtropical high, subpolar low, polar high.
Pressure differences create winds.

7.3 Wind

Horizontal movement of air from high to low pressure.
Major wind systems:
- Trade Winds
- Westerlies
- Polar Easterlies
- Monsoons
- Local winds (loo, chinook, foehn)

7.4 Humidity

Amount of water vapour in the air.
Determines cloud formation and precipitation.
Affects comfort level and heat index.

7.5 Clouds

Mass of condensed water vapour.
Classified into cirrus, cumulus, stratus, nimbus categories.
Indicators of upcoming weather.

7.6 Precipitation

Falling of moisture as rain, snow, sleet, or hail.
Types:
- Cyclonic
- Orographic
- Convectional

7.7 Solar Radiation

Earth receives energy from the sun in the form of short-wave radiation.
Albedo effect determines how much is absorbed or reflected.
Uneven heating causes atmospheric circulation.

8. FACTORS AFFECTING CLIMATE

8.1 Latitude

Determines angle of sunlight.
Tropical regions hotter; polar regions cold.

8.2 Altitude

Higher altitude → lower temperature.
Mountain climate is cooler than plains.

8.3 Distance from Sea

Coastal areas have moderate climate (maritime influence).
Inland areas have extreme climate (continentality).

8.4 Ocean Currents

Warm currents raise temperatures; cold currents lower them.
Influence rainfall distribution.

8.5 Relief / Topography

Mountains act as barriers.
Rainfall on windward side; rainshadow zone lies on leeward side.

8.6 Vegetation Cover

Dense forests reduce temperature and increase humidity.

8.7 Human Activities

Urban heat island effect.
Pollution increases concentration of greenhouse gases.
Deforestation changes regional climate.

9. GLOBAL CLIMATIC CLASSIFICATION (Brief Insight)

Although not asked directly, understanding classification helps grasp climate distribution.

Hot Equatorial Climate
Tropical Monsoon Climate
Temperate Marine Climate
Mediterranean Climate
Desert Climate
Polar Climate

Each climate type is defined by temperature, rainfall, and seasonal variability.

10. IMPORTANCE OF STUDYING ATMOSPHERIC COMPOSITION & CLIMATE

Helps predict weather and climate trends.
Essential for agriculture and water resource planning.
Supports disaster management (cyclones, droughts, floods).
Understanding greenhouse gases helps address climate change.
Influences vegetation, wildlife, and human settlements.

11. CONCLUSION

Climate is a long-term pattern of atmospheric conditions that controls major natural processes.
The atmosphere is made of gases, water vapour, and dust particles that regulate Earth’s temperature and support life.
Its layered structure creates different thermal and dynamic zones influencing weather patterns.
Elements like temperature, pressure, wind, humidity, and precipitation determine weather and climate characteristics.
Human activities are altering atmospheric composition, making climate studies crucial for sustainable development.
A deep understanding of climate helps societies manage resources, prepare for disasters, and adapt to global changes.