Chapter 3: Atmosphere and Climate

Social Science (Geography) · CBSE Class 9 · NCERT ‘Understanding Society: India and Beyond’ · Notes, Solutions & 25 Quiz

Chapter 3 — Atmosphere and Climate (Notes · Geography)

The atmosphere is the blanket of air held around the Earth by gravity. It shields us from harmful (ultraviolet) radiation, regulates temperature by trapping some of the Sun’s energy, and drives weather and climate (temperature, humidity, pressure).

The Big Questions

What is the composition of the atmosphere? How do its layers affect the Earth? What is the mechanism of the monsoon? How can we reduce our carbon footprint?

Composition of the Atmosphere

A mixture of gases: Nitrogen ≈ 78% and Oxygen ≈ 21% are the two most abundant, plus small amounts of Argon (0.93%), Carbon dioxide (0.04%) and traces of helium, neon, ozone, hydrogen, etc. It also holds water vapour (0.1–0.4%) and dust particles — vital for cloud formation and precipitation. Composition varies with altitude.

Structure (Layers) of the Atmosphere

Layers are defined by changes in temperature and density with altitude (density is highest near the surface).

• Troposphere (~12 km) — we live here; holds most water vapour and clouds; nearly all weather (rain, fog, hail) occurs here; temperature decreases with height. Ends at the tropopause.
• Stratosphere (up to ~50 km) — cloud-free, ideal for aeroplanes; contains the ozone layer that filters harmful UV radiation. Ends at the stratopause.
• Mesosphere (up to ~80 km) — temperature decreases with height; most meteorites burn up here.
• Thermosphere (80–700 km) — temperature rises rapidly; contains the ionosphere (reflects radio waves); auroras occur here.
• Exosphere — uppermost, very thin air; light gases (helium, hydrogen) escape to space.

Remember: Temperature decreases with altitude only in the troposphere and mesosphere; it increases in the stratosphere (ozone) and thermosphere (absorbs X-rays/UV).

Weather vs Climate

Weather = hour-to-hour and day-to-day condition of the atmosphere (changes quickly). Climate = the average weather of a place over a long period — usually 30 years or more.

Elements of Weather and Climate

• Temperature — depends on insolation (incoming solar energy), which decreases from the equator to the poles, so temperature also falls poleward.
• Humidity — water vapour (moisture) in the air; warm air holds more vapour → higher humidity.
• Precipitation — condensed water vapour falling as rain, drizzle, snow, sleet or hail; affected by winds, mountains and seasons.
• Atmospheric pressure — weight of air on the surface; highest at sea level, falls with altitude. Warm air rises → low pressure (cloudy/wet); cold air sinks → high pressure (clear/sunny). Air moves from high to low pressure.
• Wind — movement of air from high to low pressure (named after the direction it blows FROM, e.g., a westerly blows from the west). Land breeze blows land→sea at night; sea breeze blows sea→land by day — moderating coastal climate.

Seasons in India

India’s climate is tropical monsoon. The IMD recognises four seasons: Winter (Dec–early Apr), Summer/pre-monsoon (Apr–Jun), Monsoon/rainy — advancing (Jun–Sep), and Post-monsoon — retreating (Oct–Dec). Himalayan states also get autumn and spring. Traditionally India has six ṛtus: Vasanta, Grīṣhma, Varṣhā, Śharad, Hemanta, Śhiśhira.

The Monsoon Mechanism

The word ‘monsoon’ comes from the Arabic mausim (‘season’) — it is the seasonal reversal of wind direction.
• South-west (summer) monsoon (Jun–Sep): in summer the land heats faster than the ocean, creating low pressure over land and high pressure over the cooler Indian Ocean; moist winds blow sea→land, bringing most of India’s rainfall.
• North-east (winter) monsoon (Oct–Feb): the land cools faster than the sea, creating high pressure over land; cold dry winds blow land→sea. Passing over the Bay of Bengal they pick up moisture and bring rain to the south-east coast (Tamil Nadu, Andhra Pradesh, parts of Karnataka).

Monsoon is vital — most Indian agriculture depends on it; a good monsoon means food and water security, while excess rain causes floods and weak monsoons cause droughts. (Kālidāsa’s Meghadūtam and Varāhamihira’s Bṛihatsaṁhitā noted monsoon onset and rainfall prediction.)

Climate Change & Carbon Footprint

Climate change = long-term change in weather patterns, caused mainly by human activities — burning fossil fuels, deforestation and industrial pollution that release greenhouse gases (carbon dioxide, methane, nitrous oxide, water vapour). These trap heat, raising global temperatures and causing more floods, droughts, glacier melt, rising sea levels and biodiversity loss. A carbon footprint is the total greenhouse gases released by a person’s activities. We can reduce it by using public transport/cycling, saving electricity and water, avoiding single-use plastics, planting trees and using renewable energy.

Case Study — Punjab Floods 2025: Heavy monsoon rain (worsened by western disturbances) swelled the Satluj, Beas, Ravi and Ghaggar. Natural causes: very heavy rain in Punjab, Himachal and J&K; rivers already high. Human causes: weak/old embankments (dhūsī bāndh), houses & farms built too close to rivers, silted rivers/dams, and late flood warnings. Effects: loss of life, displacement to relief camps, destroyed paddy crops and dairy/poultry, damaged roads/bridges, and waterborne disease.

Key Terms

Gravity = attraction the Earth exerts on objects; Altitude = height above mean sea level; Insolation = incoming solar energy; Monsoon = seasonal reversal of wind direction; Nakṣhatras = 27 lunar mansions; Carbon footprint = total greenhouse gases from one’s activities; Ionosphere = part of the thermosphere that reflects radio waves.

Textbook Solutions — Questions & Activities

Full model answers to the chapter’s ‘Questions and activities’, with diagram and graph activities explained.

1What is the atmosphere? Explain its composition with the help of a pie diagram.

The atmosphere is the blanket of gases held around the Earth by gravity; it supports life, shields us from harmful UV radiation and regulates temperature. Composition (pie): Nitrogen ≈ 78%, Oxygen ≈ 21%, Argon ≈ 0.93%, Carbon dioxide ≈ 0.04%, and traces of other gases (≈ 0.03%), plus variable water vapour (0.1–0.4%) and dust. A pie chart would show nitrogen as the largest slice, oxygen next, and a thin sliver for the rest.

2Draw a labelled diagram of the structure of the atmosphere.

Draw five horizontal layers upward from the Earth’s surface, labelling heights and boundaries: Troposphere (0–~12 km, weather, ends at tropopause) → Stratosphere (~12–50 km, ozone, aeroplanes, stratopause) → Mesosphere (~50–80 km, meteors burn, mesopause) → Thermosphere (80–700 km, ionosphere, auroras) → Exosphere (outermost, thin air). Mark that temperature decreases in the troposphere & mesosphere but rises in the stratosphere & thermosphere.

3Which are the four main seasons of India?

According to the IMD: (i) Winter (December–early April), (ii) Summer / pre-monsoon (April–June), (iii) Monsoon / rainy — advancing monsoon (June–September), and (iv) Post-monsoon — retreating monsoon (October–December).

4Why do you not feel the pressure of the atmosphere?

Air presses on our bodies from all sides, and the pressure inside our bodies is equal to the atmospheric pressure outside. The inward and outward pressures balance (cancel each other), so we do not feel the heavy weight of the air above us.

5In which layer of the atmosphere do aeroplanes fly and why?

Aeroplanes fly in the stratosphere. This layer is free of clouds and weather disturbances (which occur in the troposphere below), so flight is smooth and stable, with less turbulence and better fuel efficiency.

6Distinguish between: (a) troposphere & stratosphere, (b) south-west & north-east monsoon.

(a) Troposphere vs Stratosphere: Troposphere is the lowest layer (~0–12 km) where we live and all weather occurs, and temperature decreases with height; the stratosphere lies above (~12–50 km), is cloud-free and ideal for aeroplanes, holds the ozone layer, and temperature increases with height. (b) SW vs NE monsoon: The south-west (summer) monsoon blows sea→land (Jun–Sep), is moist and brings most of India’s rainfall; the north-east (winter) monsoon blows land→sea (Oct–Feb), is cold & dry, but picks up moisture over the Bay of Bengal to rain on the south-east coast (Tamil Nadu, Andhra).

7Temperature–rainfall graphs & climate generalisations (Table 3.3 activity).

This is a graph/analysis activity (answers from Table 3.3): plot each station’s temperature (line) and rainfall (bars). Expected findings — 7.2(a) most extreme climate: Leh and Jodhpur/Delhi (wide temperature range); 7.2(b) retreating-monsoon stations: Chennai and Thiruvananthapuram (rain peaks Oct–Nov); 7.3(a) Shillong gets more rain than Kolkata because of its hill location facing moist monsoon winds; 7.3(b) Delhi gets more rain than Jodhpur as Jodhpur lies in the arid Thar; 7.4(a) Thiruvananthapuram is equable due to coastal/near-equator location; 7.4(b) Chennai rains after the main monsoon because it gets the retreating NE monsoon; 7.4(c) Leh has little precipitation year-round as it is a cold high-altitude desert. 7.5 Yes — the monsoon’s seasonal rhythm gives overall climatic unity to the whole country despite regional differences.

8Activity — houses & clothing of different regions.

Activity (answers vary): collect pictures and note how homes & clothing suit the climate/relief — e.g., sloping roofs in heavy-rain areas (Shillong/Kerala), flat roofs & thick walls in hot deserts (Rajasthan), woollen clothes & snow-proof houses in cold Leh, and light cotton clothing in hot-humid coasts. This shows human life adapts to climatic and physical conditions.

Extra Practice — HOTS & Value-based

Higher-order thinking, application and value-based questions (CBSE style).

1Why does temperature decrease from the equator towards the poles? (HOTS)

Near the equator the Sun’s rays strike the Earth almost vertically, concentrating heat over a small area, so insolation is high. Towards the poles the rays strike at a low, slanting angle, spreading the same energy over a larger area and passing through more atmosphere, so insolation and temperature fall. Hence temperature decreases from the equator to the poles.

2Explain why the south-west monsoon brings most of India’s rainfall, linking it to pressure and temperature. (HOTS)

In summer the Indian landmass heats faster than the surrounding ocean, creating an intense low-pressure area over land while the cooler ocean stays at high pressure. Since wind moves from high to low pressure, moisture-laden winds blow from the sea onto the land; as they rise over the land and mountains, the water vapour condenses and falls as heavy rain — so the SW monsoon delivers most of the country’s annual rainfall.

3The Punjab 2025 floods had both natural and human causes. Evaluate which were more responsible. (HOTS/Value-based)

Answer will vary. Natural causes (very heavy monsoon rain + western disturbances, already-high rivers) set off the flood, but human causes greatly worsened it — weak/old embankments, building homes and farms too close to rivers, silted rivers/dams reducing capacity, and late warnings. A reasoned view: nature triggered the event, but poor planning and encroachment turned it into a disaster — so better planning (strong embankments, flood zoning, desilting, early warnings) could have reduced the damage.

4‘Monsoon gives climatic unity to India despite regional diversity.’ Justify. (HOTS)

Although regions differ widely — desert Jodhpur, cold Leh, humid Kerala, rainy Shillong — the whole country’s rhythm of life revolves around the monsoon: the arrival of rain, the farming calendar, festivals and water supply follow the same seasonal cycle nationwide. This shared dependence on the monsoon binds India’s diverse climates into one overall framework, giving it climatic unity.

5Suggest a practical plan for your school/home to reduce its carbon footprint. (Value-based/Application)

Answer will vary. Practical steps: switch off lights/fans when not in use and use energy-efficient LEDs; walk, cycle or carpool to school; harvest rainwater and avoid wasting water; segregate waste, reuse and recycle, and avoid single-use plastics; plant and maintain trees; and use solar power where possible. Each small habit cuts greenhouse-gas emissions and builds a climate-smart lifestyle.