Atmosphere and Climate

This is Chapter 3 of the new integrated Grade 9 book: 22 pages (39–60, the last a blank Notes page) built round four ‘Big Questions’ — what the atmosphere is made of, how its layers affect the planet, how the monsoon works, and how we can reduce our carbon footprint. It has 14 figures (a pie chart, a layer diagram, a temperature-zone diagram, a pressure chart, a breeze diagram, two monsoon maps, a graph and photographs), 3 tables (wind speeds, traditional seasons, and monthly data for 10 stations), a dozen boxes (Think About It, Let's Recall, Don't Miss Out, Let's Explore, Let's Analyse), a case study on the 2025 Punjab floods, a Before we move on summary and 8 questions and activities (Q7 has five parts). We use eight concept sections. Every number is recomputed; our additions are marked †.

• Earlier in this book: iest101 (Understanding Social Science — how the book works, and how it treats old texts and dates) and iest102 (Shaping of the Earth's Surface — landforms and landform disasters; the GLOF and dust-storm material links to this chapter's floods).
• Matching live chapters (for depth): kegy207 (Composition and Structure of Atmosphere), kegy208 (Solar Radiation, Heat Balance and Temperature — insolation), kegy209 (Atmospheric Circulation and Weather Systems — pressure, winds, Coriolis), kegy210 (Water in the Atmosphere — humidity, precipitation), kegy211 (World Climate and Climate Change) and kegy104 (India: Climate — the monsoon in depth, seasons, rainfall).
• UPSC, honestly: climatology and the monsoon are steady Prelims and GS1 topics, and climate change is a GS3 staple — but questions are set at the depth of the Class 11 books above, so most of them sit there. This chapter is the gentle on-ramp. We link 23 questions whose core this chapter genuinely teaches (all also sit on a Class 11 chapter): the order of layers (1984, 1998), the ionosphere (1996), jets in the lower stratosphere (2011), the ozone layer (1982, 1986, 1987), water vapour (2003, 2024), insolation (1989), pressure and the barometer (1987, 1989, 2001), the monsoon's mechanism and the winter rain of Tamil Nadu (1979, 1991 ×2, 1996, 2014, 1989), the greenhouse effect (1989, 2025), and Mains GS1 on the troposphere (2022) and auroras (2024).

• Composition (Fig. 3.2): nitrogen 78%, oxygen 21%, argon 0.93%, CO₂ 0.04%, others 0.03% (= 100); plus variable water vapour and dust.
• Layers: troposphere (≈ 12 km; weather; temperature falls) → stratosphere (to 50 km; ozone; few clouds) → mesosphere (to 80 km; meteors burn; temperature falls) → thermosphere (80–700 km; ionosphere, radio waves, auroras) → exosphere.
• Weather = hour-to-hour, day-to-day; climate = average over 30+ years. Elements: temperature (insolation), humidity, precipitation, pressure, wind. Wind: high → low pressure; named after where it comes from.
• Seasons: IMD's four in the book — winter, summer/pre-monsoon, monsoon, post-monsoon; traditional six ṛtus (Vasanta … Śiśira).
• Monsoon = seasonal reversal of winds (Arabic mausim). SW (Jun–Sep): land heats → low over India → moist winds in. NE (Oct onwards): land cools → dry winds out; rain on Tamil Nadu coast. Onset Kerala 1 June; whole country 8 July; withdrawal 17 Sep → 15 Oct (IMD normals).
• Climate change: mainly human GHGs (CO₂, methane, nitrous oxide) → warming, floods, droughts, glacier melt, sea-level rise. Carbon footprint = total GHGs from one's activities. Case: Punjab floods 2025 — natural and human-made causes.

The book's account: the atmosphere is a mixture of gases. Nitrogen and oxygen are the two most abundant and are essential to life. Carbon dioxide, argon, helium, neon, krypton, xenon, ozone and hydrogen are present in smaller amounts. The air also holds water vapour — varying, generally 0.1 to 0.4 per cent in the book's figure — which forms clouds and rain, and tiny dust particles. The mix also changes with altitude.

The book's account: the atmosphere is layered, the layers being defined by how temperature and density change with height; air is densest at the surface and thins upward. Troposphere: the most important layer, on average about 12 km high; temperature falls with height; it holds the air we breathe, most of the water vapour and clouds, and nearly all weather (rain, fog, hail); the tropopause separates it from the next layer. Stratosphere: up to 50 km; called ideal for aeroplanes because it is free of clouds and weather disturbances; it contains the ozone layer, which filters the Sun's harmful ultraviolet radiation; the stratopause is its upper boundary. Mesosphere: to 80 km; temperature falls with height; most meteors burn up here. Thermosphere: 80 to 700 km; temperature rises very fast because its gas molecules absorb the Sun's X-rays and short-wave ultraviolet; it reflects radio waves back to Earth — the ionosphere is part of it — and the auroras occur here. Exosphere: the outermost, very thin layer, from which light gases such as helium and hydrogen escape into space because gravity is weak. A Don't Miss Out box stresses that temperature falls with height only in the troposphere and the mesosphere.

Weather is the hour-to-hour and day-to-day state of the atmosphere; it can change a lot from day to day and affects our mood and plans. Climate is the average weather of a place over a long time — the sum of weather conditions and variations over a large area, usually thirty years or more. The major elements are temperature, precipitation, humidity, wind and atmospheric pressure. Temperature varies between day and night and between seasons; an important control is insolation, which decreases from the equator towards the poles, so temperature does too (Fig. 3.6 shows the torrid, temperate and frigid zones). Humidity is water vapour in the air; warmer air can hold more vapour, so on hot, humid days clothes dry slowly and sweat evaporates slowly, making us uncomfortable. Precipitation happens when part of the air becomes saturated, the vapour condenses and falls under gravity — drizzle, rain, snow, sleet and hail; it depends on prevailing winds, mountains and seasons. Rain, the commonest form, cools a place; a long absence of rain makes a dry climate; most groundwater comes from rain.

Air pressure is the push of the weight of air on the Earth's surface. It is highest at sea level and falls rapidly with height. Across the surface, temperature controls it: where it is hot, air heats, rises and leaves low pressure — linked with cloudy skies and wet weather; where it is cold, heavy air sinks and builds high pressure — linked with clear, sunny skies. Air always moves from high to low pressure, and that movement is wind, gentle or strong (Fig. 3.7, whose caption adds that the Earth's rotation turns its direction). Winds are named after the direction they blow from — a westerly blows from the west. Table 3.1 grades wind by speed. Local winds such as the sea breeze (sea to land by day, especially in the afternoon, when the land is warmer and has lower pressure) and the land breeze (land to sea at night, as the land cools faster; gentler, because the temperature and pressure difference is small) keep coastal climates moderate.

Weather changes even within a day, but generalised monthly conditions divide the year into seasons. India's climate is broadly tropical monsoon. The book gives four seasons recognised by the IMD: winter (December to early April; coldest months December and January, about 10–15 °C in the north-west rising to 20–25 °C in the south-east of the mainland); summer or pre-monsoon (April to June, up to July in the north-west; hottest month April in the west and south, May in the north; 32–40 °C across most of inland India); monsoon or rainy — advancing monsoon (June to September, dominated by the humid south-west monsoon, which sweeps in from late May or early June; it begins to recede from north India in early October, when south India gets more rain); and post-monsoon — retreating monsoon (October to December; in the north-west, October and November are usually cloudless). The Himalayan states, being more temperate, add autumn and spring. A Don't Miss Out box gives the six traditional seasons (ṛtus), each about two months, from the astronomical division of the year, reflected in the traditional calendar's months (Table 3.2).

India's climate is strongly shaped by monsoon winds. Sailors in ancient times were among the first to notice the seasonal reversal of winds, which carried their ships; Arab traders named it ‘monsoon’, from the Arabic mausim, ‘season’. Monsoon winds are seasonal and of two kinds. The south-west (summer) monsoon blows from sea to land across the Indian Ocean, the Arabian Sea and the Bay of Bengal between June and September. In the book's account it is caused mainly by the unequal heating of land and sea: in summer India's landmass heats faster, making low pressure over the subcontinent while the cooler ocean keeps high pressure; moist winds flow from ocean to land and bring most of the year's rain. The north-east (winter) monsoon runs from October to February: the land cools faster, making high pressure over land and low over the seas, so cold, dry winds blow from land to sea and bring little rain — except that, crossing the Bay of Bengal, they pick up moisture and rain on the east coast, especially Tamil Nadu, Andhra Pradesh and parts of Karnataka. The monsoon underpins farming, water in rivers, reservoirs and wells, transport, festivals and rural jobs; too much rain floods, too little brings drought.

Climate change is a long-term change in weather patterns — temperature, rainfall, wind — caused mainly, the book says, by human activities: burning fossil fuels, cutting forests, and industrial pollution that releases gases such as carbon dioxide. These raise greenhouse gases — the book lists carbon dioxide, methane, nitrous oxide and water vapour — which trap heat and raise global temperatures. Results named: more frequent floods and droughts, melting glaciers, rising sea levels and loss of biodiversity; harm to health, farming and livelihoods, with heavy impacts on almost all groups including women and children. Tackling it needs collective effort — cutting carbon footprints, renewable energy, protecting forests, sustainable lifestyles. A Let's Explore box, My Carbon Footprint, lets you score your own habits.

In 2025 Punjab had severe floods from heavy monsoon rain and the swelling, one after another, of the Satluj, Beas and Ravi. Villages, fields, houses, roads and bridges were damaged; as waters receded and relief went on, the losses — economic, social and environmental — were heavy, showing the need for better flood management. Natural causes (book): very heavy monsoon rain, intensified by western disturbances that brought extra moisture; rain also in Himachal Pradesh and Jammu and Kashmir; the Satluj, Beas, Ravi and Ghaggar were already high before the heaviest rain, so hill run-off and local rain made them overflow. Human-made causes (book): weak, old embankments (dhūsī bāndh); houses and farms built too close to rivers, shrinking the space floodwater needs; silt reducing the capacity of rivers and dams; and late or unclear flood warnings in some areas. Effects: lives lost; thousands in relief camps; farmland under water and paddy damaged; poultry and dairy farms hit and animals sick or dead; roads, bridges, border fences and public buildings damaged; standing water spreading water-borne disease.

Three Think About It boxes, one Let's Recall, three Let's Explore, one Let's Analyse and a four-point Classroom Discussion. Outlines below.

Eight questions and activities after a six-point Before we move on summary; Q7 is a data exercise on Table 3.3 with five parts. Try first, then open.

Numbers

One-liners

• Air is 78% nitrogen, 21% oxygen; the tiny rest — argon, CO₂, ozone, vapour — does big jobs.
• Layers are named by temperature: falls, rises, falls, rises.
• Weather today; climate over 30 years.
• Low Sun spreads its heat thin — so the poles are cold.
• Warm air rises (low pressure, rain); cool air sinks (high pressure, clear).
• Wind blows from high to low and is named for where it comes from.
• Sea breeze by day, land breeze by night.
• Monsoon = seasonal reversal of winds: in from the sea in summer, out to sea in winter.
• The monsoon comes from the south-east and south-west and leaves from the north-west first.
• More greenhouse gases, more trapped heat — floods, droughts, melting ice, rising seas.

Traps to avoid

• Jets cruise at about 9–12 km near the tropopause, not at 25 km as Fig. 3.3 draws.
• Meteors burn up in the mesosphere; meteorites are the bits that land.
• Water vapour varies up to about 4%, not only 0.1–0.4%.
• Table 3.1's ‘storm’ (103–117 km/h) is Beaufort's violent storm.
• It is the India Meteorological Department.
• The book's season months overlap; IMD's are Jan–Feb, Mar–May, Jun–Sep, Oct–Dec.
• Withdrawal starts about 17 September in the north-west, not early October.
• Water vapour is a feedback, not a direct human emission.
• Monsoon onset is from the south (Kerala), not West Bengal.
• Jodhpur's 20.1 °C in September and Fig. 3.14's June point are slips — use the table.

Prelims — what UPSC asks

• From this chapter: order of layers (1984, 1998), the ionosphere (1996), jets in the lower stratosphere (2011, debatable wording), ozone (1982, 1986, 1987), water vapour (2003, 2024), insolation (1989), pressure and the barometer (1987, 1989, 2001), the monsoon (1979, 1989, 1991 ×2, 1996, 2014) and the greenhouse effect (1989, 2025). Learn each layer with its temperature trend and one feature; learn the monsoon's two directions with their months and rain.
• Watch book-vs-standard numbers (water vapour, troposphere height, monsoon dates, Beaufort names): quote the book, then the standard value with its source and year.

Mains — where it appears and how to answer

• GS1: why the troposphere controls weather (2022); what auroras are and how they are triggered (2024). Use a labelled layer diagram, the temperature profile and the solar-wind chain.
• For climate change and floods (GS3), give causes in two columns (natural, human), effects with a dated Indian example, and measures (NDMA guidelines, warnings, floodplain zoning); present policy positions with their holders, without taking sides.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

Open any question to attempt it in the Question bank — your attempt is saved to your progress.