Atmosphere and Climate: Class 9 Social Science · Chapter 3

Stand on a beach in Kochi on an early-June morning and you can sense the rain before it comes. A damp wind pushes in from the sea, grey clouds stack up over the coconut palms, and within an hour the lanes are running with water. The wind, the clouds, the downpour and even the smell of wet earth all belong to the thin envelope of air around our planet.

That envelope is the atmosphere (वायुमंडल). Every rainstorm, heatwave and breeze is the atmosphere at work, so it controls both weather and climate. It also keeps us alive in quieter ways. Like a blanket, it stops the ground from losing all its warmth every night. It blocks most of the Sun's harmful ultraviolet light, and it supplies the oxygen we breathe. The Earth's gravity holds all of it in place,11Gravity is the pull that every object with mass exerts on other objects. The Earth's pull keeps the Moon in its orbit and stops the air from drifting off into space. so the air is thickest near the surface and thins out higher up.

By the end of this chapter you should be able to answer four questions:

1. What is air made of, and how is the atmosphere arranged from the ground up to space?
2. What does each layer of the atmosphere do for life on the Earth?
3. Why do the monsoon winds bring rain to India, and why do they turn round each year?
4. What is changing our climate, and how can each of us cut our share of greenhouse gases?

3.1 Composition and Structure of the Atmosphere

3.1.1 Composition of the Atmosphere

Air looks empty, yet it is a mixture of many gases. Suppose you could sort a sample of dry air into 100 equal parts. About 78 of those parts would be nitrogen and about 21 would be oxygen. Every other gas has to squeeze into the single part that is left over (Fig. 3.1).

Most of that last 1 per cent is argon, a gas that hardly reacts with anything. The rest is a pinch of carbon dioxide, neon, helium, krypton, xenon, hydrogen and ozone. Tiny amounts can still matter a great deal, as Table 3.1 shows. Green plants build their food from carbon dioxide, and a thin layer of ozone high above us absorbs most of the Sun's dangerous ultraviolet rays.

Two more ingredients are left out of 'dry air' because their amounts keep changing. Water vapour can be almost absent over a cold desert, yet make up around 4 per cent of the air over a warm tropical sea. Dust, smoke, pollen and salt from sea spray also float about. These specks matter more than they seem. Water vapour condenses on them to form cloud droplets, so without them there would be far fewer clouds and less rain.

3.1.2 Structure of the Atmosphere

Take a bus up to a hill station such as Munnar or Shimla and you notice two changes. The air feels cooler, and a steep walk leaves you out of breath sooner. Both clues tell us that the atmosphere changes as we go up. The air is densest at the bottom, because the weight of all the air above squeezes it, and it thins out quickly with altitude. The mix changes as well: almost all the water vapour stays in the lowest few kilometres, while most of the ozone gathers much higher up.

Scientists sort the atmosphere into five layers. The boundaries are not drawn at round numbers: each one is placed where the temperature stops falling with height and starts rising, or the other way round. Follow the temperature line in Fig. 3.2 as you read about each layer from the ground upwards.

The troposphere (क्षोभमंडल) is the layer we live in. It holds most of the air's mass and nearly all of its water vapour, so clouds, rain, fog, hail, thunderstorms and dust storms all happen here. On average it is about 12 km thick. It bulges to about 18 km over the hot equator and shrinks to about 8 km over the cold poles. Here the air cools as you climb, by roughly 6.5 °C for every kilometre, which is why snow can lie on high peaks even in May. The troposphere ends at a boundary called the tropopause.

Above the tropopause lies the stratosphere (समतापमंडल), reaching to about 50 km. Its best-known feature is the ozone layer, most of it between about 15 and 35 km up. Ozone soaks up ultraviolet light from the Sun. That shields living things below and also warms the air, so the upper stratosphere grows warmer with height. With almost no clouds or storms, the air here is smooth and steady. Long-distance jet aircraft therefore climb to about 10–12 km, near the top of the troposphere or into the lower stratosphere, above most bad weather. The stratosphere ends at the stratopause.

On a clear night you may spot a 'shooting star'. It is a speck of rock or dust from space, a meteoroid, rushing into the air at tens of kilometres every second. Friction heats it until it glows and burns away, and most do so in the mesosphere, the layer from about 50 km to about 80 km. Here the temperature falls with height once more. Near the top it drops to about −90 °C, the coldest part of the whole atmosphere. The mesosphere ends at the mesopause.

The thermosphere begins at about 80 km and reaches up to roughly 700 km; its top rises and falls with the Sun's activity. The few gas particles here absorb the Sun's X-rays and short-wave ultraviolet rays, so the temperature climbs steeply, to many hundreds of degrees. Yet the air is so thin that it would not feel hot: there are too few particles to pass the heat on to you. A part of this layer called the ionosphere holds electrically charged particles. They bounce some radio waves back towards the ground, letting radio signals travel far beyond the horizon. Many satellites, including the International Space Station, circle the Earth inside the thermosphere, and the glowing curtains of the auroras appear here too.

The exosphere is the outermost layer, beginning near 700 km. The air is so thin that a gas particle can travel a long way without meeting another. Light, fast-moving gases such as hydrogen and helium can leak away into space from here. Nothing gets in their way, and the Earth's pull is weaker so far from its centre. The exosphere has no sharp top; it simply fades into outer space.

Each layer does a job for life below. The troposphere carries water and weather around the planet, and the stratosphere screens out harmful ultraviolet light. The mesosphere burns up most space rubble, and the thermosphere absorbs the harshest X-rays and helps radio signals travel.

3.2 Weather and Climate

Your phone's weather app says 'Thunderstorm, 31 °C' for this afternoon. An atlas says that Jaisalmer gets very little rain in a year. The first statement is about weather; the second is about climate. They are linked, but they work on very different time scales.

Weather (मौसम) is the state of the atmosphere at a place over a short time: this hour, today, this week. It can change fast. A bright morning in Bengaluru may turn into a thunderstorm by four o'clock, and a match planned for the evening is washed out.

Climate (जलवायु) is the pattern that appears when you average the weather of a place or region over a long time, usually 30 years or more. It includes the usual ups and downs, too. Weather tells you whether to carry an umbrella today; climate tells you whether a house there needs a steep roof to shed heavy rain.

3.2.1 Elements of Weather and Climate

Weather scientists, called meteorologists, describe the atmosphere with a few things they can measure, known as elements. Five matter most in our daily lives: temperature, humidity, precipitation, atmospheric pressure and wind. As you read, notice how each one depends on the others.

Temperature

Why is a January afternoon warm in Chennai but chilly in Delhi? The main reason one place is warmer than another is insolation (सूर्यातप). Near the equator the noon Sun stands high in the sky all year, so its rays strike the ground almost straight down. Towards the poles the rays arrive at a slant, and a slanting beam spreads the same energy over a larger patch of ground (Fig. 3.3). Shine a torch straight down at a table and then tilt it: the tilted spot is bigger and dimmer. So, on the whole, insolation and temperature both decrease from the equator towards the poles. Temperature also swings between day and night, and between summer and winter.

Geographers use this pattern to divide the globe into three heat zones. The torrid zone lies between the Tropic of Cancer (23½° N) and the Tropic of Capricorn (23½° S). Somewhere in this belt the noon Sun is directly overhead on some day of the year, so it stays hot. The two temperate zones lie between each tropic and the Arctic or Antarctic Circle (66½° N or S), with warm summers and cool winters. The frigid zones around the poles receive only slanting light and stay cold. The Tropic of Cancer runs through the middle of India, so the southern half of the country lies in the torrid zone.

Humidity

Take a cold bottle out of the fridge on an August afternoon in Kolkata and within a minute it is dripping with water. Do the same on a January afternoon in Jaipur and it stays almost dry. The drops come from the air itself: they show humidity (आर्द्रता), the amount of water vapour in the air. Vapour enters the air when water evaporates from seas, rivers, lakes, wet soil and leaves. Warm air can hold much more water vapour than cold air, so hot, wet places are often very humid. Humid air also makes heat harder to bear. Our bodies cool down when sweat evaporates, and in moist air it evaporates slowly, so we feel sticky and overheated.

Precipitation

When moist air rises, it cools. Cooler air cannot hold as much water vapour, so once the air is saturated the extra vapour condenses into tiny droplets or ice crystals, forming clouds. When these grow too heavy to stay up, gravity brings them down as precipitation (वर्षण). It comes in several forms:

• Rain: drops of liquid water, the commonest form.
• Drizzle: very fine drops that fall slowly.
• Snow: ice crystals that stick together as flakes.
• Sleet: a mix of rain and snow, or raindrops that freeze into ice pellets as they fall.
• Hail: balls of ice built up layer by layer inside tall thunderclouds.

Rain, the commonest form, cools the air, fills rivers and ponds, and soaks into the ground to refill groundwater, which supplies most wells and tube-wells. A place that gets little rain for long stretches develops a dry climate. Where and when precipitation falls depends on the winds that bring moist air, on mountains that force air to rise, and on the season. Mumbai, on the windward side of the Western Ghats, gets several times as much rain as Pune, which lies just beyond them.

Atmospheric pressure

Air has weight, and its push on every surface is called air pressure or atmospheric pressure (वायुदाब). Weather reports measure it in hectopascals (hPa); at sea level it averages about 1013 hPa. Climb a mountain and there is less air above you, so the pressure falls quickly. About 5.5 km up it is only half its sea-level value, which is why climbers on the highest peaks often carry oxygen.

Pressure also differs from place to place at ground level, mainly because of temperature (Fig. 3.5). Over a hot patch of land the air warms, expands, becomes lighter and rises, leaving low pressure at the surface. Rising air cools, so low pressure often brings clouds and rain. Over a cool area the air is denser and sinks, building high pressure. Sinking air warms and dries out, so high pressure usually means clear, sunny skies.

Moving air always heads from high pressure towards low pressure, just as air rushes out of a pumped-up cycle tube when you open the valve. Weather maps show this with isobars, lines that join places of equal pressure. Because the Earth spins, moving air does not travel straight from a high to a low; it is turned aside and spirals around the pressure centres. You learnt more about air pressure in Grade 8 Science (Curiosity).

Wind

Wind (पवन) is air moving from an area of high pressure to an area of low pressure. The bigger the pressure difference over a short distance, the faster it blows. A light wind makes a kite tug at its string and dries a wet floor quickly. A strong one bends coconut palms and bangs doors shut, and a cyclone can rip off tin roofs and snap electric poles.

To compare winds, sailors and weather services use the Beaufort scale, first drawn up by the British naval officer Francis Beaufort in 1805. It has 13 steps, from 0 (calm) to 12 (hurricane force), each matched to a range of speeds and to things you can see (Table 3.2).

3.2.2 Land and Sea Breezes

Spend a whole day on a beach at Puri or Chennai and you will feel the wind change sides. The reason is that land heats up and cools down much faster than water. The sun warms only a thin top layer of sand and soil. Sunlight reaches deep into the sea, and the water keeps mixing, spreading the heat through a large volume.

By late morning the land is warmer than the sea (Fig. 3.6). Air over the land rises, the pressure there falls, and cooler air flows in from the sea: a sea breeze. It is usually strongest in the afternoon, which is one reason coastal cities feel more bearable than inland towns in summer. After sunset the land loses its heat quickly and soon becomes cooler than the water. Now the pressure is lower over the sea, and air drifts out from the land: a land breeze. The night-time difference in temperature is smaller, so the land breeze is usually gentler. Together these local winds keep coastal climates moderate.

3.2.3 Climate on a Graph

How can you see a place's climate at a glance? Geographers draw a climograph, a single chart that shows the average temperature of each month as a line and the average rainfall of each month as bars. The averages come from many years of records; the five stations in Table 3.3 use the 30 years from 1991 to 2020.

Latitude and height of each station: Panaji 15°29′ N, 58 m; Hyderabad 17°27′ N, 530 m; Bhubaneswar 20°15′ N, 45 m; Ahmedabad 23°04′ N, 55 m; Guwahati 26°06′ N, 54 m. The 'Year' row gives the average temperature and the total rainfall (the sum of the rounded monthly values). Source: 1991–2020 monthly climate normals for these stations as compiled by the Japan Meteorological Agency's Tokyo Climate Center from Indian station reports; rainfall converted from millimetres to centimetres.

3.3 Seasons in India

Ask an elder in your town when the first mango blossoms appear, when the first big shower comes or when you first need a blanket at night. They will name roughly the same weeks every year. Each stretch of the year in which a similar kind of weather keeps repeating, for weeks or months at a time, is a season.

India's climate is of the tropical monsoon type. It is warm for most of the year, with one main rainy season brought by the monsoon winds and long dry spells in between. The India Meteorological Department (IMD) recognises four seasons, described in Table 3.5.

The months in the table overlap because seasons do not switch on and off on fixed dates, and they reach different regions at different times. For its weather records, though, IMD uses fixed blocks of months (Fig. 3.8). Winter is January–February, pre-monsoon is March–May, the monsoon is June–September and post-monsoon is October–December. In the cooler Himalayan states people also experience a distinct spring and autumn.

3.4 Monsoon

Our word monsoon (मानसून) comes from mausim, the Arabic word for season. Arab traders used it for a wind that reversed with the seasons over the Arabian Sea. For centuries, ships under sail could cross that sea only when the wind was right, so sailors timed their voyages to India's ports by it. Of all the forces that shape India's climate, none matters more.

3.4.1 The South-west Monsoon

The south-west monsoon, or summer monsoon, blows from June to September. It runs on the same idea as a sea breeze, unequal heating of land and water, but on the scale of a whole subcontinent (Fig. 3.9). Through April and May the Indian landmass bakes, and a broad area of low pressure develops over the north-west of the country. The Indian Ocean warms more slowly and stays relatively cool, so the pressure over it is higher. Air flows from the ocean's high towards the land's low. Crossing thousands of kilometres of warm sea, it gathers huge amounts of water vapour and reaches India from the south-west as warm, moist winds.

When these winds meet the land, and especially when hills such as the Western Ghats force them upwards, the air cools and drops its moisture as heavy rain. One branch sweeps in from the Arabian Sea onto the west coast. Another crosses the Bay of Bengal towards north-east India and then turns west along the Ganga plains. About three-quarters of India's yearly rain falls in these four months. IMD's long-term average for June to September is 87 cm, out of roughly 116 cm for the whole year (1971–2020 averages).

The monsoon does not reach the whole country at once. It normally sets in over Kerala around 1 June and then pushes north and west, reaching the far north-west of the country around 8 July. It withdraws in roughly the reverse order, leaving western Rajasthan first, around 17 September, and the south last, by about 15 October (Fig. 3.10). These are IMD's 'normal' dates, revised in 2020 from long records; in any one year the real dates can be a week or two earlier or later.

3.4.2 The North-east Monsoon

From October the pattern flips. Now the land cools faster than the sea, so pressure rises over the cold plains of northern India and falls over the warmer seas to the south. Winds blow outwards from the land, from the north-east, which is why this is called the north-east monsoon or winter monsoon; it lasts from about October to February. Air that has travelled over land is dry, so most of India gets little rain in these months.

The exception is the south-east. Winds that cross the Bay of Bengal pick up moisture on the way and drop it as rain on Tamil Nadu, coastal Andhra Pradesh and parts of Karnataka. Chennai, for example, gets its heaviest rain in October and November, when most of the country is drying out. For these areas the winter monsoon is the main rainy season.

3.4.3 Monsoon and Our Lives

Picture a farming village in Vidarbha in June. Everyone watches the sky. Once the first good rains soak the soil, families sow kharif crops such as cotton, soybean and tur, and the village tank and wells begin to fill for the dry months ahead. Across most of India the monsoon is the year's main water supply. In towns it shows up in vegetable prices, in the electricity made at hydroelectric dams and in work for farm labourers. Festivals such as Teej and Nag Panchami belong to the rainy season.

The monsoon's gifts are not always gentle:

• Too much rain in a short time causes floods and landslides, as the case study in Section 3.5.1 shows.
• A weak or late monsoon, or long dry breaks in the middle of it, can cause drought, crop failure and shortages of drinking water.
• Rural families, whose work and income depend on farming, are hit hardest when the rains fail.

3.5 Climate Change

The year 2024 was the warmest in more than 170 years of records. The Earth's climate has changed before, through ice ages and warm spells that lasted thousands of years. What worries scientists now is how fast the present change is happening, and its main cause: human activity. Climate change means long-term shifts in temperatures, rainfall, winds and other patterns of weather. Since the 1800s, burning coal, oil and gas for electricity, transport and industry, along with clearing forests, has poured heat-trapping gases into the air.

These greenhouse gases include carbon dioxide, methane, nitrous oxide and water vapour. Sunlight passes through them easily and warms the ground. The warm ground then gives off heat, and greenhouse gases absorb part of it and send some back down (Fig. 3.11). Without any greenhouse gases the Earth would be a frozen planet. The trouble is that we keep adding more, so more heat is held in and the planet warms up.

In India the signs include more frequent and fiercer heatwaves, very heavy rain on some days with longer dry gaps between, and shrinking glaciers in the Himalaya. Rising seas threaten low coasts such as the Sundarbans. Around the world, species that cannot adapt quickly enough are disappearing. The damage to health, harvests and incomes falls hardest on those with the least means to cope: poor families, farmers who depend on rain and people who work outdoors. Women and children often bear an extra share, for example when they must walk farther for water or when disasters close schools.

No single step can solve the problem, but many steps together make a difference. Countries can switch to renewable energy such as solar and wind power, protect and grow forests, and build cleaner industries and public transport. Each of us can shrink our own carbon footprint (कार्बन पदचिह्न).

3.5.1 Case Study: Punjab Floods 2025

In August and early September 2025, Punjab was hit by severe floods. Very heavy monsoon rain fell over the state and over the hills of Himachal Pradesh and Jammu and Kashmir, where its rivers begin. The Satluj, the Beas and the Ravi, along with the Ghaggar, rose to dangerous levels. They broke through embankments or spilled over their banks and spread across villages, fields, roads and bridges.

By 2 September the state government had declared all 23 districts flood-affected. Figures reported on 2–3 September counted 30 deaths and about 3.5 lakh people affected. Standing crops were damaged on more than 1.48 lakh hectares (Punjab government figures reported by AIR News and The Tribune). The toll rose further in the days that followed. Gurdaspur, Amritsar, Ferozepur, Fazilka and Pathankot were among the hardest-hit districts. The losses to farms, homes and public property ran into thousands of crores of rupees.

Why was the damage so great? Geographers look at two kinds of causes (Fig. 3.13).