NCERT Solutions for Class 9th Social Science Chapter 3 End-of-chapter exercise — Questions and activities

The atmosphere is the blanket of air surrounding the Earth. It is pulled down around the Earth by gravity, and is a mixture of gases in various proportions, vital for the survival of all living beings on the Earth. It shields us from the Sun's harmful radiation, including ultraviolet; it regulates the Earth's temperature by trapping some of the Sun's energy; and it is the key component of the Earth's weather and climate systems, influencing temperature, humidity and air pressure.

Its composition, as a pie diagram:

Two things the pie cannot show, so write them beside it:

• Besides these gases the atmosphere also contains water vapour — generally 0.1% to 0.4% — and tiny dust particles. Water vapour plays a significant role in cloud formation and precipitation.
• The remaining trace gases the chapter names are helium, neon, krypton, xenon, ozone and hydrogen.

Draw it as a vertical section, with height above sea level on the left-hand axis, the five layers stacked in order, the three '-pauses' marked as boundary lines, and the landmark feature of each layer labelled on the right.

The labels your diagram must carry, and the values to put on them:

The Indian Meteorological Department (IMD) recognises four distinct seasons in India:

Add the exception: the Himalayan states, being more temperate, experience two additional seasons — autumn and spring.

Because the pressure is balanced on both sides of you. The chapter gives both halves of the reason: air presses on us from all sides, and our bodies exert a counter-pressure in response — the pressure inside our bodies is equal to the atmospheric pressure and cancels the pressure from outside.

Two separate ideas are doing the work here, and a full answer needs both:

• The air pushes from every direction, not just downward. If it pressed only from above you would feel it as a weight on your shoulders. Because it presses equally on your back, your front and your sides, the pushes cancel out.
• Your body pushes back. The fluids and gases inside you are at the same pressure as the air outside, so the walls of your body are not squeezed inward.

Aeroplanes fly in the stratosphere — the layer above the troposphere, extending up to 50 kilometres. The chapter's reason is short and exact: this layer is ideal for flying aeroplanes because it is free of clouds and other weather disturbances.

a. The troposphere and the stratosphere

b. The south-west monsoon and the north-east monsoon

How to draw a temperature-and-rainfall graph (a climograph). Copy the method of Fig. 3.14, which is the Delhi row of Table 3.3 turned into a picture:

1. Put the twelve months along the horizontal axis, January to December.
2. Put temperature in °C on the left-hand vertical axis and rainfall in cm on the right-hand vertical axis. Use the same two scales for every station you draw, or you will not be able to compare them.
3. Draw the rainfall as bars from the base line.
4. Plot the temperature as a dot for each month and join the dots into a line.
5. Title the graph with the station's name, latitude and altitude — those three explain most of what the graph shows.

Two worked examples, drawn from Table 3.3, that show the two great families of Indian climate:

The broad generalisations you should arrive at. Every figure below is worked out from Table 3.3 itself:

Distance from the equator is simply latitude, so arrange the stations by the latitude column, nearest the equator first.

a. The two stations with the most extreme climate: Leh and Jodhpur.

'Extreme' means the temperature swings widely through the year, so the measure to use is the annual range — the warmest monthly average minus the coldest.

So the answer is Leh and Jodhpur, with Delhi so close behind Jodhpur that it is worth naming. Leh's climate is extreme in an additional sense too: it is the only station in the table with temperatures below 0°C — in January (−8.5), February (−7.2), March (−0.6) and December (−5.6).

b. Two stations influenced by the retreating monsoon: Chennai and Thiruvananthapuram.

The retreating (post-monsoon) season runs from October to December, so add each station's October, November and December rainfall and see whose share is largest.

Chennai's November alone (35.0 cm) is its wettest month of the whole year — the clearest possible sign of the retreating monsoon. Bengaluru is a reasonable third answer, being on the same side of the peninsula.

c. The two hottest stations in (i) February and (ii) June

a. Shillong (225.3 cm) against Kolkata (162.5 cm) — the difference is 62.8 cm, and the cause is relief.

The chapter names three factors that affect precipitation: prevailing winds, mountains, and seasons. Here the first two act together.

Both places are fed by the same moist south-west monsoon winds coming off the Bay of Bengal. Over Kolkata those winds travel across a flat plain only 6 m above sea level, so nothing forces them upward and only part of their moisture falls. When the same winds reach the hills on which Shillong stands, they must climb more than 1400 m. Rising air cools, its capacity to hold water vapour drops — the chapter's own rule is that 'as the air gets warmer, its capacity to hold water vapour increases', so cooling reverses it — the vapour condenses, and it rains heavily. That is why the hill station gets 1.4 times the rain of the plain.

b. Delhi (67.0 cm) against Jodhpur (36.6 cm) — Delhi gets 30.4 cm more, nearly twice as much.

Notice first what does not explain it. The two stations are almost twins in the table: Delhi is 219 m above sea level and Jodhpur 224 m, so altitude is out; and Jodhpur at 26°18'N is actually nearer the equator than Delhi at 29°N, so latitude cannot be the reason either. The answer has to lie in where each one sits along the monsoon's path, and the chapter's two maps show it.

• Fig. 3.10 (advancing monsoon): Delhi is crossed in the last week of June, while the desert corner in which Jodhpur lies is reached only by the 5 July and 8 July lines — the very last part of India the monsoon arrives in.
• Fig. 3.11 (retreating monsoon): withdrawal begins in that same north-western corner, on 17 September, and only then moves east and south.

So Jodhpur has the shortest rainy season of the two — the monsoon arrives there last and leaves there first. On top of that, by the time the winds have travelled that far inland over dry, hot land they have already given up most of their moisture, and Jodhpur lies in the desert region of western Rajasthan where there is no relief barrier to force the air upward.

a. Thiruvananthapuram has an equable climate because it is coastal and very near the equator.

Two reasons, both visible in its own row of the table:

• Latitude 8°29'N — the nearest station to the equator. Insolation there is high and, more importantly, it barely changes through the year, so there is no cold season to fall into.
• Altitude 61 m, on the coast — the sea is the moderator. Water warms and cools far more slowly than land, so the land and sea breezes described on page 46 carry that steadiness ashore. As the chapter says, land and sea breezes 'are essential in creating moderate climatic conditions in the coastal region'.

Notice the sign of the sea's work: the hottest month is April, before the monsoon, and the temperature then falls through the rainy season instead of rising. Every month lies between 26.2°C and 28.7°C — which is what 'equable' means.

b. Chennai's rain comes late because it is watered by the retreating, north-east monsoon.

During the south-west monsoon Chennai is on the sheltered, eastern side of the peninsula — the moist winds from the Arabian Sea have already crossed the land, so they arrive with much less to give. Then in October the wind reverses. The north-east monsoon blows from land to sea, and the chapter says these winds are cold and dry and bring no rain to most of India — but when they pass over the Bay of Bengal they pick up moisture and cause rainfall on the eastern coast of India, especially in Tamil Nadu, Andhra Pradesh and parts of Karnataka. Chennai is on that coast. So its rainy season begins where everyone else's is ending, which is precisely why the chapter calls the winter monsoon 'important for the rainfall of the south-eastern regions of India'.

c. Leh gets a little precipitation in nearly every month because it lies in a high, cold rain shadow, and what falls there is mostly snow.

Why so little, and why so evenly spread: Leh stands at 3506 m, the highest station in the table, beyond the great Himalayan ranges. The south-west monsoon winds have to cross those ranges to reach it, and they lose almost all their moisture climbing the southern slopes — so the monsoon that drowns Shillong hardly arrives at Leh at all. That is why there is no June–September peak. What Leh does receive comes in small amounts all year from other sources, largely as snow in the cold months: its average temperature is below 0°C in January (−8.5), February (−7.2), March (−0.6) and December (−5.6), so precipitation there cannot fall as rain. A little every month, and never much — 8.5 cm in a whole year, against Shillong's 225.3 cm.

Yes. The differences between the ten stations are large, but they are differences of degree within one shared rhythm — and Table 3.3 shows that rhythm plainly.

Evidence 1 — nearly every station takes most of its rain in the same four months. June to September is one-third of the year, yet it delivers:

These are stations 1500 km apart, on a desert, a delta, a hill range, two different coasts and a cold high plateau, and nine out of ten of them have their wettest month between June and September. Nothing but a single, country-wide wind system could do that.

Evidence 2 — even the exception is the monsoon. Chennai's rain comes in October and November, but it comes from the north-east monsoon picking up moisture over the Bay of Bengal. It is not outside the system; it is the other half of the same reversal.

Evidence 3 — the temperature curves bend the same way. At station after station the hottest month comes before the rains and the temperature then falls while the rain falls, exactly as the chapter says rain 'lowers the temperature of a place':

Evidence 4 — the calendar itself is built on it. The IMD's four seasons are named after the monsoon: pre-monsoon, monsoon (advancing), post-monsoon (retreating). A country whose seasons are named after one wind has already conceded that the wind organises its year.

Method. Collect from several sources so that the set is genuinely varied — old newspapers and magazines, the tourism pages of state government websites, your school library's atlas and picture books, family photographs from journeys, and postcards. Pick at least five regions that differ sharply in climate and relief. For each picture, write beneath it three things: (i) the region, (ii) the climate and relief of that region, taking your evidence from Table 3.3 wherever the station is listed, and (iii) the feature of the house or the clothing you think answers that climate.

What a good answer must contain. Not a caption saying 'this is a house in Rajasthan', but a claim with a reason: which feature answers which element of weather — temperature, rainfall, humidity or wind. And be careful to note where the answer is not climate at all: much of what people wear and build also comes from tradition, from what materials are locally available and from what they can afford. Say so where you think it applies. That honesty is part of the answer.

Sample answer: