NCERT Solutions Class 8 Science Chapter 6 Pressure Winds Storms and Cyclones

F R E E S T U D Y M AT E R I A L F O R E V E R Y S T U D E N T

CLASS 8 · SCIENCE

NCERT Solutions

Chapter 6: Pressure, Winds,
Storms, and Cyclones

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

80 – 97 19 58 English

Solutions, notes, sample papers & more at 49 pages

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

CLASS 8 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 6: Pressure, Winds, Storms,
and Cyclones
Complete NCERT Solutions for Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones from the
NCERT textbook Curiosity. Every question the chapter asks is answered — the Probe and ponder openers,
Table 6.1, all six Activities (6.1 to 6.6), every in-text question from pages 81 to 89, all 13 questions of Keep
the curiosity alive and the Discover, design, and debate tasks — with each answer tracing the mechanism:
force spread over an area, pressure differences, and the air movement they drive.

TEXTBOOK BOOK PAGES

Curiosity (Class 8) 80 – 97

SECTIONS QUESTIONS

19 58

MEDIUM

English

Probe and ponder — Page 80
Chapter opener — the questions this chapter sets out to answer

PROBE AND PONDER

Q1 Why are winds stronger on some days than on others?

Because the difference in air pressure between two places is larger on some days than on
others, and wind speed depends on that difference.
Wind is simply air flowing from a region of higher pressure to a region of lower pressure. When
the Sun heats one stretch of land strongly, the air above it becomes warm and lighter, rises, and
leaves a low-pressure region behind. Air from the surrounding high-pressure regions moves in
to take its place. If the region has warmed only a little, the pressure difference is small and the
air drifts in slowly — a calm day. If the heating is strong, or moist air is rising fast and
condensing, the pressure at the centre falls much lower and air rushes in — a windy day.

Why it happens: Activity 6.5 shows this directly. Air flowed from the inflated balloon
to the uninflated one only while their pressures differed, and it stopped the moment
the pressures became equal. As the book notes, if we could measure the speed of
the escaping air we would find it higher when the pressure difference is higher.
The wind outdoors obeys the same rule on a much bigger scale.

Page 1 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Q2 Why are water tanks usually placed at a height?

So that the water column above the taps is tall, because the pressure a liquid exerts depends on
the height of its column.
Activity 6.1 shows that the balloon bulges more as the water column above it gets taller, and
that the width of the pipe makes no difference. The water in a tank behaves the same way: the
greater the height of water above a tap, the greater the pressure at that tap. That pressure is
what pushes the water out. A tank standing on the ground would give only a weak trickle; the
same tank on the roof gives a good, strong stream.

Tip: notice that it is the height, not the amount of water, that matters. A wide tank
holding twice as much water at the same height gives exactly the same pressure at
the tap.

Q3 Can air pressure really crush us?

The atmosphere presses on us with an enormous force, but it does not crush us, because the
pressure inside our body balances it.
The book gives the figure: the air column standing over an area of just 15 cm × 15 cm pushes
down with a force of about 2250 N — the same as the force of gravity on a 225 kg mass. Every
patch of our skin carries a load like that. We feel nothing because the fluids and gases moving in
our tissues and organs keep the pressure inside the body equal to the atmospheric pressure
outside. The two pressures push against each other and cancel.

Why it happens: a pressure difference, not pressure by itself, is what squeezes or
moves things. The sucker in Activity 6.4 sticks only because the air inside it has been
pushed out and the outside pressure is now greater. If the pressure inside our
bodies could suddenly be removed, the outside air really would crush us.

Page 2 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Q4 What causes storms and cyclones? If the Earth stopped rotating, would cyclones
still form?

Storms and cyclones both begin with warm air rising and leaving a low-pressure region
behind. A cyclone, however, also needs the Earth's rotation to make that inrushing air spin — so
if the Earth stopped rotating, cyclones as we know them would not form.
How a storm forms: the ground (or the sea) heats up, the warm and moist air above it becomes
lighter and rises, creating a low-pressure area. Cooler air from surrounding high-pressure areas
flows in, gets heated and rises in turn, so a continuous wind circulation is set up. The rising air
expands and cools, its moisture condenses into water droplets and clouds form; the droplets
merge into heavier drops that fall as rain, hail or snow. Strong winds together with this rain
make a storm.
How a cyclone forms: over warm ocean water the same rising continues, but each time water
vapour condenses into raindrops it releases the heat that was taken up during evaporation.
That heat warms the ascending air further, so it rises even more, and the pressure at the centre
falls even lower. Air from all around rushes in and begins rising too. The Earth's rotation makes
this moving air spin, and the cycle repeats until there is a very low-pressure centre with high-
speed winds revolving around it — a cyclone.

Why it happens: the spin is not produced by the storm itself. The book says plainly
that Earth's rotation causes the moving air to spin. On a non-rotating Earth, air
would still rush straight in towards a low-pressure centre and there would still be
heavy rain and strong winds — but the inflowing air would have no reason to curve,
so it could not organise into a rotating system with a calm eye at its centre.

In-text Questions — Page 81
Chapter opening text · 6.1 Pressure

Q1 Have you ever wondered why fallen leaves rise in the air or trees sway or bend?

Because moving air pushes on them — the wind exerts a force on every surface it meets.
A leaf lying on the ground has a large surface for its very small weight. When wind blows across
it, the air pushes on that surface, and even a modest push is enough to lift the leaf and carry it
along. A tree is far heavier, but its leaves and branches present a very large area to the wind, so
the total force on the crown becomes large enough to bend the trunk.

Page 3 of 49

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Q3 Can the shape or size of the straps really make a difference?

Pawan — narrow straps Megha — broad straps

Fig. 6.1, page 81 — redrawn sketch: Megha and Pawan carrying equally heavy bags.
Pawan's bag hangs from narrow straps, Megha's from broad straps.

Yes — not to the weight of the bag, but to the pressure that weight produces on the shoulders.
Megha's and Pawan's bags are equally heavy, so gravity pulls both down with the same force.
But that force is passed to the shoulders through the straps. Pawan's narrow straps deliver the
whole force over a small area of skin, while Megha's broad straps spread the same force over a
much larger area. Since pressure is force divided by area, the pressure under the narrow straps
is much greater, and it is high pressure on the skin that hurts.

Pressure = Force / Area

Same force, smaller area → larger pressure (Pawan's bag)

Same force, larger area → smaller pressure (Megha's bag)

In-text Questions — Page 82

Page 5 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

6.1 Pressure — broad and narrow handles, head loads

Q1 Can you now understand why it feels easier to lift a water-filled bucket with a broad
handle than with a narrow handle (Fig. 6.2)?

Broad handle Narrow handle

Fig. 6.2, page 82 — redrawn sketch: two identical buckets, one carrying a broad handle
and the other a narrow handle.

Yes. Both buckets pull down on the hand with the same force, but the broad handle spreads
that force over a larger area of the palm and fingers, so the pressure on the skin is smaller.
A narrow handle touches the hand along a thin line. The whole weight of the bucket is delivered
through that thin strip, so the pressure there is very high and it bites into the skin. A broad
handle rests across the whole width of the fingers. The weight is the same, the area is several
times larger, and the pressure falls in the same proportion.
The round cloth pad that people place under a pot or a vegetable basket on the head (Fig. 6.3)
works in exactly the same way. Without it, the rim of the pot presses on a small ring of the scalp;
with it, the load is spread over a much wider patch. In both cases the aim is the same — reduce
the pressure by increasing the area over which the weight acts.

Table 6.1: Record your observations — Page 82

Page 6 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

6.1 Pressure — activities to be done under the supervision of an adult

ACTIVITY

Q1 Driving an iron nail — by the head of the nail, and by the pointed end of the nail.
Easy or difficult to perform? Give reasons.

Driving the nail by its pointed end is easy; driving it by its head is very difficult.

MODE OF ACTION EASY OR REASON
DIFFICULT?

Hammering so that the Easy The tip has a tiny area, so the hammer's force gives a
pointed end enters the wood very large pressure there and the wood fibres part
easily.

Hammering so that the head Difficult The flat head has a much larger area. The same force
is pressed against the wood now gives a small pressure, which is not enough to
push the nail in.

Why it happens: the hammer blow is the same in both cases. Only the area over
which it is delivered changes, and pressure is force divided by area. The point
concentrates the force into a small patch of wood; the head spreads it out.

Safety first: the book asks that the activities in Table 6.1 be done only under the
supervision of an adult.

Q2 Cutting an apple with a knife — using the sharp edge of the knife, and using the
blunt edge of the knife. Easy or difficult to perform? Give reasons.

The sharp edge cuts easily; the blunt edge does not.

Page 7 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

MODE OF EASY OR REASON
ACTION DIFFICULT?

Using the Easy The edge is extremely thin, so its area of contact with the apple is
sharp edge minute. Even a gentle push gives an enormous pressure and the
skin and flesh separate.

Using the blunt Difficult The blunt back of the blade has a much wider surface. The same
edge push is spread over it, the pressure is low, and the apple is only
squashed, not cut.

Did you know? This is why knives are sharpened rather than made heavier.
Sharpening does not increase the force your hand can apply — it reduces the area,
and that is what raises the pressure.

Q3 What can you conclude from your observations in Table 6.1?
Table 6.1: Record your observations

ACTIVITY MODES OF ACTION EASY OR DIFFICULT TO
PERFORM? GIVE
REASONS.

Driving an iron
nail

By the head of By the pointed
the nail end of the nail

Cutting an apple
with a knife

Using the sharp Using the blunt
edge of the edge of the knife
knife

When the same force acts over a smaller area, the pressure produced is greater — and a
greater pressure makes certain tasks easier.

Page 8 of 49

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Both rows of the table say the same thing in different words. The pointed end of a nail and the
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In-text Questions — Page 83

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

6.1 Pressure — overhead tanks; do liquids exert pressure?

Q1 You must have seen overhead water tanks (Fig. 6.4) in your locality, or on the
rooftops of houses used for water supply. Why are these tanks always placed at a
height?

Overhead tank

Fig. 6.4, page 83 — redrawn sketch: an overhead water tank standing on a tall steel
tower, with the supply pipe running down to the ground.

Because the pressure a liquid exerts depends on the height of its column, and it is that pressure
which drives water out of the taps.
Water in the tank is connected to every tap in the house by pipes. The taller the column of water
standing above a tap, the greater the pressure at that tap. Placing the tank on the roof makes
this column as tall as possible, so the water leaves the taps in a strong stream and can also
reach taps on the upper floors.

Why it happens: Activity 6.1 shows that a taller water column bulges the balloon
more, and that widening the pipe changes nothing. So it is height alone — not the
size of the tank or the amount of water in it — that sets the pressure at the outlet.

Page 10 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Q2 Do liquids also exert pressure?

Yes. A liquid presses on the bottom of its container, on the walls of the container, and in fact in
all directions.
Activity 6.1 makes the pressure at the bottom visible: the balloon tied to the lower end of a
water-filled pipe bulges outwards, and it bulges more when the water column is made taller.
Activity 6.2 shows the sideways pressure: water spurts out of holes made in the side wall of a
bottle. Between them, the two activities establish that a liquid exerts pressure everywhere it
touches its container.

Check it yourself: the base of a dam is built much broader than its top for exactly
this reason. The horizontal pressure of the stored water is largest near the bottom,
and the broad base is needed to withstand it (Fig. 6.8).

Activity 6.1: Let us try and find out — Page 83
6.1 Pressure — pressure of a liquid column (Fig. 6.5)

ACTIVITY

Q1 Do both balloons bulge? Do they bulge to the same extent?

Yes, both balloons bulge — and they bulge to exactly the same extent, even though one
pipe is broad and the other narrow.
Both pipes are about 25 cm long and are filled with water to the same level, roughly half way.
The balloon at the lower end of each pipe pushes out, and the two bulges look alike. This is
surprising at first, because the broad pipe holds much more water than the narrow one, and
therefore a much greater weight of water.

Page 11 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Stand / clamp

same water level

Narrow pipe Broad pipe

Equal heights of water column produce the same bulge in both balloons, although the broad pipe
holds far more water.

Q2 What can you infer from this activity?

That the bulge is produced by the pressure of the water column, and that this pressure depends
on the height of the column, not on the weight of water above the balloon.
Reason it through. The two pipes have different diameters, so they hold different weights of
water. If the weight of water were responsible for the bulge, the broad pipe's balloon would
have bulged much more. It does not — the two bulges are equal. So weight cannot be the
cause. What is the same in the two pipes is the height of the water column, and that is what the
bulge follows.

Why it happens: the extra water in the broad pipe also rests on a proportionately
larger area of the pipe's cross-section. Force and area grow together, so force divided
by area — the pressure — stays the same. This is why equal water column heights
produce equal bulges despite different diameters.

In-text Questions — Page 84

Page 12 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

6.1 Pressure — height of the liquid column (Fig. 6.6)

Q1 What will happen to the bulge of the balloon if we increase the height of the water
column?

The bulge will get bigger, and it will keep growing each time more water is poured in.
Pour a little more water into one of the pipes of Fig. 6.5 and the balloon at its lower end pushes
out further. Repeat it a few times and the bulge increases every time. The reason is that the
pressure at the bottom of the pipe depends on the height of the water standing above it: a taller
column presses harder on the rubber, and the rubber stretches more.

Height of water column increases

→ pressure at the bottom of the pipe increases
→ balloon bulges more

Q2 Do you see any relation between the amount of bulge of the rubber balloon and the
height of the water column in the pipe?

Yes — the bulge increases as the height of the water column increases. The two go up
together.
Taken with Activity 6.1, this gives the complete rule for the pressure of a liquid in a vessel: it
does not depend on the width of the vessel or on how much liquid it holds, but it does depend
on the height of the liquid column. The bulge of the balloon is simply a way of seeing that
pressure.

Tip: this single result explains the overhead tank, the strong jet from a tap on the
ground floor, and the broad base of a dam. Whenever you meet a liquid-pressure
question, first ask: how tall is the column above the point in question?

Page 13 of 49

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

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the column of water above the first-floor
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Activity 6.2: Let us find out — Page 84
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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

6.1 Pressure — liquid pressure on the walls of a container (Fig. 6.7)

ACTIVITY

Q1 What do you observe?

Water flows out through all four holes at the same time, in streams that shoot sideways
away from the bottle.
The four holes are made at the same height near the bottom, sealed with tape, and the bottle is
filled with water. When all the tapes are removed together, water jets out of each hole. Because
the holes are at the same height, the four streams are alike; and as the water level in the bottle
falls, all four streams weaken together.

Bottle, cap removed

water jets water jets

Four holes, all at the same height
Water leaves every hole in the side wall, so the liquid must be pressing outwards on the wall.

Q2 What can you infer from this observation?

That liquids exert pressure on the sides of a container as well as on its bottom — in fact, a liquid
exerts pressure in all directions.

Page 15 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

The holes are in the vertical wall of the bottle. Water can only be pushed out through them if it
is pressing horizontally, outwards, against that wall. And since all four holes at the same height
give similar streams, the sideways pressure is the same all round at a given depth.

Why it matters: this is why water spurts like a fountain from a leaking joint or a hole
in a water pipe — the water inside the pipe is pressing on the pipe wall, and
wherever the wall is broken, that pressure drives the water out.

In-text Questions — Page 85
6.1 Pressure — leaking pipes · 6.2 Pressure Exerted by Air

Q1 You must have seen water spurting out like a fountain from leaking joints or holes
in water pipes. Can you explain why this happens? Is it due to the pressure exerted
by water on the walls of the pipes?

Yes — it is the pressure of the water on the pipe wall that drives the fountain.
Water in a supply pipe is under pressure, because the pipe is fed from a tank standing high
above it. That water presses outwards on the pipe wall everywhere, exactly as it pressed on the
wall of the bottle in Activity 6.2. Where the wall is unbroken, the metal or plastic pushes back
and holds the water in. At a crack or a loose joint there is nothing to push back, so the water is
driven out through the gap.

Why it shoots up: the opening is small, so all the water forced out has to pass
through a tiny area at high speed, and it leaves in whatever direction the hole faces.
The taller the tank feeding the pipe, the greater the pressure and the higher the
fountain — another sign that liquid pressure depends on the height of the column
above.

Activity 6.3: Let us explore — Page 85

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

6.2 Pressure Exerted by Air — lifting a covered paper plate (Fig. 6.9)

ACTIVITY

Q1 In which case is the lifting easier, with the folded or the unfolded chart paper
covering the paper plate?

(b) (c)

Fig. 6.9 (b) and (c), page 86 — redrawn: (b) the inverted paper plate covered with the
chart paper folded twice; (c) the same plate covered with the unfolded chart paper. In
both, the stick passes out through a hole at the centre of the sheet.

Lifting is easier when the plate is covered with the folded chart paper. With the unfolded
sheet, distinctly more effort is needed.
The two chart-paper sheets are identical — about 70 cm × 56 cm each — so their weights are
the same. Folding one twice does not make it lighter; it only makes it cover a smaller area of the
plate. Yet the folded sheet lifts easily and the spread-out sheet resists. The difference cannot
come from weight, so it must come from area.

Q2 What can you infer from this?

That air presses down on the covering sheet, and that the total force it exerts grows as the area
of the sheet grows. Force per unit area is pressure — so air exerts pressure.

Page 17 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Set the two cases side by side. Same sheet, same weight, same stick, same plate. The only thing
that changed was how much area the sheet presented to the air above it. When that area was
made larger, the effort needed to lift the plate became larger. Something above the sheet must
therefore be pushing down on it, and the push must be proportional to the area — which is
precisely how a pressure behaves.

Folded sheet → small area → small downward force → easy to lift

Unfolded sheet → large area → large downward force → hard to lift

Force ∝ Area, so Force / Area = a constant pressure — the atmospheric pressure

Why it happens: air is not empty space. It is a real substance surrounding the plate,
and it presses on every object it touches. The pressure exerted by the air around us
is called the atmospheric pressure.

In-text Questions — Page 86
6.2 Pressure Exerted by Air — blowing up a balloon (Fig. 6.10)

Q1 You must have experienced that when you blow air into a balloon, it gets inflated.
Why?

Because the air you blow in presses outwards on the inner walls of the balloon, and the rubber
stretches under that pressure.
Every breath you blow adds more air into the same small space. The air inside pushes on the
rubber from within; the atmosphere outside pushes back. As long as the inside pressure is
greater, the rubber is stretched and the balloon grows. It stops growing when the stretched
rubber is taut enough to balance the extra pressure inside.

Q2 Can we say that air exerts pressure in all directions?

Yes — and the balloon itself is the proof, because it swells in every direction, not just upwards or
sideways.
If the air inside pushed only downwards, the balloon would sag at the bottom and stay flat
elsewhere. Instead it rounds out equally all over, which tells us the air inside is pressing equally
on every part of the wall. The same is true of the air outside: it presses on the top, the sides and

Page 18 of 49

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

the underside of every object around us.
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Did you know? Liquids behave the same way — Activity 6.2 showed water pressing

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ACTIVITY

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Q1 Do you realise that it sticks to the surface?

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atmospheric pressure outside. The outside air now presses the sucker against the surface from

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Why the surface must be smooth: on a rough surface, air keeps leaking back in
through the tiny gaps between the cup and the bumps. The pressure inside never
falls, no pressure difference is set up, and the sucker will not stick.

Q2 Now, try to pull it off. Do you find it difficult to pull it off?

Yes, it is surprisingly hard to pull off — and how hard tells us how large the atmospheric
pressure is.
To lift the sucker you must overcome the pressure difference between the air outside it and the
thin air inside it. Your pull has to be greater than the force with which the atmosphere is holding
the cup down. Since the atmosphere presses on every square centimetre with a large force,
even a small sucker takes a firm tug.

Air outside: atmospheric pressure (high)

Air trapped inside: pressure reduced by pressing the cup down (low)

Sucker held on by the pressure difference

To remove it: applied force must overcome that difference

Did you know? The book puts a number to the atmosphere's push: over an area of
just 15 cm × 15 cm, the air column presses with about 2250 N — the force of gravity
on a 225 kg mass.

In-text Questions — Page 87
6.2 Pressure Exerted by Air · 6.3 Formation of Wind

Q1 Do you know how large the atmospheric pressure is?

It is very large. Over an area of 15 cm × 15 cm, the atmospheric air column pushes with a
force of about 2250 N — nearly the force of gravity on an object of mass 225 kg.
Work out what that means for the area itself:

Page 20 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Area = 15 cm × 15 cm = 0.15 m × 0.15 m = 0.0225 m2

Pressure = Force / Area = 2250 N ÷ 0.0225 m2

= 100000 N/m2 = 1 × 105 Pa

So every square metre of ground, roof or skin carries about a hundred thousand newtons of
atmospheric push. We survive it because the pressure inside our bodies — produced by the
movement of fluids and gases in our tissues and organs — is also equal to the atmospheric
pressure, and it balances the pressure from outside.

Tip: in weather work this pressure is usually written in the practical units given in A
step further: 1 millibar (mb) = 1 hectopascal (hPa) = 100 Pa, so 100000 Pa is about
1000 mb — which is why the isobars in Fig. 6.19 read 994 mb, 996 mb, 998 mb and
1008 mb.

Q2 You must have seen that when an inflated balloon is kept without closing its
mouth, the air from the balloon escapes. Recall that when there is a puncture in the
bicycle tube, the air escapes and the tube collapses. In both of these cases, does air
move from a high pressure region to a low pressure region?

Yes. In both cases the air inside is at a higher pressure than the air outside, and it flows out until
the two pressures are equal.
In the balloon, the stretched rubber squeezes the air within. In the bicycle tube, the pump has
forced a large amount of air into a small volume. Both hold air at a pressure well above
atmospheric. Open the balloon's mouth or puncture the tube, and you give that high-pressure
air a path to the low-pressure air outside — so out it goes, and the balloon or tube collapses.

Why it matters: this is the rule that produces wind. Air anywhere on the Earth
behaves the same way — it moves from a region of high air pressure towards a
region of low air pressure. Activity 6.5 demonstrates it in a controlled way, with two
balloons joined by a straw.

In-text Questions — Page 88

Page 21 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

6.3 Formation of Wind — how do winds form?

Q1 How do winds form?

Wind is air on the move from a region of higher pressure to a region of lower pressure.
Wherever such a pressure difference appears, air begins to flow, and that flow is what we call
wind.
The pressure difference is usually created by uneven heating. When one region is heated more
than its surroundings, the air over it becomes warm and lighter and rises, leaving a region of
low pressure behind. Air from the cooler, higher-pressure surroundings moves in horizontally
to take its place. That horizontal movement is the wind we feel.

Uneven heating → warm air rises → low pressure forms

Surrounding cooler air is at higher pressure

→ air flows from high pressure to low pressure = wind

Q2 Does the difference in air pressure have anything to do with the formation of
winds?

Yes — the pressure difference is the whole cause of wind, and its size decides how strong the
wind is.
Activity 6.5 settles both points. Air flows from the inflated balloon to the uninflated one only
while their pressures differ; the moment the pressures become equal, the flow stops and both
balloons stay the same size. And as the book adds, if we could measure the speed of the
escaping air we would find it higher when the pressure difference is higher.
The sea breeze and land breeze you studied in Curiosity, Grade 7, are the same effect out of
doors. By day the land heats faster than the sea, so the air above the land becomes warmer and
lighter, rises, and creates a low-pressure area; air from the higher-pressure region over the sea
blows in as the sea breeze. At night the water stays warmer than the land, the low pressure
now forms over the sea, and the wind blows from land to sea as the land breeze.

Activity 6.5: Let us observe — Page 88

Page 22 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

6.3 Formation of Wind — air moves from high to low pressure (Fig. 6.12)

ACTIVITY

Q1 Predict what would happen to the balloons.

Prediction: the inflated balloon will shrink and the uninflated one will swell, until both are of
about the same size.
The reason for predicting this is the rule we already have: air moves from a region of high
pressure to a region of low pressure. The inflated balloon holds air at a pressure higher than
that in the limp balloon at the other end of the straw. As soon as the mouth is released, the
straw becomes a path between them, so air should travel through it from the inflated balloon to
the uninflated one.

Tip: a prediction is worth writing down before the activity. If the observation matches
it, you have confirmed the rule; if it does not, you have found something new to
explain.

Q2 Observe what happens to both the balloons. Did it happen as predicted?

Yes, it happens as predicted. The inflated balloon starts getting smaller and the uninflated one
starts filling up.
Air travels along the straw from the inflated balloon into the limp one. You can often hear the
rush of air and see both balloons changing at the same moment. After a short while the
movement slows and stops, and the two balloons are left almost equal in size.

straw
inflated uninflated
high pressure low pressure

air flows this way

Page 23 of 49

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Air travels through the straw from the high-pressure balloon to the low-pressure one — the same
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rule that makes wind blow.

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Yes. Both balloons change size, and

The inflated balloon shrinks — it becomes visibly smaller as air leaves it.
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g
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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

The conclusion of the activity: air moves from a region of high air pressure to a
region of low air pressure, and it keeps moving only as long as a pressure
difference exists. This is exactly why wind blows — and why it dies down when the
pressure evens out.

In-text Questions — Page 89
6.4 High-Speed Winds Result in Lowering of Air Pressure

Q1 I have read that high-speed winds can blow off roofs. I wonder how?

Because high-speed winds are accompanied by reduced air pressure. When a fast wind sweeps
over a roof, the pressure above the roof falls below the pressure of the still air inside the house,
and the roof is pushed upwards from below.
Think of the roof as a lid with air on both sides. Under it, inside the house, the air is calm and at
ordinary atmospheric pressure. Above it, the wind is racing past, and where air moves fast its
pressure is lower. The air below therefore pushes up harder than the air above pushes down. If
this difference is large and the roof is weak, the roof is lifted and carried away (Fig. 6.14 a).

Wind speeds up over the roof → pressure above the roof falls

Pressure inside the house stays high

Upward push from below > downward push from above

→ weak roof is blown off

What to do about it: the book's advice is to keep the doors and windows of
houses open during storms with high-speed winds. The wind can then move over
the roof and through the house, so the pressure inside falls too. The difference
between inside and above is greatly reduced, and the roof stays in place (Fig. 6.14 b).

Check it yourself: Activity 6.6 shows the same effect on a small scale — blow
between two hanging balloons and they swing towards each other instead of apart.

Activity 6.6: Let us observe — Page 89

Page 25 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

6.4 High-Speed Winds Result in Lowering of Air Pressure (Fig. 6.13)

ACTIVITY

Q1 What happens to the balloons? Note down your observations.

The two balloons move towards each other and the gap between them closes, even though
you are blowing air into that gap.
Two equally sized inflated balloons hang from a stick with a gap of 6–10 cm between them.
Most people expect the blast of air to push them apart. Instead, the moment you blow into the
narrow space, the balloons swing inwards and may even touch.

Q2 Now blow harder and observe.

Blowing harder makes the balloons come together faster and more strongly.
The harder you blow, the higher the speed of the air rushing between the balloons, and the
lower the pressure it creates there. The difference between the outside pressure and the
pressure in the gap therefore grows, so the inward push on each balloon grows too, and they
approach each other more quickly.

Tip: this is a fair test — nothing has changed except the speed of the air. That is what
lets you link the effect to speed alone.

Q3 What can you infer from this activity?

That high-speed winds are accompanied by a reduced air pressure.
Blowing between the balloons creates a region of fast-moving air, and therefore a region of low
pressure, in the gap. The air surrounding the balloons on their outer sides is still at ordinary
atmospheric pressure, which is now higher. That higher pressure pushes each balloon inwards,
towards the low-pressure region — so they meet in the middle.

Page 26 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Air blown fast through the gap → pressure in the gap falls

Pressure outside the balloons = atmospheric (higher)

Net push on each balloon is inwards → balloons move together

Blow harder → bigger pressure difference → they move faster

Where this leads: the same rule explains roofs being blown off in a storm, holes cut
in banners and hoardings, and why loose sheets and papers are pulled towards a
fast-moving vehicle rather than pushed away from it.

Keep the curiosity alive — Page 94

Page 27 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Chapter-end exercise, Questions 1–13

KEEP THE CURIOSITY ALIVE

Q1 Choose the correct statement. (i) Look at Fig. 6.21 carefully. Vessel R is filled with
water. When pouring of water is stopped, the level of water will be ____________. (a)
the highest in vessel P (b) the highest in vessel Q (c) the highest in vessel R (d) equal
in all three vessels.

P Q R

Fig. 6.21, page 94 — redrawn sketch: three vessels P, Q and R of different shapes and
widths, joined to one another by tubes near the bottom. Water is being poured into R.

(ii) A rubber sucker (M) is pressed on a flat smooth surface and an identical sucker
(N) is pressed on a rough surface: (a) Both M and N will stick to their surfaces. (b)
Both M and N will not stick to their surfaces. (c) M will stick but N will not stick. (d)
M will not stick but N will stick. (iii) A water tank is placed on the roof of a building
at a height ‘H’. To get water with more pressure on the ground floor, one has to (a)
increase the height ‘H’ at which the tank is placed. (b) decrease the height ‘H’ at
which the tank is placed. (c) replace the tank with another tank of the same height
that can hold more water. (d) replace the tank with another tank of the same height
that can hold less water. (iv) Two vessels, A and B contain water up to the same
level as shown in Fig. 6.22. Pₓ and Pₕ is the pressure at the bottom of the vessels. Fₓ
and Fₕ is the force exerted by the water at the bottom of the vessels A and B.

Page 28 of 49

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

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agas Activity 6.1 showed with a broad and a narrow pipe.
difference to the pressure,

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

(iv) The two vessels hold water to the same level, so the height of the water column is the same
and the pressure at the bottom is the same: PA = PB. But vessel B is wider, so the area of its
base is larger. Since force = pressure × area, the same pressure acting on a larger base gives a
larger force.

PA = PB (equal heights of water column)

Force = Pressure × Area, and AreaB > AreaA

→ F A < FB

Q2 State whether the following statements are True [T] or False [F]. (i) Air flows from a
region of higher pressure to a region of lower pressure. (ii) Liquids exert pressure
only at the bottom of a container. (iii) Weather is stormy at the eye of a cyclone. (iv)
During a thunderstorm, it is safer to be in a car.

(i) True • (ii) False • (iii) False • (iv) True

STATEMENT T/ REASON
F

(i) Air flows from a region of True This is the conclusion of Activity 6.5. Air moved from the inflated
higher pressure to a region of balloon to the uninflated one and stopped only when the two
lower pressure. pressures became equal. Wind is this same flow on a large
scale.

(ii) Liquids exert pressure only False Activity 6.2 shows water spurting out of holes in the side wall
at the bottom of a container. of a bottle. Liquids exert pressure on the walls as well as the
bottom — in fact in all directions.

(iii) Weather is stormy at the eye False The eye is the region of lowest pressure at the centre, and there
of a cyclone. the wind is calm. It is the region surrounding the eye that has
strong winds and heavy rainfall.

(iv) During a thunderstorm, it is True The book states that if you are inside a bus or a car, you are
safer to be in a car. comparatively safer. The metal body carries the charge around
the people inside instead of through them.

Page 30 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Q3 Fig. 6.23 a shows a boy lying horizontally, and Fig. 6.23 b shows the boy standing
vertically on a loose sand bed. In which case does the boy sink more in sand? Give
reasons.

(a) (b)

Fig. 6.23, page 95 — redrawn sketch: (a) the boy lying horizontally on a loose sand bed;
(b) the same boy standing vertically on the same sand.

The boy sinks more when he is standing vertically (Fig. 6.23 b).
His weight is exactly the same in both pictures — lying down does not make him lighter. What
changes is the area of sand supporting that weight. Standing, he touches the sand only with
the soles of his two feet, a very small area. Lying down, his back, legs, arms and head all rest on
the sand, an area many times larger. Since pressure is force divided by area, the pressure on the
sand is far greater when he stands, and loose sand grains give way under high pressure — so
he sinks in deeper.

Weight of the boy = same in both cases

Standing: small contact area → large pressure → sinks more

Lying: large contact area → small pressure → sinks less

Page 31 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Check it yourself: this is why a person crossing soft sand or marshy ground is told
to crawl or lie flat rather than walk, and why camels have broad padded feet for
walking on desert sand.

Q4 An elephant stands on four feet. If the area covered by one foot is 0.25 m2, calculate
the pressure exerted by the elephant on the ground if its weight is 20000 N.

The elephant exerts a pressure of 20000 N/m2, that is 20000 Pa, on the ground.
The elephant stands on all four feet, so its whole weight is shared over the total area of the four
feet. First find that total area, then divide the weight by it.

Total area = 4 × 0.25 m2 = 1 m2

Weight (force) = 20000 N

Pressure = Force / Area = 20000 N ÷ 1 m2

= 20000 N/m2 = 20000 Pa

Why the number of feet matters: if the elephant lifted one foot and stood on
three, the area would fall to 0.75 m2 and the pressure would rise to about 26667 Pa.
The weight would not have changed at all — only the area over which it acts.

Q5 There are two boats, A and B. Boat A has a base area of 7 m2, and 5 persons are
seated in it. Boat B has a base area of 3.5 m2, and 3 persons are seating in it. If each
person has a weight of 700 N, find out which boat will experience more pressure on
its base and by how much?

Boat B experiences the greater pressure on its base — greater by 100 N/m2 (100 Pa).
For each boat, first add up the weight of the persons sitting in it, then divide by the base area.

Page 32 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Boat A

Force = 5 × 700 N = 3500 N

Area = 7 m2

Pressure = 3500 ÷ 7 = 500 N/m2

Boat B

Force = 3 × 700 N = 2100 N

Area = 3.5 m2

Pressure = 2100 ÷ 3.5 = 600 N/m2

Difference = 600 − 500 = 100 N/m2

Notice that boat A carries the greater load — 3500 N against 2100 N — and yet feels the smaller
pressure. Its base is twice as wide, so the load is spread over twice the area.

Tip: pressure questions can never be answered by comparing forces alone. Always
compare force per unit area. Here we have taken only the weight of the persons into
account, as the question intends.

Q6 Would lightning occur if air and clouds were good conductors of electricity? Give
reasons for your answer.

No, lightning would not occur. Lightning depends on air being an insulator, and on the cloud
being able to hold separated charges.
Lightning happens in three stages. First, strong up-and-down winds rub ice particles against
water droplets so that the cloud becomes charged. Second, the charges separate and build up
— light positive ice particles collect in the upper part of the cloud, heavier negative water
droplets in the lower part — and the negative base makes the ground below positively charged.
Third, when the build-up becomes very large, the insulating property of air breaks down and the
charges flow suddenly across, producing a bright flash.
Now change the materials. If the clouds were good conductors, the positive and negative
charges could not stay apart within the cloud; they would move through it and neutralise each
other as fast as they were produced, so no large build-up could ever form. If the air were a

Page 33 of 49

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

good conductor, the charge would leak away steadily and continuously into the surrounding air
co m
and the ground, instead of being stored until air suddenly gives way. Either way there would be
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discharge. .c
no huge accumulation and no sudden discharge — and lightning is precisely that sudden

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deliberately offers an easy conducting path so the charge reaches the ground
quietly instead of striking the building. a

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Q7 What will happen to the two identical balloons A and B as shown in Fig. 6.24 when
water is filled into the bottle up to a certain height. Will both the balloons bulge? If
yes, will they bulge equally? Explain your answer.

A B

Fig. 6.24, page 96 — redrawn sketch: water is poured through a funnel into a bottle that
carries two identical balloons on short side tubes. (The printed English figure does not
letter the balloons; the letters A and B follow the Hindi edition, which marks them ‘क’
and ‘ख’.)

Yes, both balloons will bulge — and they will bulge equally.
In Fig. 6.24 the two balloons are fixed to short side tubes near the bottom of the bottle, and
both tubes are at the same height from the base. When water is poured in through the funnel,
it fills the bottle and pushes into both side tubes.
Water presses on the walls of its container in all directions, so it presses outwards into each side
tube and stretches the rubber — that is why both balloons bulge. How much each bulges is
decided by the pressure at the tube's mouth, and that pressure depends only on the height of
the water column standing above it. Since the two tubes are at the same height, the column
above each is the same, the pressure at each is the same, and the two identical balloons stretch
by the same amount.

Page 35 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Both tubes at the same height in the bottle

→ same height of water column above each

→ same liquid pressure at both openings
→ identical balloons bulge equally

Why height and not quantity: Activity 6.1 made this point with a broad and a
narrow pipe. Equal water column heights gave equal bulges even though the two
pipes held very different amounts of water. Pour more water into the bottle here and
both balloons will bulge more, still equally, because the column above both has risen
by the same amount.

Q8 Explain how a storm becomes a cyclone.

A storm becomes a cyclone when it forms over warm ocean water, where the heat released by
condensation keeps feeding it, so the central pressure falls lower and lower and the Earth's
rotation sets the inrushing air spinning.
Follow the steps:

1. Warm moist air rises. The ocean water is heated, the warm and moist air above it becomes
lighter and rises, and a low-pressure region forms below.
2. Water vapour condenses and releases heat. As the moist air rises it cools and the vapour
condenses into raindrops. During evaporation water had taken up heat; on condensing, that
heat is released back into the atmosphere.
3. The rising becomes stronger. This released heat warms the ascending air further, so it rises
even higher, and the pressure at the centre becomes even lower than before.
4. Air rushes in from all around. Air from the surrounding higher-pressure regions rushes
towards the centre, and it too begins to rise — bringing more moisture with it, so the cycle
repeats and strengthens.
5. The Earth's rotation makes it spin. The moving air does not travel straight in; the rotation
of the Earth makes it turn, so the whole system begins to revolve around the low-pressure
centre.
6. A cyclone is born. The result is a very low-pressure area with high-speed winds revolving
around it — a spinning system of clouds, winds and rain, with a calm eye at the centre.

Page 36 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

High pressure

1008 mb
998 mb
996 mb
eye
994 mb

High pressure
Winds spiral in towards the lowest pressure

Isobars around a cyclone, with the values printed in Fig. 6.19. Pressure falls from 1008 mb at the
outside to 994 mb at the centre, and air spirals in along that fall.

The key difference: an ordinary storm over land runs out of warm moist air quickly.
A cyclone sits over an ocean that keeps supplying it, and each round of condensation
releases fresh heat that drives the next round. That is why the book says a cyclone
loses strength once it reaches land — the source of moist air is cut off.

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Q9 Fig. 6.25 shows trees along the sea coast in a summer afternoon. Identify which side
is land — A or B. Explain your answer.

A B

Fig. 6.25, page 96 — redrawn sketch: coconut palms growing along a sea coast on a
summer afternoon. A marks one end of the ground line and B the other.

Side A is the land (and side B is the sea).
Read the wind direction from the trees first. Their trunks lean towards A and their fronds are
swept towards A, so the wind must be blowing from B towards A.
Now use the time of day. It is a summer afternoon, when land heats up much faster than water.
The air over the land becomes warm and lighter, rises, and leaves a low-pressure area over the
land. The air over the sea stays cooler and is at higher pressure. Air therefore blows from the
sea to the land — this is the sea breeze. Since the wind here blows from B to A, B must be the
sea and A must be the land.

Page 38 of 49

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

What it shows: the air pressure in the inflated balloon was higher than in the uninflated one, so
air travelled through the straw from the high-pressure balloon to the low-pressure balloon. The
flow continued only while the pressures differed, and stopped as soon as they became equal.
This is exactly how wind blows outdoors.

Tip: a simpler version is to let go of an inflated balloon without tying it, or to feel the
air rushing out of a punctured bicycle tube. Both show high-pressure air escaping to
the low-pressure surroundings, but the two-balloon set-up is better because you can
also see the flow stop when the pressures become equal.

Q11 What is a thunderstorm? Explain the process of its formation.

A thunderstorm is a storm accompanied by lightning and thunder. (A storm itself is strong
winds accompanied by rain.)
How it forms:

1. Heating and rising. The land gets heated, and the warm, moist air above it — being lighter
— rises, creating a low-pressure area.
2. Circulation sets in. Cooler air from the surrounding high-pressure areas flows in to take its
place, gets heated in turn and rises. This makes a continuous circulation of wind.
3. Clouds form. The rising air expands and cools, the moisture in it condenses into water
droplets, and clouds are formed. The droplets merge into heavier drops which fall as rain,
hail or snow. Strong winds together with this rain make a storm. Such storms are frequent in
hot, humid and tropical regions like India.
4. Ice particles appear. Under certain conditions the warm air rises to such great heights that
the low temperature there converts the water droplets into ice particles.
5. The cloud gets charged. Strong winds blowing upwards and downwards make the ice
particles and water droplets rub against each other. As you learnt in Exploring Forces, rubbing
charges objects — so static electric charges develop within the cloud.
6. Lightning and thunder. Positively charged lighter ice particles gather in the upper part of
the cloud and negatively charged heavier water droplets in the lower part. When the build-
up is large enough, air stops insulating and a sudden flow of charge gives a flash of
lightning; the air it heats expands violently and produces the loud sound we call thunder. A
storm with lightning and thunder is a thunderstorm.

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Did you know? Isolated, localised thunderstorms have local names in India —
Kalboishakhi in West Bengal, Bihar and Jharkhand, and Bordoisila in Assam.
Coming before the monsoon, they help kharif crops grow. In Kerala, Karnataka and
Tamil Nadu they are called mango showers because they help mangoes ripen, and
in Karnataka local thunderstorms help coffee plants grow.

Q12 Explain the process that causes lightning.

Lightning is the sudden flow of charge that takes place when the charge built up in a cloud
becomes so large that air can no longer insulate it.

1. Charging by rubbing. Inside a tall storm cloud, strong winds blow upwards and downwards.
They make ice particles and water droplets rub against each other, and — as with any two
objects rubbed together — the rubbing leaves them electrically charged.
2. Charge separation. The positively charged, lighter ice particles are carried upwards and
occupy the upper part of the cloud. The negatively charged, heavier water droplets stay in
the lower part. The cloud now has its charges separated, top from bottom.
3. Induced charge on the ground. When the negatively charged lower part of the cloud
comes closer to the ground, it makes the ground and nearby objects such as trees and
buildings become positively charged.
4. Air breaks down. Normally air acts as an electrical insulator and keeps the opposite charges
apart. But when the build-up of charge becomes very large, this insulating property of air
breaks down.
5. The flash. A sudden flow of charges takes place, producing a bright flash of light — that is
lightning. It can occur as opposite charges collide within a cloud, between clouds, or
between a cloud and the ground. The flash heats the air around it very rapidly, the air
expands, and the loud sound produced is thunder.

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

+++++++
light ice particles rise

−−−−−−−
heavier water droplets settle

lightning

ground +++++++++

Charge separation inside the cloud, and the positive charge induced on the ground below. When the
build-up is large enough, air stops insulating and the charge flows across.

Safety, as the book gives it: during lightning, stay away from tall objects, find a
low-lying open area and crouch down, and minimise contact with the ground. Do not
lie down flat. Avoid using an umbrella with a metallic rod. If you are in water, get out
of it. If you are inside a bus or a car, you are comparatively safer.

Q13 Explain why holes are made in banners and hoardings.

Holes are made so that wind can pass straight through the banner instead of piling up
against it — which keeps the pressure difference across the two faces small, and stops the
banner from being torn or blown away.
A banner is a large flat sheet held up in the open. When a fast wind strikes it, the air moving
across the front face is at a reduced pressure, because high-speed winds are accompanied by
lower pressure, while the sheltered air behind the banner stays at ordinary atmospheric
pressure. The difference in pressure between the two faces acts over the whole large area of the
banner, so it produces a very large force — enough to rip the cloth, snap the ropes or pull down
the frame.

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

The holes let air flow through from the high-pressure side to the low-pressure side. The two
faces are then at nearly the same pressure, the net force on the banner falls sharply, and the
banner stays put.

No holes: wind blocked → pressure on the two faces differs → large force on a large area

With holes: air passes through → pressures nearly equal → small force → banner is safe

Where you have met this before: it is the same reasoning as keeping doors and
windows open during a storm. Letting the air move through the house instead of
only over the roof reduces the pressure difference between the inside and the top of
the roof, and the roof is not blown off (Fig. 6.14 b).

Discover, design, and debate — Page 97
Interdisciplinary projects — Science and Society

DISCOVER, DESIGN, AND DEBATE

Q1 Hold a strip of paper, 18 cm long and 2 cm wide, between your thumb and forefinger
so that it hangs freely. Predict what you will observe if you blow over the paper.
Perform the activity now. Note down your observations and interpret your results.

Prediction most students make: the air will push the strip down. What actually happens: the
strip rises and flutters up towards the stream of air.
How to do it: cut a strip of paper about 18 cm long and 2 cm wide. Hold one end between your
thumb and forefinger, just below your lower lip, and let the rest hang down freely. Now blow
steadily over the top surface of the strip, not at its edge.
Observations to record:

When you blow gently, the free end lifts a little and quivers.
When you blow harder, the strip rises further and may become almost horizontal.
The moment you stop blowing, the strip falls back down.

Interpretation: blowing makes the air above the strip move fast, and high-speed air is
accompanied by reduced pressure. The air below the strip is still, so it remains at ordinary
atmospheric pressure. The pressure below is now greater than the pressure above, and this
difference pushes the strip up. Blowing harder increases the speed, lowers the pressure above
still more, and lifts the strip higher — which matches the result of Activity 6.6.

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Relief teams (NDRF and state forces) were positioned in advance with food, drinking water
and medicines; after the cyclone they cleared fallen trees from roads and restored power
lines.
Community volunteers helped move elderly people, children and cattle, and ran community
kitchens in the shelters.

Two suggestions for the local government (choose your own, and give a reason for each):

1. Build or upgrade cyclone shelters within walking distance of every coastal village, with
a raised plinth, stored drinking water and a solar light, and connect them by an all-weather
road. Evacuation fails when people cannot reach a shelter in time.
2. Protect and replant mangroves along the shoreline. Mangrove belts slow the storm surge
and hold the soil, reducing both flooding and the salt damage to farmland that the chapter
describes.

What the book asks you to remember: stay updated on weather reports and the
periodic alerts and warnings issued by the IMD. If you live in a cyclone-prone area,
keep an emergency kit ready with essential items, and during a cyclone quickly move
to a nearby designated cyclone shelter.

Q3 Collect data on the strength of thunderstorms for various regions of India. Compare
your findings and identify which regions are more prone to thunderstorms. Can you
give reasons for your findings?

How to collect the data: use IMD's pre-monsoon and monsoon reports and its lightning
bulletins, and the annual lightning reports published by disaster-management agencies. For
each state note (i) the number of thunderstorm days in a year, (ii) the months in which they
occur, and (iii) the number of lightning strikes recorded. Put your figures into one table so that
the states can be compared fairly — always compare the same quantity over the same period.

Page 45 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

REGION WHEN WHY
THUNDERSTORMS ARE
FREQUENT

West Bengal, Bihar, March–May, before the Strong pre-monsoon heating of the land, with
Jharkhand, Odisha monsoon plenty of moist air drawn in from the Bay of
Bengal. These are the storms called Kalboishakhi.

Assam and the March–May Moist air from the Bay of Bengal is forced upwards
North-East by the hills, so tall clouds build quickly. Locally
called Bordoisila.

Kerala, Karnataka, Pre-monsoon months Warm, humid coastal air rises readily. These are
Tamil Nadu the mango showers, which help mangoes ripen;
local thunderstorms in Karnataka help coffee
plants grow.

North-western dry Few thunderstorm days The air is hot but dry. Without enough moisture
regions (for there is little condensation, so tall storm clouds
comparison) rarely form.

The pattern and the reason: thunderstorms are most frequent where two things occur
together — strong heating of the land and plenty of moisture in the air. Heating makes the air
rise and creates the low pressure that draws in more air; moisture supplies the water vapour
that condenses, forms tall clouds and releases heat that drives the rising further. That is why the
eastern and north-eastern states, close to the warm Bay of Bengal, record far more
thunderstorm days than the dry interior of the north-west. The chapter states the same
conclusion in one line: the important requirements for the formation of thunderstorms are
moisture and strong winds.

Tip: when you present your comparison, say clearly which year your figures are for
and where you got them. A conclusion drawn from data of different years for
different states is not a fair comparison.

Chapter at a glance
Pressure is force per unit area, P = F/A. The same weight pressing on a smaller area gives
a larger pressure — which is why a broad strap is comfortable, a pointed nail drives easily
and a sharp knife cuts. The SI unit is N/m2, also called the pascal (Pa).
A liquid exerts pressure at the bottom of its vessel and on the walls — in fact in all
directions. The pressure depends on the height of the liquid column, not on the width of

Page 46 of 49

Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

the vessel or on the weight of liquid it holds. That is why overhead tanks are placed high
and why a dam's base is broader than its top.
Air also presses on everything around it. The pressure of the air surrounding us is the
atmospheric pressure; the force it exerts on just a 15 cm × 15 cm patch is about 2250 N —
the weight of a 225 kg mass. We are not crushed because the pressure of the fluids and
gases inside the body balances it.
Wind is air flowing from a region of high pressure to a region of low pressure. Warm air
is lighter, so it rises and leaves a low-pressure region behind; cooler air from surrounding
high-pressure regions moves in. The bigger the pressure difference, the faster the wind —
this is what makes sea breeze, land breeze and storms.
High-speed winds are accompanied by reduced air pressure. When fast wind blows over
a roof, the pressure above it becomes lower than the pressure below, and a weak roof can
be lifted off. Keeping doors and windows open during a storm reduces this difference; holes
in banners and hoardings do the same job.
In a thunderstorm, strong up-and-down winds rub ice particles against water droplets and
charge the cloud. Charges separate — positive above, negative below — and when air stops
insulating, the sudden flow of charge is lightning; the air it heats expands and gives
thunder.
A cyclone is a spinning system of clouds, winds and rain over warm ocean water.
Condensation releases heat, the air rises further, the pressure at the centre drops still more,
air rushes in and the Earth's rotation makes it spin. The calm centre is the eye; the IMD
tracks cyclones and issues the warnings we must follow.

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Class 8 Science Chapter 6 Pressure, Winds, Storms, and Cyclones AglaSem · NCERT Solutions

Quick revision

TERM WHAT IT MEANS WHERE IT EXAMPLE FROM THE BOOK
COMES IN
THE CHAPTER

Pressure The force acting perpendicular to a Section 6.1, A bag with broad straps presses
surface, divided by the area of that pages 81–82 less on the shoulders than one
surface: P = F/A with narrow straps (Fig. 6.1)

Pascal (Pa) The SI unit of pressure; 1 Pa = 1 Section 6.1, 100 N on 2 m2 of cardboard gives
N/m2 page 82 50 N/m2 = 50 Pa

Millibar (mb) / Practical units of air pressure; 1 mb A step further, The isobars 994 mb, 996 mb, 998
hectopascal (hPa) = 1 hPa = 100 Pa page 87 mb, 1008 mb in Fig. 6.19

Liquid pressure Pressure exerted by a liquid Activities 6.1 Equal water heights bulge both
column; it increases with the height and 6.2, pages balloons equally (Fig. 6.5); water
of the column and acts in all 83–85 spurts from all four side holes
directions (Fig. 6.7)

Atmosphere The envelope of air surrounding the Section 6.2, It extends up to many kilometres
Earth — nitrogen, oxygen, argon, page 85 above the Earth's surface
carbon dioxide and other gases in
small quantities

Atmospheric The pressure exerted by the air Section 6.2, Air pushes a rubber sucker onto a
pressure around us pages 86–87 smooth surface (Activity 6.4, Fig.
6.11)

Wind Air in motion from a region of high Section 6.3, Air flows from the inflated balloon
pressure to a region of low pressure pages 87–89 to the uninflated one through a
straw (Activity 6.5, Fig. 6.12)

Sea breeze and Daytime wind from sea to land, Section 6.3, Land heats faster by day, so the
land breeze night-time wind from land to sea, pages 88–89 low pressure forms over the land
caused by pressure differences over
land and water

Storm Strong winds accompanied by rain Section 6.5, Storms are frequent in hot,
page 90 humid, tropical regions like India

Thunderstorm A storm accompanied by lightning Section 6.5, Kalboishakhi in West Bengal,
and thunder page 91 Bihar and Jharkhand; Bordoisila in
Assam; mango showers in Kerala,
Karnataka and Tamil Nadu

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TERM WHAT IT MEANS WHERE IT EXAMPLE FROM THE BOOK

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COMES IN

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THE CHAPTER
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down the insulating property of air ground (Fig. 6.16)

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conductor building's walls, pointed page 92 path into the ground (Fig. 6.18)
the highest point and other end

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