Pressure, Winds, Storms
and Cyclones
Complete, step-wise solutions for every in-text question and end-of-chapter exercise — with diagrams and worked-out numericals.
In-text & Probe-and-Ponder Questions
These are the curiosity questions, activity-prompts, and thought-bubbles scattered across the chapter — answered directly, in the order they appear.
Probe and Ponder (Chapter Opener)Why are winds stronger on some days than on others?
Wind speed depends on the difference in air pressure between two regions. On days when the temperature difference between two areas (such as land and sea, or two regions at different altitudes) is large, the pressure difference created is also large. Air rushes faster from the high-pressure region to the low-pressure region to balance this difference, producing strong winds. On calmer days, the pressure difference between regions is small, so the wind blows gently.
Why are water tanks usually placed at a height?
The pressure exerted by a liquid column increases with the height of the column. By placing the tank at a height, a tall column of water is created above the level of the taps. This extra height increases the water pressure in the pipes, so water gushes out of the taps with greater force and a steady stream, instead of just trickling out.
Can air pressure really crush us?
Air pressure is genuinely enormous — the atmospheric air column over just a 15 cm × 15 cm patch presses down with a force equal to the weight of a 225 kg object (about 2250 N). Yet we are not crushed, because our body also has an internal pressure (created by fluids and gases inside our tissues and organs) that is equal to the atmospheric pressure pushing in from outside. These two pressures balance each other out, so we feel nothing unusual under normal conditions.
What causes storms and cyclones? If the Earth stopped rotating, would cyclones still form?
Storms form when warm, moist air rises rapidly over a heated surface, creating a low-pressure area; cooler air rushes in, also rises, and the rising moist air condenses into clouds and rain, accompanied by strong winds.
Cyclones form over warm ocean water through the same rising-air process, but on a much larger scale, with continuous release of latent heat keeping the air rising and the pressure dropping further.
If the Earth stopped rotating, the basic updraft of warm moist air and the resulting low-pressure system could still occur — so storms would still form. However, the characteristic spinning/swirling motion of a cyclone is caused by the Earth’s rotation (which deflects moving air). Without rotation, air would simply rush straight into the low-pressure centre rather than spiralling around it — so true rotating cyclones, as we know them, would not form.
Can the shape or size of bag straps really make a difference to how heavy a bag feels?
Yes. Although both bags weigh the same, a narrow strap spreads the weight (force) over a smaller area of the shoulder, producing higher pressure (since Pressure = Force ÷ Area). A broad strap spreads the same weight over a larger area, producing lower pressure. This is why Pawan’s bag with narrow straps hurts more, while Megha’s broad-strap bag feels comfortable.
Suppose you live on the second floor of a three-storeyed building and an overhead tank sits on the top floor. Will you or your friend on the first floor get a more powerful stream of tap water?
Your friend on the first floor will receive water with greater pressure. The pressure of a liquid column depends on the height of the column above the point being measured. Since the first floor is farther below the overhead tank than the second floor, the height of the water column above the first-floor tap is greater than that above the second-floor tap — giving a stronger, more powerful stream on the first floor.
Water sometimes spurts out like a fountain from leaking joints or holes in water pipes. Why does this happen?
Yes — this happens because of the pressure exerted by water on the walls of the pipe. Water inside a pipe is under pressure (especially if it is connected to a tank placed at a height). Liquids exert pressure in all directions, including sideways on the walls of their container. At a hole or leaking joint, this confined pressure suddenly finds an opening, and the water is forced out forcefully, often spurting like a fountain.
“I have read that high-speed winds can blow off roofs. I wonder how?”
When high-speed winds blow over a roof, they create a region of low pressure just above the roof (because high-speed winds are associated with reduced pressure, as shown in Activity 6.6). Meanwhile, the air pressure inside the house (below the roof) remains at the normal, higher atmospheric pressure. This pressure difference pushes the roof upward and outward from below, and if the roof is weak or the difference is large enough, it can be torn off and blown away.
This is also why it is recommended to keep doors and windows open during a storm — it lets the wind pass through the house, reducing the pressure difference between inside and outside, which helps keep the roof intact.
Keep the Curiosity Alive — Exercise Solutions
Direct, fully-worked answers to all 13 end-of-chapter questions, including step-wise numericals.
Look at Fig. 6.21 carefully. Vessel R is filled with water. When the pouring of water is stopped, the level of water will be ____.
P, Q and R are connected to one another at their base, even though they differ in shape and width. Liquids exert pressure that depends only on the height of the column, and a liquid always settles so that the pressure at any common connected level is the same throughout. As a result, water finds its own level — it will rise to the same height in all three connected vessels, regardless of their individual shapes or widths.
A rubber sucker (M) is pressed on a flat smooth surface, and an identical sucker (N) is pressed on a rough surface. What happens?
A sucker sticks because pressing it out most of the air between the sucker and the surface, so the air pressure inside it drops. The higher atmospheric pressure outside then holds it firmly against the surface. On a smooth flat surface (M), a near-perfect seal is formed, so very little air remains inside — the sucker sticks well. On a rough surface (N), tiny gaps and irregularities prevent a good seal — air leaks back in, the pressure inside does not stay low, and the sucker fails to stick.
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:
Liquid pressure depends only on the height of the liquid column, not on the amount (volume) of liquid or the width of the container. So options (c) and (d), which only change how much water the tank holds at the same height H, will not change the pressure at the ground floor. Increasing H directly increases the pressure delivered at the bottom.
Two vessels A and B contain water up to the same level, as shown in Fig. 6.22. Pₛ and P_B are the pressures at the bottom; F_A and F_B are the forces exerted by water at the bottom of A and B.
Since both vessels are filled to the same height, the pressure at the bottom is equal: \(P_A = P_B\) (pressure depends only on column height). However, Force = Pressure × Area, and vessel B has a larger base area than vessel A. So even though the pressure is the same, the total force on the larger base of B is greater: \(F_A < F_B\).
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. TRUE
This is exactly how wind is formed — air always moves from high-pressure to low-pressure regions.
(ii) Liquids exert pressure only at the bottom of a container. FALSE
Liquids exert pressure in all directions — at the bottom and on the side walls of the container (proved in Activity 6.2, where water spurted from holes made on the sides of a bottle).
(iii) Weather is stormy at the eye of a cyclone. FALSE
The eye of the cyclone is the calmest part — it is the region of lowest pressure at the centre, where winds are calm. The surrounding region (the eyewall) experiences the strong winds and heavy rain.
(iv) During a thunderstorm, it is safer to be in a car. TRUE
As mentioned in the chapter, being inside a bus or car offers comparatively greater safety during lightning, since the metal body helps conduct any electric charge around the occupants and into the ground.
Fig. 6.23a shows a boy lying horizontally, and Fig. 6.23b shows the boy standing vertically on a loose sand bed. In which case does the boy sink more in sand? Give reasons.
The boy sinks more in Fig. 6.23(b), when he is standing vertically.
Reason: The boy’s weight (force) is the same in both cases. But when lying down, his weight is spread over the entire area of his body touching the sand — a large area — producing low pressure. When standing, the same weight acts only through the small area of his two feet — a much smaller area — producing much higher pressure (since Pressure = Force ÷ Area). This higher pressure makes him sink deeper into the loose sand while standing.
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.
Weight of elephant (Force), \(F = 20000\ \text{N}\)
Area of one foot \(= 0.25\ \text{m}^2\)
Number of feet on the ground \(= 4\)
\[ A = 4 \times 0.25\ \text{m}^2 = 1\ \text{m}^2 \]
\[ \text{Pressure} = \dfrac{\text{Force}}{\text{Area}} = \dfrac{F}{A} \]
\[ \text{Pressure} = \dfrac{20000\ \text{N}}{1\ \text{m}^2} \]
Boat A has a base area of 7 m² with 5 persons; Boat B has a base area of 3.5 m² with 3 persons. Each person weighs 700 N. Which boat experiences more pressure on its base, and by how much?
Total force, \(F_A = 5 \times 700\ \text{N} = 3500\ \text{N}\)
\[ P_A = \dfrac{F_A}{A_A} = \dfrac{3500\ \text{N}}{7\ \text{m}^2} = 500\ \text{N/m}^2 \]
Total force, \(F_B = 3 \times 700\ \text{N} = 2100\ \text{N}\)
\[ P_B = \dfrac{F_B}{A_B} = \dfrac{2100\ \text{N}}{3.5\ \text{m}^2} = 600\ \text{N/m}^2 \]
\[ P_B – P_A = 600 – 500 = 100\ \text{N/m}^2 \]
Would lightning occur if air and clouds were good conductors of electricity? Give reasons for your answer.
No, lightning would not occur in the form we observe it.
Lightning happens because air normally acts as an insulator. This lets positive and negative charges build up and separate within clouds (positive ice particles on top, negative water droplets below) until the charge becomes very large. Only then does the air’s insulation suddenly break down, releasing a sudden, dramatic flash of light.
If air and clouds were good conductors instead, electric charges would not be able to accumulate or stay separated — they would continuously and gradually leak away or neutralise as soon as they formed. Without this build-up of a large charge difference, the sudden, powerful discharge we know as lightning would never form.
What will happen to the two identical balloons A and B in Fig. 6.24 when water is filled into the bottle up to a certain height? Will both balloons bulge? If yes, will they bulge equally?
Yes, both balloons A and B will bulge, and they will bulge equally.
Reason: Liquids exert pressure on the side walls of their container, and this pressure depends only on the height of the liquid column above that point — not on the shape of the container or the direction in which the wall faces. Since both balloons are fixed at the same height from the base of the bottle, the water pressure acting on them is identical. Equal pressure produces an equal outward push, so both balloons bulge out by the same amount (just as seen in Activity 6.2, where water spurted equally from holes made at the same height around a bottle).
Explain how a storm becomes a cyclone.
A storm develops into a cyclone over warm ocean waters through the following chain of events:
- Heated ocean water causes warm, moist air above it to rise rapidly.
- As this moist air rises and cools, the water vapour condenses into raindrops — and condensation releases extra heat back into the surrounding air.
- This released heat warms the rising air even further, making it rise faster and creating an even lower pressure at the centre.
- Air from the surrounding high-pressure regions rushes in to fill this low-pressure area, and this incoming air also begins to rise — the cycle repeats and intensifies continuously.
- Because of the Earth’s rotation, this inrushing air does not move straight to the centre but is deflected, causing it to spin around the low-pressure core.
The end result is a large, organised, spinning system of clouds, high-speed winds, and rain — with a calm low-pressure “eye” at the centre — which we call a cyclone. A regular storm only becomes a cyclone once this self-sustaining, rotating circulation is established over warm ocean water.
Fig. 6.25 shows trees along the sea coast in a summer afternoon. Identify which side is land — A or B. Explain your answer.
Side A is the land, and side B is the sea.
Reason: During a summer afternoon, land heats up faster than water. The air above the land becomes warm, lighter, and rises, creating a low-pressure area over the land. The relatively cooler, high-pressure air over the sea then blows towards the land to fill this gap — this is the sea breeze, and it blows from the sea towards the land during the day.
In the figure, the trees are bent towards side A, which tells us the wind is blowing from B (the sea) to A (the land). Hence, A is the land and B is the sea.
Describe an activity to show that air flows from a region of high pressure to a region of low pressure.
Materials: Two similar thin rubber balloons and a drinking straw.
- Insert one end of the straw into the mouth of one (uninflated) balloon and tie it securely with a thread or rubber band so no air can leak.
- Inflate the second balloon, and while pinching its neck shut with your fingers, insert the free end of the straw into its mouth and tie it securely too.
- You now have an inflated balloon and an uninflated balloon connected by a straw.
- Release your fingers and observe both balloons.
Observation: The inflated balloon gradually deflates while the previously uninflated balloon gradually inflates, until after some time both balloons reach almost the same size, after which the flow stops.
Inference: Air pressure inside the inflated balloon is higher than in the uninflated one. Air therefore flows through the straw from the high-pressure balloon to the low-pressure balloon, continuing until the pressure in both becomes equal. This demonstrates that air always flows from a region of high pressure to a region of low pressure.
What is a thunderstorm? Explain the process of its formation.
A thunderstorm is a storm accompanied by thunder (loud sound) and lightning (a bright flash of light), caused by the build-up and sudden discharge of electric charge within clouds.
Formation process:
- Heated land causes warm, moist air to rise rapidly, creating a low-pressure area; cooler air from the surroundings rushes in to take its place and is heated in turn — setting up continuous wind circulation.
- As the rising air expands and cools, the moisture in it condenses into water droplets, forming clouds; these droplets merge into bigger drops that eventually fall as rain, hail, or snow.
- In some clouds, warm air rises to great heights where the very low temperature converts the water droplets into tiny ice particles.
- Strong winds blowing upward and downward inside the cloud cause the ice particles and water droplets to rub against each other repeatedly, generating static electric charges (just as rubbing charges two objects, studied in ‘Exploring Forces’).
- The lighter, positively charged ice particles collect in the upper part of the cloud, while the heavier, negatively charged water droplets settle in the lower part — creating a charge separation.
- When the charge build-up becomes large enough, air’s insulating property breaks down at that point, and a sudden flow of charge occurs — producing lightning, followed by the sound of thunder.
A storm that is accompanied by this lightning and thunder is, by definition, called a thunderstorm.
Explain the process that causes lightning.
Inside a storm cloud, strong upward and downward winds cause ice particles and water droplets to continuously collide and rub against one another. This friction causes static electric charges to develop:
- The lighter, positively charged ice particles rise and gather in the upper part of the cloud.
- The heavier, negatively charged water droplets settle in the lower part of the cloud.
This separation of charge also causes the ground and nearby tall objects (trees, buildings) below the negatively charged base of the cloud to become positively charged by induction.
Normally, air is a poor conductor (an insulator) and prevents these opposite charges from meeting. But once the amount of accumulated charge becomes very large, the insulating property of the air breaks down at that point. A sudden, massive flow of charge takes place — within a cloud, between two clouds, or between a cloud and the ground — producing a bright flash of light called lightning. The intense heat of this discharge makes the surrounding air expand rapidly, producing the loud sound we hear as thunder.
Explain why holes are made in banners and hoardings.
Banners and hoardings are large flat sheets that face strong wind, especially during storms. If the sheet were completely solid, wind blowing against it would be entirely blocked, building up a large pressure difference between the front (windward) and back (leeward) faces. This high force, acting on a large area, generates an enormous total force (Force = Pressure × Area) that can easily tear the banner or even pull down the entire hoarding structure.
By making holes in the banner, wind is allowed to pass through these openings instead of being completely blocked. This reduces the pressure difference between the two faces of the banner, lowering the net force exerted by the wind on the structure — exactly the same reason doors and windows of a house should be kept open during a storm, so that the roof is not blown off.
Discover, Design, and Debate
Model answers and guidance for the open-ended activity and project questions.
Hold an 18 cm × 2 cm paper strip between your thumb and forefinger so it hangs freely. Predict what you will observe if you blow over the top of the paper.
Prediction: Instead of being pushed down, the free end of the paper strip will rise upward, towards the stream of air being blown over it.
Why this happens: Blowing air over the top surface of the paper makes the air above it move at high speed, while the air below the paper (in still room air) remains at normal, slower-moving conditions. As learnt in Activity 6.6, high-speed winds are accompanied by reduced air pressure. So the pressure above the paper drops, while the pressure below stays at the higher, normal atmospheric value. This pressure difference pushes the paper strip upward, from the high-pressure region below to the low-pressure region above — exactly the same principle that helps explain how a roof can be lifted off a house by fast winds, and is part of the explanation for how aircraft wings generate lift.
List three major cyclones that have occurred in India in the last 20 years, two major effects of destruction caused by each, the measures taken, and two suggestions for the local government.
• Widespread destruction of kutcha houses, power and communication networks in coastal Odisha.
• Severe damage to standing crops and the Puri town’s infrastructure.
Measures taken: Mass evacuation of over 1 million people to cyclone shelters before landfall, supported by early IMD warnings, kept the death toll very low for a storm of this intensity.
• Extensive flooding and embankment breaches in the Sundarbans region.
• Heavy loss of trees, electricity infrastructure, and agricultural land due to saltwater intrusion.
Measures taken: Pre-emptive evacuation of lakhs of residents to shelters, deployment of NDRF teams, and post-cyclone relief and embankment-repair operations.
• Damage to power lines, uprooted trees, and disruption of fishing and port activities along the Kutch coast.
• Crop damage in coastal agricultural belts.
Measures taken: Large-scale evacuation of coastal villages, suspension of fishing operations and rail/flight services in advance, based on accurate satellite tracking by IMD.
Two suggestions for local governments:
- Build and regularly maintain more multi-purpose cyclone shelters along vulnerable coastlines, with stocked emergency kits, and conduct periodic mock-drills with coastal communities.
- Strengthen mangrove cover and embankments along the coast, since mangroves naturally reduce the impact of storm surges and act as a buffer against flooding.
Note: For a school project, replace these with the most recent verified data and local-government action reports for accuracy.
Collect data on the strength of thunderstorms across regions of India, compare findings, and identify which regions are more prone to thunderstorms. Give reasons.
India experiences some of its strongest, most frequent localised thunderstorms in the pre-monsoon season (March–May), particularly in:
- West Bengal, Bihar, and Jharkhand — where these storms are locally known as Kalboishakhi, often with violent winds and hail.
- Assam and the North-East — known as Bordoisila, marking the start of pre-monsoon agricultural activity.
- Kerala, Karnataka, and Tamil Nadu — experience “mango showers,” thunderstorms that help ripen mango crops; in Karnataka, local thunderstorms also support coffee plant growth.
Reason these regions are more prone: These areas combine high humidity (due to proximity to seas/rivers or dense vegetation) with intense surface heating before the monsoon arrives. This produces strong, fast-rising warm moist air — the essential ingredients (moisture + strong updrafts) needed for thunderstorm formation, as explained in Section 6.5.