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Pressure, Winds, Storms, and Cyclones

NCERT Class 8 · Science Based on NCERT Class 8 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores the concept of pressure, how it is exerted by solids, liquids, and gases, and how differences in air pressure lead to the formation of winds, storms, thunderstorms, and cyclones. It explains why overhead water tanks are placed at a height, why broad straps are more comfortable, and how high-speed winds can blow off roofs. The one big idea a student must walk away with is that air moves from a region of high pressure to a region of low pressure, and this movement, combined with heat and moisture, creates all weather phenomena from a gentle breeze to a destructive cyclone.
Key Concepts (16)
Pressure
Pressure is defined as the force per unit area. Pressure = Force / Area.
e.g. A bag with broad straps is more comfortable to carry than one with narrow straps because the same weight (force) is spread over a larger area, reducing pressure on the shoulders.
Pascal (Pa)
The SI unit of pressure is newton per metre squared (N/m²), which is also called a pascal (Pa).
e.g. If a force of 100 N is applied on an area of 2 m², the pressure is 50 N/m² or 50 Pa.
Atmospheric Pressure
The pressure exerted by the air around us is known as atmospheric pressure.
e.g. When a rubber sucker is pressed against a smooth surface, most of the air inside is pushed out, creating low pressure inside. The higher atmospheric pressure outside keeps the sucker stuck.
Millibar (mb)
A practical unit of air pressure, where 1 millibar (mb) is equal to 100 Pa.
e.g. Weather reports often give air pressure in millibars, e.g., 994 mb near the eye of a cyclone.
Hectopascal (hPa)
Another unit of air pressure equal to 100 Pa.
e.g. Atmospheric pressure is often expressed in hectopascals (hPa), which is numerically the same as millibars.
Atmosphere
The envelope of air surrounding the Earth is called the atmosphere.
e.g. The atmosphere contains nitrogen, oxygen, argon, carbon dioxide, and other gases, and extends up to many kilometres above the Earth's surface.
Wind
Wind is the movement of air from a region of high air pressure to a region of low air pressure.
e.g. During the day, land heats up faster than the sea, creating low pressure over land. Air from the high-pressure area over the sea blows towards the land, forming a sea breeze.
Storm
A storm is a strong wind accompanied by rain.
e.g. In hot, humid, and tropical regions like India, storms are more frequent, such as the Kalboishakhi in West Bengal before the monsoon.
Thunderstorm
A storm accompanied by lightning and thunder is called a thunderstorm.
e.g. A thunderstorm occurs when warm, moist air rises, cools, and forms clouds, and strong winds cause rubbing between water droplets and ice particles, leading to charge separation and lightning.
Lightning
A sudden flow of electric charges between opposite charges within a cloud, between clouds, or between a cloud and the ground, producing a bright flash of light.
e.g. When the build-up of charges in a cloud becomes very large, the insulating property of air breaks down, and a bright flash called lightning occurs.
Thunder
The loud sound produced when lightning rapidly heats the air around it, causing the air to expand.
e.g. After seeing a flash of lightning, we hear the loud sound of thunder because light travels faster than sound.
Cyclone
A large storm that forms over warm ocean waters, with a very low-pressure area at the centre (the eye) and high-speed winds revolving around it.
e.g. Cyclone Amphan in 2020 had peak wind speeds of 270 km/h and caused massive destruction in coastal areas.
Eye of the Cyclone
The region of lowest pressure at the centre of a cyclone, where the wind is calm.
e.g. In a cyclone, the eye is calm, but the surrounding region experiences strong winds and heavy rainfall.
Lightning Conductor
A metallic rod installed along the walls of a building, with one end pointed and higher than the highest point of the building, and the other end buried deep in the ground, providing an easy path for electric charges to transfer into the ground.
e.g. Lightning conductors protect buildings from damage by safely directing the lightning strike into the ground.
Sea Breeze
The wind that blows from the sea towards the land during the day, caused by the land heating faster than the sea, creating low pressure over land.
e.g. On a summer afternoon, cool air from the sea blows towards the hot land, giving relief from the heat.
Land Breeze
The wind that blows from the land towards the sea at night, caused by the water being warmer than the land, creating low pressure over the sea.
e.g. At night, the land cools faster than the sea, so the wind blows from the land to the sea.
How It Works
How pressure depends on area
  1. Force is applied on a surface.
  2. If the area over which the force acts is small, the pressure is high.
  3. If the area over which the force acts is large, the pressure is low.
  4. This is why it is easier to drive a nail using its pointed end (small area, high pressure) and easier to cut an apple with the sharp edge of a knife (small area, high pressure).
💡 Pressure is inversely proportional to area. A common mistake is to think that a larger area increases pressure — it actually decreases it.
How liquid pressure depends on height
  1. Take two pipes of different diameters but same height, with balloons attached at the bottom.
  2. Fill both pipes with water to the same level.
  3. Both balloons bulge to the same extent, showing that pressure depends on the height of the water column, not the weight or diameter.
  4. If more water is added to increase the height of the column, the bulge increases, showing that pressure increases with height.
💡 Liquid pressure depends only on the height of the liquid column, not on the shape or width of the container. Overhead tanks are placed at a height to increase water pressure in taps.
How air exerts pressure (atmospheric pressure)
  1. Air surrounds the Earth and exerts force on all objects.
  2. In the paper plate activity, lifting the plate covered with an unfolded chart paper requires more effort than with a folded chart paper, because the larger area experiences more force from air.
  3. This shows that air exerts pressure, known as atmospheric pressure.
  4. A rubber sucker sticks to a smooth surface because the air pressure inside is reduced, and the higher atmospheric pressure outside holds it in place.
💡 We are not crushed by atmospheric pressure because the pressure inside our bodies is equal to the atmospheric pressure, balancing the external force.
How winds form (air moves from high to low pressure)
  1. Take two balloons — one inflated (high pressure) and one uninflated (low pressure).
  2. Connect them with a straw.
  3. Air moves from the inflated balloon (high pressure) to the uninflated balloon (low pressure).
  4. The flow stops when the pressure in both balloons becomes equal.
  5. This shows that air moves from a region of high pressure to a region of low pressure, which is how winds form.
💡 Wind speed is higher if the pressure difference is higher. The direction of wind is always from high pressure to low pressure.
How high-speed winds lower air pressure
  1. Hang two inflated balloons close to each other.
  2. Blow air into the narrow space between them.
  3. The balloons move towards each other.
  4. This happens because the fast-moving air between them creates a low-pressure area, and the higher pressure outside pushes them together.
💡 High-speed winds are accompanied by reduced air pressure. This is why roofs can be blown off during storms — the pressure above the roof is lower than the pressure inside the house.
How a thunderstorm forms
  1. Land gets heated, causing warm and moist air to rise, creating a low-pressure area.
  2. Cooler air from surrounding high-pressure areas flows in, gets heated, and rises.
  3. The rising air expands, cools, and moisture condenses to form clouds.
  4. Water droplets merge to form heavier drops, which fall as rain, hail, or snow.
  5. Strong winds blowing upwards and downwards cause rubbing between water droplets and ice particles, creating static electric charges.
💡 Thunderstorms require moisture and strong winds. They are more frequent in hot, humid, tropical regions like India.
How lightning occurs
  1. Strong winds cause rubbing between water droplets and ice particles in clouds, creating static electric charges.
  2. Positively charged lighter ice particles move to the upper part of the cloud.
  3. Negatively charged heavier water droplets move to the lower part of the cloud.
  4. The negatively charged lower part of the cloud induces a positive charge on the ground and nearby objects.
  5. When the charge build-up becomes very large, the insulating property of air breaks down.
  6. A sudden flow of charges occurs, producing a bright flash of light called lightning.
  7. Lightning rapidly heats the air, causing it to expand and produce a loud sound called thunder.
💡 Lightning can occur within a cloud, between clouds, or between a cloud and the ground. During lightning, stay away from tall objects, avoid water, and do not lie flat on the ground.
How a cyclone forms
  1. Warm ocean water heats the air above it, causing warm and moist air to rise.
  2. As the moist air rises, water vapour condenses into raindrops, releasing heat into the atmosphere.
  3. This heat further warms the ascending air, causing it to rise even more, creating an even lower pressure.
  4. Air from surrounding regions rushes in and also starts rising.
  5. Earth's rotation causes the moving air to spin.
  6. This cycle repeats, creating a very low-pressure area with high-speed winds revolving around it — a cyclone.
💡 The centre of a cyclone is called the eye, where the wind is calm. Cyclones lose strength once they reach land because the source of moist air is cut off.
Exam Traps — Don't Make These Mistakes
⚠ Confusing force and pressure
✗ Pressure is the force acting on an object.
→ Pressure is not just force; it is force per unit area. A large force spread over a large area can produce low pressure, while a small force on a tiny area can produce high pressure.
✓ Pressure is defined as the force per unit area acting perpendicular to a surface.
⚠ Thinking liquid pressure depends on the amount of water
✗ The pressure at the bottom of a wide pipe is less than at the bottom of a narrow pipe because the wide pipe has more water.
→ The activity with two pipes of different diameters shows that the bulge in the balloons is the same for the same height of water column, even though the weight of water is different. Pressure depends on the height of the liquid column, not the volume or weight.
✓ The pressure exerted by a liquid depends on the height of its column, not on the shape or width of the container.
⚠ Misunderstanding why we are not crushed by atmospheric pressure
✗ We are not crushed by atmospheric pressure because air is light.
→ Atmospheric pressure is actually very large — the force on an area of 15 cm × 15 cm is equal to the weight of a 225 kg object. We are not crushed because the pressure inside our bodies is equal to the atmospheric pressure, balancing it.
✓ We are not crushed by atmospheric pressure because the pressure inside our bodies is also equal to the atmospheric pressure, balancing the pressure exerted from outside.
⚠ Thinking the eye of a cyclone has the strongest winds
✗ The eye of a cyclone has the strongest winds and heaviest rainfall.
→ The eye of a cyclone is the region of lowest pressure, but it is actually calm with no wind. The strongest winds and heaviest rainfall occur in the region surrounding the eye.
✓ The eye of a cyclone is the region of lowest pressure at the centre, where the wind is calm. The surrounding region experiences strong winds and heavy rainfall.
⚠ Confusing sea breeze and land breeze directions
✗ Sea breeze blows from land to sea during the day.
→ During the day, land heats faster than water, creating low pressure over land. Air from the high-pressure area over the sea blows towards the land, so sea breeze blows from sea to land. At night, the opposite happens.
✓ During the day, sea breeze blows from the sea to the land. At night, land breeze blows from the land to the sea.
⚠ Thinking lightning only occurs between clouds and the ground
✗ Lightning only occurs when a cloud discharges to the ground.
→ Lightning can occur within a single cloud, between two clouds, or between a cloud and the ground. All three types are possible.
✓ Lightning can occur as opposite charges collide within a cloud, between clouds, or between clouds and the ground.
Compare & Contrast
Time of occurrence and direction of wind
Sea Breeze
Land Breeze
Sea breeze occurs during the day when wind blows from the sea to the land because land heats faster. Land breeze occurs at night when wind blows from the land to the sea because water cools slower.
Scale and location of formation
Thunderstorm
Cyclone
A thunderstorm is a localised storm with lightning and thunder, often forming over land. A cyclone is a large-scale storm that forms over warm ocean waters, with a very low-pressure centre (eye) and high-speed revolving winds.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (3 marks), 1 Long Answer (5 marks) — total approximately 10-12 marks.
Most asked: Definition and formula of pressure (Force/Area) with numerical problems · Factors affecting liquid pressure (height of column) · Atmospheric pressure and its effects (sucker, balancing inside body) · Formation of wind (air moves from high to low pressure) · High-speed winds and reduced pressure (roof blowing off) · Formation of thunderstorms and lightning · Formation of cyclones and the eye of the cyclone · Safety measures during lightning and cyclones
Don't skip: Numerical problems on pressure (Force/Area) — often 3 marks; Explain the formation of a cyclone with a diagram — often 5 marks; Differentiate between sea breeze and land breeze — often 3 marks; Explain why a roof is blown off during high-speed winds — often 3 marks
One-Line Revision (read aloud before the exam)
  1. Pressure is defined as force per unit area, and its SI unit is the pascal (Pa), which is N/m².
  2. A bag with broad straps is more comfortable because the same force is spread over a larger area, reducing pressure.
  3. Liquid pressure depends on the height of the liquid column, not on the shape or width of the container.
  4. Overhead water tanks are placed at a height to increase water pressure in taps.
  5. Atmospheric pressure is the pressure exerted by the air around us.
  6. We are not crushed by atmospheric pressure because the pressure inside our bodies balances it.
  7. Air moves from a region of high pressure to a region of low pressure, forming wind.
  8. High-speed winds are accompanied by reduced air pressure, which can blow off roofs.
  9. A thunderstorm forms when warm, moist air rises, cools, and forms clouds, with strong winds creating electric charges.
  10. Lightning is a sudden flow of electric charges between opposite charges in clouds or between a cloud and the ground.
  11. During lightning, stay away from tall objects, avoid water, and crouch low in an open area.
  12. A cyclone forms over warm ocean waters when rising moist air releases heat, creating a very low-pressure area with spinning winds.
  13. The eye of a cyclone is the calm centre with the lowest pressure, surrounded by strong winds and heavy rain.
  14. Cyclones lose strength when they reach land because the source of moist air is cut off.
  15. A lightning conductor protects buildings by providing an easy path for electric charges to go into the ground.
  16. Sea breeze blows from sea to land during the day; land breeze blows from land to sea at night.

MCQs — 10 Questions with Answers

Q1. A force of 50 N is applied over an area of 2 m². What is the pressure exerted?

  • A. 25 N/m² (Pa) ✓
  • B. 100 N/m² (Pa)
  • C. 2 N/m² (Pa)
  • D. 0.04 N/m² (Pa)

Answer: A — Using Pressure = Force ÷ Area = 50 ÷ 2 = 25 N/m².

Q2. Why is it easier to cut an apple with a sharp knife than a blunt knife?

  • A. The sharp knife is heavier
  • B. The sharp edge has a smaller area, creating higher pressure ✓
  • C. The blunt knife is harder
  • D. The apple is softer at the edges

Answer: B — A sharp edge concentrates force over a smaller area, producing higher pressure that cuts more easily.

Q3. The SI unit of pressure is:

  • A. Newton (N)
  • B. Metre² (m²)
  • C. Pascal (Pa) or N/m² ✓
  • D. Kilogram (kg)

Answer: C — Pascal (Pa), equivalent to N/m², is the SI unit of pressure.

Q4. In Activity 6.1, when water height increases in the pipe, what happens to the balloon?

  • A. It shrinks
  • B. It bulges more ✓
  • C. It remains unchanged
  • D. It twists

Answer: B — Greater water column height produces greater pressure, causing the balloon to bulge more.

Q5. Megha's broad-strapped bag and Pawan's narrow-strapped bag weigh the same. Why is Pawan's bag more uncomfortable?

  • A. The narrow straps have more weight
  • B. Narrow straps concentrate the same weight over a smaller area, increasing pressure ✓
  • C. The bag material is different
  • D. Gravity affects narrow straps differently

Answer: B — Same force (weight) over a smaller area (narrow straps) creates higher pressure, causing discomfort.

Q6. A person carries a heavy basket on their head by placing a round cloth underneath. This reduces discomfort because:

  • A. The cloth absorbs the weight completely
  • B. It increases the contact area, reducing pressure on the head ✓
  • C. It makes the basket lighter
  • D. It changes the direction of force

Answer: B — The cloth spreads the weight over a larger area of the head, reducing pressure and discomfort.

Q7. Why are overhead water tanks always placed on the rooftops of buildings?

  • A. To save space on the ground
  • B. To keep water cool
  • C. To increase water column height and thus pressure in the taps below ✓
  • D. To prevent water contamination only

Answer: C — Greater height of the water column increases pressure, ensuring a strong stream of water from taps.

Q8. In Activity 6.2, water flows out from the holes in the bottle sides because:

  • A. Holes create a vacuum inside
  • B. Gravity pulls water downward only
  • C. Liquids exert pressure in all directions, including on the sides ✓
  • D. Air pressure forces water out

Answer: C — Liquids exert pressure not just downward but in all directions, pushing water through the side holes.

Q9. In Activity 6.3, it was harder to lift the paper plate when covered with unfolded paper than folded paper because:

  • A. Unfolded paper is thicker
  • B. Unfolded paper covers more area, so atmospheric pressure acts over a larger area, creating more force ✓
  • C. The stick became weaker
  • D. Unfolded paper is heavier

Answer: B — Larger surface area of unfolded paper means atmospheric pressure acts over more area, increasing the downward force.

Q10. Why is the base of a dam constructed broader than its top?

  • A. To collect more water
  • B. For aesthetic reasons
  • C. To withstand the large horizontal water pressure that increases near the bottom ✓
  • D. To allow fish to live at the base

Answer: C — Water pressure increases with depth; a broad base provides more surface area to resist the high horizontal pressure at the bottom.

Flashcards

Define pressure and give its formula.

Pressure is force per unit area; Formula: Pressure = Force ÷ Area.

What is the SI unit of pressure?

The SI unit of pressure is newton per metre² (N/m²), also called pascal (Pa).

Why do broad bag straps feel more comfortable than narrow straps?

Broad straps spread the weight over a larger area, reducing pressure on shoulders.

In Activity 6.1, why did both balloons bulge equally despite different pipe diameters?

Both pipes had the same water column height, so pressure at the bottom was equal.

Why are overhead water tanks placed at a height?

Height increases water column pressure, creating a stronger stream of water from taps.

In which directions does liquid exert pressure inside a container?

Liquids exert pressure in all directions—downward at the bottom and horizontally on the sides.

Why is a dam's base broader than its top?

The broad base withstands the large horizontal water pressure that increases near the bottom.

What did Activity 6.3 prove about air?

Air exerts force and pressure; this force increases with the area of surface contact.

Which requires more effort to lift: a paper plate covered with folded or unfolded paper?

Unfolded paper requires more effort because it covers a larger area, increasing atmospheric pressure effect.

What is the atmosphere?

The atmosphere is the envelope of air surrounding Earth containing nitrogen, oxygen, argon, carbon dioxide, and other gases.

Important Board Questions

Define pressure. Write its formula and SI unit. [1 mark]

Pressure is force per unit area; Formula: P = F/A; SI unit: N/m² or Pa (pascal).

Why do narrow bag straps hurt our shoulders more than broad straps, even though both bags have the same weight? Explain using the concept of pressure. [2 marks]

Narrow straps concentrate weight over smaller area → higher pressure → discomfort. Broad straps spread weight over larger area → lower pressure → comfort.

Describe Activity 6.1 with observations. What does it prove about liquid pressure? Why did both balloons bulge equally despite different pipe diameters? [3 marks]

Two pipes of different diameters filled to same water height; both balloons bulged equally because pressure depends on height of liquid column, not its weight or volume.

Draw and label a diagram of Activity 6.3 setup (inverted paper plate with unfolded chart paper). Explain why more effort is needed to lift the plate when covered with unfolded paper. What does this prove about air pressure? [5 marks]

Diagram: inverted plate, stick, unfolded paper covering it with air above. Unfolded paper has larger area → atmospheric pressure force acts over more surface → greater total downward force → harder to lift. Proves air exerts pressure.

Assertion-Reason Questions for Pressure, Winds, Storms, and Cyclones

Q1. Assertion (A):

Overhead water tanks are always placed at a height.

Reason (R):

Placing tanks at a height increases the water column height, which increases pressure at the taps, ensuring a strong flow of water.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. The reason correctly explains why tanks are placed high—to increase water pressure through greater column height, enabling water to flow forcefully from taps. This is the fundamental principle behind overhead tank placement.

Q2. Assertion (A):

A sharp knife cuts an apple more easily than a blunt knife when the same force is applied.

Reason (R):

The sharp knife has a larger contact area with the apple, which allows the force to penetrate the surface more easily.

Answer: C — A is true but R is false — The assertion is true—sharp knives do cut more easily. However, the reason is false. A sharp knife has a SMALLER contact area, not larger, which concentrates force into higher pressure. The reason misrepresents the mechanism, so it cannot be the correct explanation.

Q3. Assertion (A):

The pressure exerted by water at the bottom of a pipe depends on the diameter of the pipe.

Reason (R):

A larger diameter means more water volume, which increases the weight pushing down and thus increases pressure.

Answer: D — A is false but R is true — Both the assertion and reason are false. Activity 6.1 demonstrates that pressure depends only on the water column height, not the pipe's diameter. Even though different diameters have different water volumes, the pressure at the same height is identical. Students often confuse volume with pressure.

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Case-Based Questions (CBQ) for Pressure, Winds, Storms, and Cyclones

Case Study:

Rajesh's family recently installed a new overhead water tank on their four-story building. The tank is located on the roof, approximately 12 meters above the ground floor. Rajesh notices that the water pressure in taps on the ground floor is much stronger than on the third floor. He wonders why this difference exists. His sister explains that the height difference between the tank and each floor affects the water pressure at those taps. The tank supplies water to all floors, but the pressure varies depending on the distance from the tank.

Q1. Why does the ground floor receive stronger water pressure than the third floor?

  • A. The ground floor has thicker pipes that allow more water to pass
  • B. The water column height is greater from the tank to the ground floor than to the third floor ✓
  • C. The tank is designed to send more water to the ground floor
  • D. The ground floor taps are newer and have better mechanisms

Answer: B

Q2. If the tank is filled to a height of 2 meters, and the ground floor is 12 meters below the tank, what is the approximate height of the water column creating pressure at the ground floor tap?

  • A. 2 meters
  • B. 10 meters
  • C. 12 meters
  • D. 14 meters ✓

Answer: D

Q3. What would happen if the tank were moved from the roof to the third floor (reducing the height difference)?

  • A. Water pressure would increase throughout the building
  • B. Water pressure would decrease on all floors below the tank ✓
  • C. The third floor would have the strongest water pressure
  • D. There would be no change in water pressure

Answer: B

Q4. To improve water pressure on the fourth floor (the highest floor), the best solution would be to

  • A. Install the water tank higher above the fourth floor to increase the water column height ✓
  • B. Use a thicker pipe on the fourth floor
  • C. Use a wider tap opening on the fourth floor
  • D. Add more water to the tank

Answer: A

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