Chapter 6: Pressure, Winds, Storms, and Cyclones
📚 Chapter Index
🎯 Learning Objectives
- Define pressure and state its formula and SI unit
- Explain how liquids and gases exert pressure in all directions
- Describe atmospheric pressure and its magnitude
- Explain how differences in air pressure cause wind formation
- Understand how high-speed winds lower air pressure
- Explain the formation of storms, thunderstorms, and lightning
- Describe the formation, structure, and effects of cyclones
- Identify safety measures during lightning and cyclones
🔍 Probe and Ponder
- Why are winds stronger on some days than on others?
- Why are water tanks usually placed at a height?
- Can air pressure really crush us?
- What causes storms and cyclones? If the Earth stopped rotating, would cyclones still form?
- Wind exerts force on fallen leaves, trees, doors, windows, and clothes — the force exerted by wind creates wind pressure
- In this chapter, we explore the relationship between force and pressure, and understand how they shape powerful natural events like thunderstorms and cyclones
6.1 Pressure
The Megha and Pawan Story
Fig. 6.1 — Megha and Pawan carrying their bags. Pawan’s narrow straps hurt his shoulders; Megha’s broad straps are comfortable — same weight, different pressure!
- Megha and her brother Pawan carry identical bags to a picnic — both bags are equally heavy
- Pawan’s bag hurts his shoulders; Megha’s does not — the difference: Pawan’s bag has narrow straps, Megha’s has broad straps
- The weight of the bag with narrow straps acts on a smaller area — more pressure on Pawan’s shoulders
- The weight of the bag with broad straps is spread over a larger area — less pressure on Megha’s shoulders
- Since the area over which force acts is involved, we define a quantity called pressure — the force per unit area
Pressure = Force ÷ Area
- We consider only those forces which act perpendicular to the surface on which pressure is computed
Fig. 6.2 — Buckets with broad and narrow handles. Broad handle reduces pressure on fingers.
Fig. 6.3 — A round piece of cloth is placed under the load to increase the area and reduce pressure.
- It is easier to lift a water-filled bucket with a broad handle than a narrow handle — broad handle increases area, reducing pressure on fingers
- People carrying loads on their heads place a round cloth under the load — to reduce pressure by increasing area
SI Unit of Pressure
- SI unit of force = newton (N); SI unit of area = metre² (m²)
- Therefore, SI unit of pressure = newton/metre² (N/m²), also called a pascal (Pa)
📐 Worked Example
If a force of 100 N is applied on a cardboard of area 2 m²:
Show Solution
Pressure = Force ÷ Area = 100 N ÷ 2 m² = 50 N/m² (50 Pa)
⚠️ Safety First
The activities listed in Table 6.1 should be conducted under the supervision of an adult.
Table 6.1 — Record Your Observations
| Activity | Modes of Action | Easy or Difficult? Give Reasons. |
|---|---|---|
| Driving an iron nail | By the head (large area) vs. by the pointed end (small area) | Easier by pointed end — smaller area means greater pressure with same force |
| Cutting an apple with a knife | Using the sharp edge (small area) vs. blunt edge (large area) | Easier with sharp edge — smaller area creates greater pressure, cutting easily |
Conclusion: When area is smaller, pressure is higher for the same force — making certain tasks easier.
Do Liquids Also Exert Pressure?
Fig. 6.4 — Overhead water tank placed at a height to increase water pressure in taps below.
🔬 Activity 6.1 — Let Us Try and Find Out
Fig. 6.5 — Equal heights of water columns produce the same bulge in balloons, despite different pipe diameters.
- Two transparent pipes of the same length but different diameters — rubber balloons attached at bottom of each
- Fill both pipes with water to the same level
- Observation: Both balloons bulge to the same extent
- Inference: The weight of water is different (different diameters), but the bulge is equal → it is the pressure of the water column, not the weight, that causes the bulge
Fig. 6.6 — Higher heights of water column produce a bigger bulge of the balloon.
- Adding more water (increasing column height) causes the balloon to bulge more
- Conclusion: The pressure exerted by a liquid depends on the height of its column
- This is why overhead tanks are placed at a height — greater height → greater pressure → good stream of water from taps
🔬 Activity 6.2 — Let Us Find Out
Fig. 6.7 — Liquid exerts pressure on the walls of the container — water shoots out in all directions from the side holes.
- Make four small holes near the bottom around the sides of a plastic bottle (at the same height) — fill with water, remove tape
- Observation: Water flows out through holes on the sides of the bottle
- Conclusion: Liquids exert pressure in all directions — at the bottom, on the sides, and in every direction
💡 Ever Heard Of — Why Dams Have Broader Bases?
Fig. 6.8 — The base of a dam is made broader to withstand the very large horizontal water pressure near the bottom.
- The base of a dam is much broader than the top
- Water exerts horizontal pressure on the walls and vertical pressure on the floor
- The horizontal pressure is very large near the bottom
- To withstand this large pressure, the base is made broader
6.2 Pressure Exerted by Air
- Air is all around us — the envelope of air surrounding the Earth is called the atmosphere
- Contains nitrogen, oxygen, argon, carbon dioxide, and other gases in small quantities
- The atmosphere extends up to many kilometres above the surface of the Earth
🔬 Activity 6.3 — Let Us Explore (Does Air Exert Pressure?)
Fig. 6.9 — Inverted paper plate with chart paper covering: with larger unfolded sheet (more area), more effort is needed to lift — proving air exerts pressure proportional to area.
- With folded chart paper (smaller area) — less effort needed to lift paper plate
- With unfolded chart paper (larger area) — more effort needed to lift paper plate
- The weight of the sheet has not changed — only the area increased
- Inference: Air exerts force on the covering sheet — this force increases with increasing area
- Conclusion: Air exerts pressure on all objects. Pressure exerted by air around us = atmospheric pressure
- Air exerts pressure in all directions — that is why an inflated balloon expands in all directions
🔬 Activity 6.4 — Let Us Perform (Magnitude of Atmospheric Pressure)
Fig. 6.11 — A rubber sucker pressed on a smooth surface: air pushed out reduces internal pressure; higher atmospheric pressure outside holds it firmly in place.
- Press a rubber sucker firmly against a smooth flat surface — it sticks; difficult to pull off
- When pressed, most air inside is pushed out → air pressure inside is reduced
- Higher atmospheric pressure outside holds the sucker against the surface
- To pull it off, force must overcome the pressure difference
How Large is Atmospheric Pressure?
- Force exerted by atmospheric air column over an area of 15 cm × 15 cm ≈ weight of 225 kg (2250 N)
- We are not crushed because the pressure inside our body equals atmospheric pressure — the two balance each other
- Internal pressure is caused by movement of fluids and gases in tissues and organs of the body
📈 A Step Further — Units of Air Pressure
- SI unit: N/m² = pascal (Pa)
- Practical unit: millibar (mb) = 100 Pa
- Also expressed in hectopascal (hPa) = 100 Pa
🧠 Check Your Understanding — Pressure (6.1 & 6.2)
LOTS (Lower Order Thinking)
1. Define pressure. Write its formula and SI unit.
Show Answer
Pressure is defined as the force per unit area, where the force acts perpendicular to the surface.
Formula: Pressure = Force ÷ Area
SI unit: newton/metre² (N/m²), also called pascal (Pa)
Formula: Pressure = Force ÷ Area
SI unit: newton/metre² (N/m²), also called pascal (Pa)
2. Why are overhead water tanks placed at a height?
Show Answer
Overhead tanks are placed at a height because liquid pressure depends on the height of its column. Greater height means greater pressure at the bottom, pushing water through pipes with greater force and resulting in a good stream of water from taps. A friend on the first floor receives more water pressure than someone on the second floor, since the water column above them is taller.
Medium
3. In Activity 6.1, the two pipes had different diameters but the balloons bulged equally. What does this prove about liquid pressure?
Show Answer
This proves that liquid pressure does not depend on the weight or volume of liquid — it depends only on the height of the liquid column. Both pipes had water filled to the same height, so the pressure at the bottom was the same in both, causing equal bulges. A narrow pipe and a broad pipe filled to the same height exert the same pressure at the bottom.
4. Why does a sucker stick to a smooth surface but not to a rough surface?
Show Answer
On a smooth surface, pressing the sucker expels most air inside, creating a partial vacuum (low pressure). Higher atmospheric pressure outside keeps the sucker pressed tightly. On a rough surface, uneven texture allows air to seep back under the sucker — internal pressure stays close to atmospheric pressure — so the pressure difference is too small to hold the sucker in place.
HOTS (Higher Order Thinking)
5. The base of a dam is broader than its top. Using the concept of liquid pressure, explain why this design is necessary.
Show Answer
Water pressure increases with depth — the pressure near the bottom of the dam is very large. A broader base provides a larger area to resist this enormous horizontal pressure (Pressure = Force ÷ Area — for the same force, larger area means less stress on the structure). If the base were as narrow as the top, the walls near the bottom would experience enormous pressure per unit area, likely causing the dam to crack or collapse, leading to catastrophic flooding.
6. We are not crushed by atmospheric pressure even though it equals the weight of 225 kg on a 15×15 cm area. Explain why. What would happen if body pressure suddenly dropped to zero?
Show Answer
We are not crushed because the pressure inside our body is equal to atmospheric pressure — the internal and external pressures balance each other perfectly, giving zero net force. If internal body pressure suddenly dropped to zero, the atmospheric pressure (~2250 N on just a 15×15 cm area) would exert an enormous inward force with nothing to balance it — causing blood vessels to collapse, organs to fail, and the body to be crushed. This is why astronauts wear pressurised suits in the vacuum of space.
📐 Numerical Practice Questions
🔢 Numerical Questions
Q1 (Basic)
A force of 200 N is applied on a surface of area 4 m². Calculate the pressure exerted.
Show Answer
Pressure = Force ÷ Area = 200 ÷ 4 = 50 Pa (50 N/m²)
Q2 (Medium)
A girl weighing 500 N stands on her toes. Total toe area = 0.005 m². Calculate pressure on toes. Compare with standing flat-footed (area = 0.04 m²).
Show Answer
On toes: Pressure = 500 ÷ 0.005 = 1,00,000 Pa
Flat-footed: Pressure = 500 ÷ 0.04 = 12,500 Pa
Standing on toes exerts 8× more pressure — this is why high heels leave marks on soft floors that heavier flat-shoe wearers don’t.
Flat-footed: Pressure = 500 ÷ 0.04 = 12,500 Pa
Standing on toes exerts 8× more pressure — this is why high heels leave marks on soft floors that heavier flat-shoe wearers don’t.
Q3 (HOTS — NCERT Q4)
An elephant stands on four feet. Area of one foot = 0.25 m². Weight = 20,000 N. Calculate pressure exerted on the ground.
Show Answer
Total area = 4 × 0.25 = 1 m²
Pressure = 20,000 ÷ 1 = 20,000 Pa (20,000 N/m²)
Elephants don’t sink into soft ground as much as expected because their large feet distribute the enormous weight over a large area, reducing pressure.
Pressure = 20,000 ÷ 1 = 20,000 Pa (20,000 N/m²)
Elephants don’t sink into soft ground as much as expected because their large feet distribute the enormous weight over a large area, reducing pressure.
6.3 Formation of Wind
- On some days, wind blows strongly; on other days, it is calm
- Recall: air escapes from an inflated balloon when opened, and from a punctured bicycle tube — in both cases, air moves from high pressure to low pressure
🔬 Activity 6.5 — Let Us Observe (Air Moves from High to Low Pressure)
Fig. 6.12 — Air moves from the inflated balloon (high pressure) to the uninflated balloon (low pressure) through the straw, until pressure equalises.
- One end of straw inside an uninflated balloon; other end inside an inflated balloon
- Observation: Air moves from the inflated balloon (high pressure) to the uninflated balloon (low pressure) — both eventually reach the same size; flow stops when pressures are equal
- Conclusion: Air moves from a region of high pressure to a region of low pressure
How Wind Forms — Sea Breeze and Land Breeze
- During the day: land heats faster → warm air above land rises → low pressure over land → cool sea air rushes in = sea breeze
- At night: water is warmer than land → warm air above sea rises → low pressure over sea → land air blows towards sea = land breeze
- Land breeze and sea breeze are due to pressure differences over land and sea
- The speed of air is higher when the pressure difference is higher
Key Principle:
- Warm air rises → creates low pressure area
- Cool air from surrounding high pressure areas moves in → forms wind
- The greater the pressure difference, the stronger the wind
6.4 High-Speed Winds Result in Lowering of Air Pressure
🔬 Activity 6.6 — Let Us Observe (High Speed Winds & Pressure)
Fig. 6.13 — Blowing between two hanging balloons creates a low-pressure area between them; surrounding higher pressure pushes them together. Blowing harder brings them closer faster.
- Inflate two balloons; hang from a stick with a gap of 6–10 cm; blow air into the narrow space between them
- Observation: The balloons move towards each other; blowing harder = approach faster
- Explanation: Blowing creates a low-pressure area between balloons; higher surrounding pressure pushes them together
- Conclusion: High-speed winds are accompanied by reduced air pressure
Why High-Speed Winds Can Blow Off Roofs
Fig. 6.14 — (a) Roof blown away when high-speed winds create low pressure above and doors/windows are closed; (b) Roof intact when windows are open, equalising pressure.
- High-speed winds over houses create a low-pressure area above roofs
- Air pressure above roofs is lower than pressure below — if pressure difference is large and roof is weak, roof gets blown away
- Safety measure: Keep doors and windows open during storms — this equalises pressure inside and outside, preventing roof from blowing off
🧠 Check Your Understanding — 6.3 & 6.4
LOTS (Lower Order Thinking)
1. In which direction does air move — from high to low pressure, or from low to high pressure?
Show Answer
Air moves from a region of high air pressure to a region of low air pressure. Demonstrated in Activity 6.5 where air moved from the inflated balloon (high pressure) to the uninflated balloon (low pressure) until pressures became equal.
2. What safety measure should you take with doors and windows during a storm? Why?
Show Answer
Keep doors and windows open. High-speed winds create a low-pressure area above the roof. If windows are closed, higher pressure inside the house pushes the roof upward, while lower pressure above pulls it up too — the roof can be blown off. When windows are open, wind passes through the house, reducing the pressure difference and protecting the roof.
Medium
3. Explain how sea breeze and land breeze form using the concept of air pressure differences.
Show Answer
Sea breeze (daytime): Land heats faster than water → warm air over land rises → low pressure over land. Cooler high-pressure sea air blows towards land → sea breeze.
Land breeze (nighttime): Water retains heat; warmer than land at night → warm air over sea rises → low pressure over sea. Cooler high-pressure land air blows towards sea → land breeze.
Land breeze (nighttime): Water retains heat; warmer than land at night → warm air over sea rises → low pressure over sea. Cooler high-pressure land air blows towards sea → land breeze.
HOTS (Higher Order Thinking)
4. Using Activity 6.6’s principle, explain why an aircraft wing generates lift.
Show Answer
An aircraft wing is curved on top and flatter below — air flowing over the top surface travels a longer distance and thus moves faster. According to Activity 6.6’s principle, high-speed air creates lower pressure. So pressure above the wing is lower than below. This pressure difference creates an upward force called lift — the same principle as the balloons being pushed together by surrounding higher-pressure air when low-pressure air was created between them.
6.5 Storms, Thunderstorms, and Lightning
How a Storm Forms
Fig. 6.15 — Strong winds going up and down inside a storm cloud facilitate rubbing between water droplets and ice particles, generating electric charges.
⛈️ Storm Formation — Step by Step
1
Land gets heated → warm and moist air rises → creates a low pressure area
2
Cooler air from surrounding high-pressure areas flows in → gets heated and rises → continuous wind circulation
3
Rising air expands, cools → moisture condenses to form water droplets → clouds form
4
Water droplets merge to form heavier drops → come down as rain, hail, or snow
5
Strong winds + rain = a storm. In hot, humid, tropical regions like India, storms are more frequent
6
Under certain conditions, warm air rises to great heights → low temperature converts water droplets into ice particles
How Charges Develop in Clouds — Leading to Lightning
Fig. 6.16 — Lightning: positive charges accumulate at the top of the cloud (lighter ice particles) and negative charges at the bottom (heavier water droplets). A sudden discharge produces the bright flash.
- Strong winds blowing upwards and downwards cause water droplets and ice particles to rub → static electric charges develop in clouds
- Positively charged lighter ice particles → upper part of cloud
- Negatively charged heavier water droplets → lower part of cloud → this is charge separation
- Negatively charged lower cloud makes the ground and nearby objects (trees, buildings) positively charged
- Normally, air is an electrical insulator — but when charge build-up becomes enormous, insulating property breaks down
- A sudden flow of charges → bright flash of light = lightning
- Lightning can occur within a cloud, between clouds, or between cloud and ground
- Lightning heats surrounding air → rapid expansion → loud sound = thunder
- A storm with lightning and thunder = thunderstorm
📈 A Step Further — Regional Names for Thunderstorms
- Kalboishakhi — West Bengal, Bihar, Jharkhand
- Bordoisila — Assam (both before monsoon, help kharif crops)
- Mango showers — Kerala, Karnataka, Tamil Nadu (support mango ripening)
- Local thunderstorms in Karnataka help coffee plant growth
Safety Measures During Lightning
Fig. 6.17 — Safe position during lightning: crouch low in an open area, minimise ground contact, do not lie flat.
Fig. 6.18 — A lightning conductor provides an easy path for electric charges to safely flow into the ground, protecting the building.
⚡ What To Do During Lightning
- Stay away from tall objects (trees, buildings, poles)
- Find a low-lying open area and crouch down
- Minimise contact with the ground — do not lie down flat
- Avoid using an umbrella with a metallic rod
- If in water, get out immediately
- If inside a bus or car, you are comparatively safer
💡 Ever Heard Of — Lightning Conductor
- A metallic rod installed along building walls — pointed top higher than the building’s highest point, other end buried in the ground
- Provides an easy path for electric charges to flow safely into the ground
🧠 Check Your Understanding — 6.5 Storms, Thunderstorms & Lightning
LOTS (Lower Order Thinking)
1. What is a thunderstorm? What are the important requirements for its formation?
Show Answer
A thunderstorm is a storm accompanied by lightning and thunder. Important requirements: (1) Moisture — water vapour in warm air that condenses to form droplets and ice particles; (2) Strong winds — blowing upwards and downwards, facilitating rubbing between water droplets and ice particles to generate electric charges in clouds.
2. Explain the process that causes lightning.
Show Answer
(1) Strong winds cause water droplets and ice particles to rub → static electric charges develop; (2) Positive charges (lighter ice) rise to upper cloud; negative charges (heavier water droplets) settle at lower cloud — charge separation; (3) Lower cloud makes the ground positively charged; (4) When charge build-up is enormous, the insulating property of air breaks down; (5) Sudden discharge between cloud and ground = bright flash = lightning; (6) Lightning heats surrounding air → rapid expansion = thunder.
Medium
3. Why is it safer to be inside a car during a lightning storm rather than standing under a tree?
Show Answer
A tree is a tall object that easily attracts lightning — if struck, the charge can travel to a person standing beneath it, causing death. A car‘s metal body forms a conducting cage (Faraday cage) — lightning striking the car travels through the metal exterior into the ground without reaching passengers inside.
HOTS (Higher Order Thinking)
4. Why are holes made in banners and hoardings?
Show Answer
Holes are made to reduce wind pressure. Without holes, high-speed wind exerts a large force on the entire solid surface area, potentially tearing the banner or toppling the structure. With holes, wind passes through — effectively reducing the area resisting the wind. Since Pressure = Force ÷ Area, smaller effective area means less total force. Same principle as keeping windows open during storms to reduce pressure difference on roofs.
5. Would lightning occur if air and clouds were good conductors of electricity?
Show Answer
No, lightning would not occur. Lightning depends on the build-up of enormous electric charges because air is normally an insulator that prevents charges from meeting. If air and clouds were good conductors, charges would flow continuously and gradually — never building up to the level required for the sudden, massive discharge that produces lightning.
6.6 Cyclone
🌀 What is a Cyclone?
- Cyclones are large storms that form over warm ocean waters
- The region of lowest pressure at the centre = eye of the cyclone — wind is calm at the eye
- The surrounding region experiences strong winds and heavy rainfall
- This spinning system of clouds, winds, and rain is called a cyclone
How a Cyclone Forms
Fig. 6.19 — Winds blowing from surrounding high-pressure areas (1008 mb) towards the central low-pressure area (994 mb) of a cyclone — Earth’s rotation causes the moving air to spin.
🌀 Cyclone Formation — Step by Step
1
Ocean water gets heated → warm and moist air above rises
2
Moist air rises → water vapour condenses to raindrops → heat released back into atmosphere
3
Released heat causes further warming of ascending air → air rises even higher → creates 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
Cycle repeats → very low-pressure area with high-speed spinning winds → this is a cyclone
7
Cyclone moves from ocean to land → generates higher wind speeds than regular thunderstorms
8
Cyclone reaches land → source of moist air cut off → cyclone gradually loses strength
Fig. 6.20 — A cyclone: the eye at the centre is calm, surrounded by a wall of high-speed spinning winds and spiral cloud bands. Once it moves over land, the moist ocean air supply is cut off and it weakens.
🌊 Destruction Caused by Cyclones
- Even as a cyclone loses strength over land, it leaves a trail of destruction taking months or years to repair
- Example: Amphan cyclone (2020) — peak wind speeds of 270 km/h
- Strong winds push ocean water towards shore → wall of water 3–12 metres high → floods coastal areas
- Heavy rainfall → rivers overflow and landslides
- Seawater inland → contaminates drinking water, damages farmland (salt makes soil less fertile)
- Roads blocked by fallen trees; power outages lasting days
🛡️ How to Protect Yourself During Cyclones
- Stay updated on alerts and warnings by the India Meteorological Department (IMD)
- Weather monitoring satellites can track cyclones and predict their path
- Keep an emergency kit ready if you live in a cyclone-prone area
- During a cyclone, move quickly to a nearby designated cyclone shelter
Let Us Wrap Up — Storm and Cyclone Formation
- Warm air rises → low-pressure area
- Cool air rushes in to fill the low-pressure area
- Warm air cools → water vapour condenses → clouds
- Bigger drops fall as rain, hail, or snow
- Strong winds (up and down) → charges develop in clouds
- Positive and negative charges meeting → lightning
- Under certain conditions, storms develop into cyclones
🧠 Check Your Understanding — 6.6 Cyclone
LOTS (Lower Order Thinking)
1. What is the ‘eye of the cyclone’? What is the weather like at the eye?
Show Answer
The eye of the cyclone is the region of lowest pressure at the centre of a cyclone. At the eye, the wind is surprisingly calm with no rain or strong wind. The region surrounding the eye experiences very strong winds and heavy rainfall.
2. Why does a cyclone lose its strength once it reaches land?
Show Answer
A cyclone loses strength once it reaches land because the source of warm, moist ocean air is cut off. The entire energy of a cyclone comes from heat released when water vapour condenses. Without warm moist ocean air, the rising air current weakens, pressure equalises, the spinning slows, and the cyclone gradually dissipates.
Medium
3. Explain how a storm becomes a cyclone.
Show Answer
Over warm ocean waters: (1) Warm moist ocean air rises rapidly → very low pressure area; (2) When water vapour condenses, heat released warms ascending air further → even lower pressure; (3) More air rushes in from surrounding high-pressure regions; (4) Earth’s rotation causes this air to spin; (5) Cycle intensifies — rising warm air, condensation, heat release, more air rushing in and spinning. Result: enormous rotating system with very low pressure at centre and extremely high-speed spinning winds = cyclone. Key differences from a regular storm: warm ocean as continuous energy source, and Earth’s rotation creating the spin.
HOTS (Higher Order Thinking)
4. If the Earth stopped rotating, would cyclones still form? Give a reasoned answer.
Show Answer
No, cyclones would not form. The chapter states that Earth’s rotation causes the moving air to spin — this spin is the defining characteristic of a cyclone. Without Earth’s rotation, air rushing into a low-pressure area would flow straight inward rather than spinning. There would still be storms and thunderstorms (which don’t require rotation), but the organised spinning vortex structure that makes a cyclone so powerful would not develop. This is why cyclones don’t form near the equator (where the effect of Earth’s rotation is minimal).
📌 Snapshots
- Pressure is defined as force per unit area
- The SI unit of pressure is N/m², also called pascal (Pa)
- Liquids and gases exert pressure on the walls of a container
- The pressure exerted by the air around us is known as atmospheric pressure
- Differences in air pressure cause winds to blow
- Warm air rises, creating a low-pressure area. Cooler air from surrounding higher-pressure regions moves in
- Important requirements for thunderstorms: moisture and strong winds
- Strong winds moving upwards and downwards facilitate rubbing of ice particles with water droplets → electric charges develop in clouds
- Collision of electric charges within a cloud, between clouds, or between cloud and ground → lightning
- Lightning strikes can cause destruction to life and property
- Lightning conductors protect buildings from the effects of lightning
- The India Meteorological Department (IMD) constantly monitors cyclones and thunderstorms in India
📝 Keep the Curiosity Alive
1. Choose the correct statement.
- Vessel R is filled with water. When pouring stops, the level of water will be ___
(a) highest in P (b) highest in Q (c) highest in R (d) equal in all three vessels - Sucker M on smooth surface, sucker N on rough surface:
(a) Both stick (b) Both don’t stick (c) M sticks but N does not (d) N sticks but M does not - To get more water pressure on ground floor:
(a) Increase height H (b) Decrease H (c) Replace with larger tank at same height (d) Replace with smaller tank - Vessels A and B with water at same level — PA, PB = pressure at bottom; FA, FB = force at bottom:
(a) PA=PB, FA=FB (b) PA=PB, FA<FB (c) PA<PB, FA=FB (d) PA>PB, FA>FB
2. True [T] or False [F]:
- Air flows from higher pressure to lower pressure. [T]
- Liquids exert pressure only at the bottom of a container. [F]
- Weather is stormy at the eye of a cyclone. [F] — the eye is calm
- During a thunderstorm, it is safer to be in a car. [T]
3. In which case does the boy sink more in sand — lying horizontally or standing vertically? Give reasons.
Show Answer
The boy sinks more when standing vertically. Same weight acts on a smaller area (feet only) → greater pressure → more sinking. When lying horizontally, the same weight is distributed over a much larger area → less pressure → less sinking.
4. An elephant stands on four feet. Area of one foot = 0.25 m², weight = 20,000 N. Calculate pressure exerted on the ground.
📐 Solution
Show Answer
Total area = 4 × 0.25 = 1 m²
Pressure = 20,000 ÷ 1 = 20,000 Pa
Pressure = 20,000 ÷ 1 = 20,000 Pa
5. Boat A: area 7 m², 5 persons. Boat B: area 3.5 m², 3 persons. Each person weighs 700 N. Which boat has more pressure on its base and by how much?
📐 Solution
Show Answer
Boat A: Force = 5×700 = 3500 N; Pressure = 3500 ÷ 7 = 500 Pa
Boat B: Force = 3×700 = 2100 N; Pressure = 2100 ÷ 3.5 = 600 Pa
Boat B experiences more pressure by 100 Pa
Boat B: Force = 3×700 = 2100 N; Pressure = 2100 ÷ 3.5 = 600 Pa
Boat B experiences more pressure by 100 Pa
6. Would lightning occur if air and clouds were good conductors of electricity? Give reasons.
Show Answer
No. Lightning requires charge build-up until the insulating property of air breaks down. If air and clouds were good conductors, charges would flow continuously and gradually — never building up enough for the sudden massive discharge that produces lightning.
7. What will happen to balloons A and B at the bottom of a bottle when water is filled to a certain height? Will they bulge equally?
Show Answer
Yes, both balloons will bulge equally. Both are attached at the same depth below the water surface, so the pressure acting on them is identical (pressure depends on height of water column, not vessel diameter). Equal pressure → equal bulge.
8. Explain how a storm becomes a cyclone.
Show Answer
Over warm ocean waters: (1) Warm moist air rises → low pressure; (2) Water vapour condenses → heat released → air rises faster → even lower pressure; (3) More air rushes in; (4) Earth’s rotation causes the rushing air to spin; (5) The cycle intensifies continuously creating an enormous rotating system of very low pressure with extremely high-speed winds revolving around it = cyclone.
9. Trees along the sea coast in a summer afternoon bend towards side A. Which side is land — A or B? Explain.
Show Answer
Side A is the land side. Trees are bent towards A → wind blows from B (sea) to A (land). In a summer afternoon, land heats faster → warm air over land rises → low pressure over land → cool sea air blows from sea (B) to land (A) = sea breeze. Trees bend in the direction the wind is blowing towards — side A.
10. Describe an activity to show that air flows from high pressure to low pressure.
Show Answer
Activity 6.5: Take two thin rubber balloons and a drinking straw. Insert one end of the straw into an uninflated balloon; inflate the second balloon and insert the free end of the straw into it. Observation: Air moves from the inflated balloon (high pressure) to the uninflated one (low pressure); both eventually reach the same size; flow stops when pressures equalise. Conclusion: Air moves from high to low pressure.
11. What is a thunderstorm? Explain the process of its formation.
Show Answer
A thunderstorm is a storm accompanied by lightning and thunder. Formation: (1) Land heats → warm moist air rises → low pressure; (2) Cooler air flows in → heated → rises (continuous circulation); (3) Rising air cools → moisture condenses → clouds → rain/hail/snow; (4) Under certain conditions ice particles form at height; (5) Strong winds cause rubbing → static charges develop; (6) Positive charges (ice) at top; negative (water droplets) at bottom — charge separation; (7) Ground becomes positively charged; (8) Charge build-up breaks air’s insulation → sudden discharge = lightning; (9) Lightning heats air → expansion = thunder.
12. Explain the process that causes lightning.
Show Answer
(1) Strong winds cause water droplets and ice particles to rub → static electric charges develop; (2) Positive charges (lighter ice) → upper cloud; negative charges (heavier water) → lower cloud; (3) Lower cloud makes ground positively charged; (4) When charge build-up is enormous, air’s insulation breaks down; (5) Sudden massive charge discharge = bright flash (lightning); (6) Lightning heats air → rapid expansion = loud thunder.
13. Explain why holes are made in banners and hoardings.
Show Answer
Holes reduce the wind pressure on banners. Without holes, wind exerts a large force on the entire surface area, potentially tearing the banner or toppling the hoarding. With holes, wind passes through — reducing the effective area resisting the wind. Since Pressure = Force ÷ Area, smaller effective area means less total force on the structure.
🧠 Final Review — Cyclone & Big Picture
LOTS (Lower Order Thinking)
1. Name the organisation in India that monitors cyclones and thunderstorms.
Show Answer
The India Meteorological Department (IMD) constantly monitors cyclones and thunderstorms in India. Weather monitoring satellites help track cyclones and predict their path, reducing impact on life and property.
Medium
2. List any four types of destruction caused by cyclones.
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(1) Storm surge — walls of water 3–12 m high flooding coastal areas; (2) River flooding and landslides from heavy rainfall; (3) Contamination of water and farmland — salt in seawater makes soil infertile; (4) Infrastructure damage — roads blocked, power outages disrupting emergency services.
HOTS (Higher Order Thinking)
3. A cyclone weakens once it moves over land. Identify the specific factor that is “cut off” and explain why removing it stops intensification.
Show Answer
The factor cut off is the supply of warm, moist air from the warm ocean surface. A cyclone’s energy comes from a continuous cycle: warm ocean air rises → condenses → heat released → air rises even faster → even lower pressure → more air rushes in. Over land, the warm ocean surface is no longer below — no continuous warm moist air supply means no condensation heat released — the upward current weakens, pressure equalises, spinning slows, and the cyclone dissipates. Like an engine losing fuel — without ocean heat, the engine stops.
🔬 Discover, Design, and Debate
- Hold a strip of paper (18 cm long, 2 cm wide) between thumb and forefinger hanging freely. Predict what happens when you blow over the paper. Perform the activity and interpret your results. (Hint: high-speed winds lower air pressure!)
- List three major cyclones in India in the last 20 years. List two major destructions caused by each. What measures did local government/communities take? Suggest two improvements to propose to the government.
- Collect data on thunderstorm strength for various regions of India. Identify which regions are more prone and give reasons for your findings.