Pressure, Winds, Storms, and Cyclones — Class 8 Science (Curiosity)
"Both our bags are equally heavy. Why does your bag hurt, and mine doesn't?" — Megha, Curiosity, Grade 8, page 81
1. About the Chapter
This is Chapter 6 of Curiosity (pages 80–97, Reprint 2026-27). It opens with fallen leaves swirling, trees bending and doors slamming, and names the thing they have in common: The force exerted by wind creates wind pressure which causes these effects.
| Section | Question |
|---|---|
| 6.1 | What is pressure? Do liquids exert it? |
| 6.2 | Does air exert pressure? |
| 6.3 | How does wind form? |
| 6.4 | What do high-speed winds do to the pressure? |
| 6.5 | Storms, thunderstorms and lightning |
| 6.6 | Cyclones |
The single thread. Pressure is force per unit area. Everything after page 82 is that one idea applied to liquids, to air, and finally to weather — and the closing insight is that pressure by itself never damages anything. Only a pressure difference does.
2. Pressure
The bag straps
Megha and Pawan carry equally heavy bags. Pawan's hurts; Megha's does not. Pawan works out why himself: My bag has narrow straps while your bag has broad straps.
The weight of the bag with narrow straps acts on a smaller area of our shoulders, whereas the weight of the bag with broad straps is spread out over a larger area.
The force is identical in the two cases, so the force cannot be the explanation. Only the area has changed — which forces a new quantity into existence:
Pressure = Force ÷ Area
At this stage, we will consider only those forces which act perpendicular to the surface on which the pressure is to be computed.
The unit
Force is measured in newton, area in metre². So pressure is measured in newton/metre² (N/m²), also called the pascal (Pa). You never need to memorise it separately — it falls straight out of the formula.
Worked example from the chapter: a force of 100 N on 2 m² gives 100 ÷ 2 = 50 N/m².
The same idea, used both ways
| Want LOW pressure → make the area BIG | Want HIGH pressure → make the area SMALL |
|---|---|
| Broad schoolbag straps | The pointed end of a nail |
| Broad bucket handle | The sharp edge of a knife |
| Cloth ring under a head-load | Needles, pins, drawing pins |
| Wide tractor tyres, snowshoes | Studs on sports shoes |
Table 6.1 asks you to test the second column: a nail driven by its point goes in easily, by its head it does not; a knife cuts with its sharp edge, not its blunt one.
When the area over which a force applied is smaller, the resulting pressure is higher, making it easier to do certain tasks.
Nothing here reduces the load. Broad straps do not make a bag lighter — they lower the pressure. Keeping force and pressure apart is the whole of section 6.1.
3. Liquids Exert Pressure
Activity 6.1 — and why it is designed that way
Two pipes of the same length but different diameters, balloons tied to their lower ends, filled with water to the same level.
Because the pipes differ in width, they hold different weights of water — so the two candidate explanations make different predictions:
| If the cause is… | Then… |
|---|---|
| the weight of the water | the wide pipe's balloon should bulge more |
| the height of the column | both should bulge the same |
Both bulge equally. This means that the weight of water in the pipes could not be responsible for the extent of the bulge of the balloons.
Then pour more water into one pipe. The bulge grows.
The pressure exerted by a liquid in a vessel depends on the height of its column.
That is the whole result, and it is counter-intuitive: the quantity of water is irrelevant. A wide tank holding far more water, at the same height, gives exactly the same pressure at the tap.
Activity 6.2 — sideways too
Four small holes near the bottom of a bottle, all at the same height, sealed with tape. Fill it, remove all the tapes at once.
Water spurts from all four.
Liquids exert pressure not only at the bottom of the container, but also on its sides. In fact liquids exert pressure in all directions.
What follows
- Overhead tanks are placed high so the column above the taps is taller — resulting in a good stream of water from the taps.
- The ground floor gets a stronger stream than the top floor, because the column above it is taller.
- Water spurts from leaking pipe joints in any direction, because water presses outward on the pipe wall everywhere.
- A dam's base is broader than its top. The pressure which acts horizontally is very large near its bottom — so the wall is thin where the push is small and thick where it is large. The shape of a dam is a drawing of the pressure it must resist.
4. Air Exerts Pressure
The envelope of air surrounding the Earth is called atmosphere — nitrogen, oxygen, argon, carbon dioxide and other gases in small quantities, extending up to many kilometres.
Activity 6.3 — one sheet, folded and unfolded
An inverted paper plate with a stick, covered first by a chart-paper sheet folded twice, then by an identical sheet unfolded. Lift by the stick each time.
The unfolded sheet is harder to lift — and here is the point of using the same sheet: the weight of the covering sheet has not changed. Only the area has.
Air exerts force on the covering sheet ... this force increases with increase in the area of covering sheets. As force per unit area is pressure, we can conclude that air exerts pressure.
The pressure exerted by the air around us is atmospheric pressure. And it acts in all directions — which is why an inflating balloon expands in all directions.
Activity 6.4 — the sucker
Press a rubber sucker onto a smooth flat surface. It sticks, and is hard to pull off.
Most of the air between its cup and the surface is pushed out and the air pressure inside it is reduced. The sucker sticks because the pressure of air surrounding the sucker is higher than the pressure exerted by the air inside.
Nothing is pulling it in — the outside air is pushing it in. On a rough surface air leaks back in, the difference disappears, and it falls off.
How big is atmospheric pressure?
The force exerted by the atmospheric air column over an area 15 cm × 15 cm is nearly equal to the force of gravity on an object of mass 225 kg (2250 N).
Check it: 0.15 m × 0.15 m = 0.0225 m², so 2250 ÷ 0.0225 = 1,00,000 N/m² — which is 1000 hPa, sitting squarely inside the 994–1008 mb range on Fig. 6.19. The book's figure is internally consistent.
So why are we not crushed? The pressure inside our bodies is also equal to the atmospheric pressure. This balances the pressure exerted from outside.
This is the key idea of the whole chapter. Pressure alone never hurts anything. Only a difference does — which is what holds the sucker on, and what will lift a roof off a house four pages later.
1 millibar (mb) = 1 hectopascal (hPa) = 100 Pa. Weather maps use these because the interesting differences are only a few hundred pascals.
5. How Wind Forms
Activity 6.5
One inflated balloon and one empty one, joined by a straw. Predict first, then watch.
The inflated one shrinks, the empty one swells — and then the flow stops, with both nearly the same size.
The air flow stops when the pressure in both balloons becomes equal.
Air moves from a region of high air pressure to a region of low air pressure.
The stopping is the more informative half. That air moved shows a flow happened. That it stopped exactly when the pressures matched — with plenty of air still in both balloons — identifies the pressure difference as the cause.
And: the speed of the air is higher if the pressure difference is higher.
Sea breeze and land breeze
| Day — sea breeze | Night — land breeze | |
|---|---|---|
| Warmer | Land | Sea |
| Air rises over | Land | Sea |
| Low pressure over | Land | Sea |
| Wind blows | Sea → land | Land → sea |
Nothing about the mechanism changes between day and night. What reverses is which side is warmer. A breeze is named for where it comes from.
6. Fast Wind Means Low Pressure
Activity 6.6
Two balloons hung 6–10 cm apart. Blow into the gap between them.
They move towards each other — and blowing harder brings them together faster.
When you blow air between the balloons, a low pressure area is created between them. The higher air pressure surrounding the balloon pushes them towards each other.
High speed winds are accompanied by a reduced air pressure.
Almost everyone predicts the opposite. That blowing harder strengthens the effect rules out the obvious alternative — your breath is not simply shoving them.
Three consequences
Roofs blown off. Fast wind over a house lowers the pressure above the roof, while the still air inside stays at high pressure. The roof is pushed up from below — not pulled off from above.
So open the doors and windows. It sounds backwards, and it is exactly right: a sealed house preserves the high indoor pressure that does the lifting. Letting the wind through means the pressure difference between inside of the houses and over the roofs is reduced to a large extent.
Holes in banners and hoardings. Same problem, flat sheet. The holes let air through so the two faces come closer to the same pressure, and the force on the structure drops.
7. Storms, Thunderstorms and Lightning
A storm. Heated land → warm moist air rises → low pressure → cooler air flows in and is heated in turn → continuous circulation. The rising air cools, moisture condenses into clouds, drops merge and fall as rain, hail or snow. The strong winds accompanied by rain is called a storm.
Charges. Air rising high enough turns droplets into ice particles. Strong up-and-down winds rub ice and water together — and rubbing charges things, as you learnt in Exploring Forces.
| Particles | Charge | Where |
|---|---|---|
| Ice particles (lighter) | Positive | Upper part of the cloud |
| Water droplets (heavier) | Negative | Lower part of the cloud |
The negative cloud base then makes the ground and nearby objects positively charged.
Lightning. Normally, air acts as an electrical insulator and does not let opposite charges meet. That is why charge accumulates instead of leaking away. But when the build up of charges becomes very large, the insulating property of air breaks down. A sudden flow of charges takes place, producing a bright flash of light called lightning.
It happens within a cloud, between clouds, or between a cloud and the ground. The flash heats the air, which expands and makes thunder.
A storm accompanied by lightning and thunder is called a thunderstorm.
Requirements: moisture and strong winds.
Local Indian thunderstorms
| Name | Region | Use |
|---|---|---|
| Kalboishakhi | West Bengal, Bihar, Jharkhand | Pre-monsoon; helps kharif crops |
| Bordoisila | Assam | Pre-monsoon; helps kharif crops |
| Mango showers | Kerala, Karnataka, Tamil Nadu | Support the ripening of mangoes |
| (local storms) | Karnataka | Help coffee plants grow |
Safety, as the chapter gives it
- Stay away from tall objects.
- Find a low-lying open area and crouch down, minimising contact with the ground.
- Do not lie down flat.
- Avoid an umbrella with a metallic rod.
- Get out of water.
- If you are inside a bus or a car, you are comparatively safer.
A lightning conductor is a metal rod running the height of a building, its pointed end above the highest point and its other end buried deep in the ground. It does not stop lightning — it provides easy path for the transfer of electric charges into the ground.
8. Cyclones
Cyclones are large storms that form over warm ocean waters.
- Warm, moist air over the ocean rises.
- Water vapour condenses into raindrops.
- Heat is released by that condensation — causing further warming of the ascending air leading it to rise even further, creating an even lower pressure. This is a feedback loop, and it is what makes a cyclone so much stronger than an ordinary storm.
- Surrounding air rushes in.
- Earth's rotation causes the moving air to spin.
- The cycle repeats → a very low-pressure area with high-speed winds revolving around it.
This spinning system of clouds, winds, and rain is called a cyclone.
The eye. In a cyclone, the region of lowest pressure is at the centre — and at the eye of the cyclone, the wind is calm, while the surrounding region has strong winds and heavy rain. Not a contradiction: wind is driven by a pressure difference, and at the exact centre the pressure is equally low all around.
Over land it weakens, because the source of moist air is cut off and the condensation feedback starves. But it leaves behind a trail of destruction that can take months or even years to repair.
The chapter's figures: Amphan (2020) reached peak wind speeds of 270 km/h; a storm surge can be 3–12 metres high.
The damage is mostly not the wind. Storm surge and flooding, rivers overflowing, landslides, contaminated drinking water, salt in the farmland, roads blocked by fallen trees, and power outages lasting days.
Protection: stay updated on IMD alerts, keep an emergency kit ready beforehand, and move to a designated cyclone shelter. Satellites let cyclones be tracked and their paths predicted — a cyclone cannot be prevented, but the warning time can be.
9. Summary
- Pressure = Force ÷ Area, in N/m² or pascal.
- Liquid pressure depends on the height of the column, and acts in all directions.
- Air exerts atmospheric pressure — about 1,00,000 Pa, balanced by the pressure inside our bodies.
- Air flows high pressure → low pressure; that is wind, and it is faster when the difference is bigger.
- High-speed winds are accompanied by reduced pressure — roofs, banners, hanging balloons.
- A storm needs moisture and strong winds; a thunderstorm adds ice particles, charge separation and the breakdown of insulating air; a cyclone adds warm ocean water and Earth's rotation.
- The IMD monitors cyclones and thunderstorms in India.
The one sentence to carry away: nothing in this chapter is damaged by pressure. Everything is damaged by a pressure difference.
Appendix — What Belongs Elsewhere, Not to This Chapter
An earlier version of this page had the right title and the wrong chapter. It taught geography and disaster management in place of the physics, and it carried figures with no source.
| Topic on the old page | Where it belongs |
|---|---|
| India's summer and winter monsoon systems in detail | Social Science / Geography |
| Global wind systems | Higher grades |
| Cyclone category table (Saffir–Simpson, "modified for India") | Not in this book — the chapter gives no categories |
| A list of six named Indian cyclones with a death toll | Not in this book — the chapter names one, Amphan 2020 |
| Institutions and named meteorologists | Not in this chapter, which names only the IMD |
| Atmospheric composition percentages | Not in this book — the chapter lists the gases without percentages |
And the unsourced numbers, all removed: "~80% of India's annual rainfall", "70% of cultivated land", "10,000+ deaths", "Cherrapunji ~12,000mm", "IMD 1875, world's oldest national met service". None appears in the chapter and none was sourced. If you want figures like these, take them from a current IMD or government publication and cite it.
What was missing, and is now restored: Activities 6.1 to 6.6, liquid pressure and its dependence on column height, that liquids press in all directions, the overhead-tank and dam reasoning, the rubber sucker, the whole of section 6.4 on high-speed winds, and the lightning conductor. Those sections are where eight of the thirteen exercise questions come from.
