Pressure, Winds, Storms, and Cyclones
Why This Matters
Think of a windy day. Fallen leaves swirl up into the air. Doors slam shut on their own. Clothes on the line flutter hard. A really strong wind can even bend a tree or tear a roof off a house.
Stop and ask: how does wind do all this? Wind is just moving air. We cannot even see it. Yet it can lift leaves and push down trees. That means moving air is pushing — it is putting a force on things.
But here is a deeper question. Why does wind blow harder on some days and stay calm on others? And why do gentle winds sometimes grow into terrifying storms and cyclones that flood whole towns?
The answers all start with one simple idea: pressure. Once you understand pressure — how a push spreads over an area, how liquids and air push, and how air rushes from one place to another — the whole story of winds, storms, and cyclones falls neatly into place. Let us begin.
The Big Idea
Pressure is how hard a push lands on each bit of a surface. It is the force divided by the area it acts on. Squeeze the same force into a smaller area and the pressure shoots up — that is why a sharp nail pierces but a blunt one does not. Liquids and even the air around us push too. And here is the key that unlocks everything else: air always flows from a place of high pressure to a place of low pressure. That flowing air is what we call wind. When warm air rises and leaves low pressure behind, air rushes in to fill it — and under the right conditions, that simple rush of air can grow into a thunderstorm or a giant spinning cyclone.
Let’s Break It Down
What pressure really means
Imagine two friends, Megha and Pawan, walking to a picnic. Their bags are exactly the same weight. But Pawan keeps shifting his bag — it hurts his shoulders. Megha’s bag feels fine. Why?
Look closely and you spot the difference. Pawan’s bag has narrow straps. Megha’s bag has broad straps. Same weight, but a very different feeling. How can that be?
The bag pulls down on the shoulder with a certain force (its weight). A narrow strap presses that force onto a small strip of skin. A broad strap spreads the same force over a wider strip of skin. Spread out, the push on each bit of skin is gentler. That gentler push-per-bit is what we mean by lower pressure.
So we get the central idea of this chapter. Pressure is the force acting on each unit of area:
Pressure = Force / Area
The bigger the area, the more the force gets spread out, so the smaller the pressure. The smaller the area, the more the force gets concentrated, so the bigger the pressure. This is why broad straps, a broad bucket handle, or a cloth ring under a head-load all feel easier — they all increase the area and cut down the pressure.
(One small note: we only count the force that pushes straight onto the surface, at a right angle to it. That is the force that creates pressure.)
Let us put real numbers on it so the formula feels solid.
We push a force of 100 newtons on a cardboard sheet of area 2 square metres. The pressure is:
Pressure = 100 N / 2 m² = 50 N/m²
The unit here, newton per square metre (N/m²), has a special name: the pascal (Pa). So this is 50 Pa. (A newton, N, is the science unit of force; you met it earlier. One newton is roughly the weight of a small apple.)
Why a smaller area gives more pressure
Here is the part NCERT states quickly but never really explains: why does a smaller area give a bigger pressure for the same force? Let us slow down and see it.
Picture the total force as a fixed pile of sand you must spread on the floor. If you spread it over a big area, the layer is thin everywhere. If you dump that same pile into a tiny patch, it piles up high. The force is like that sand. Spread over a big area, the pressure on each spot is small. Squeezed into a tiny area, the pressure on each spot is huge — because all of the force is crowding into a small place.
That is exactly why a sharp, pointed nail goes into wood so easily, while you could never push a blunt rod in with the same hand-push. The hammer gives the same force either way. But the sharp point has an incredibly tiny tip area. The whole force crowds into that tiny tip, making a giant pressure right there — big enough to tear the wood fibres apart. Figure 6.1 below shows this side by side.
Now you can explain many everyday things. The sharp edge of a knife cuts an apple easily because the edge has a tiny area, giving high pressure. The blunt back of the knife has a wider area, lower pressure, and barely cuts. Press a drawing pin and its sharp point digs in while the broad head stays comfortable under your thumb.
A camel has wide, flat feet, while a horse has small, hard hooves. On soft desert sand, why does the camel sink less even though it is heavier?
Pressure is force divided by area. The camel is heavier, so its force (weight) is more. But its feet are very wide, so that force is spread over a large area. This makes the pressure on the sand quite low, so it does not sink much. A horse’s small hooves press its weight into a tiny area, giving high pressure on the sand, so it would sink more. Wide feet are the camel’s natural trick for low pressure on soft sand.
Do liquids push too?
Solids clearly press down. But what about water? Does a liquid push as well?
Yes — and it pushes in a special way. Take two clear pipes of the same length but different widths (one broad, one narrow), each closed at the bottom with a stretchy balloon. Fill both with water to the same height. You might expect the broad pipe, which holds more water and so is heavier, to bulge its balloon more. But it does not. Both balloons bulge by the same amount.
That is a surprise, and it tells us something important. The bulge is not decided by the weight of water. It is decided by the height of the water column. Same height of water, same bulge — even though one pipe holds far more water.
So a liquid pushes on the bottom of its container with a pressure that depends on its height, not on how much it holds. Pour in more water to raise the level, and the balloon bulges more. Raise the height, raise the pressure. Now think about overhead water tanks placed up on rooftops. Figure 6.2 below explains why height is the key.
This is exactly why water tanks sit high up. The taller the column of water above your tap, the greater the water pressure at the tap — and the stronger the stream that gushes out. A friend living on a lower floor, with more water height above her tap, gets an even more powerful stream than someone higher up.
But does a liquid only push down? No. Make four small holes around the side of a filled bottle, near the bottom. Water spurts out sideways from every hole. So water also pushes on the walls of its container. In fact, a liquid pushes in all directions — down, sideways, everywhere. This is why a leaking pipe sprays water out of its side, and why the base of a dam is built much thicker than its top: the sideways water pressure is largest near the bottom, so the wall must be strongest there to hold it back.
A water tank on the top floor of a building supplies two flats. One tap is on the ground floor, the other on the third floor. Which tap gives water with more force, and why?
The ground-floor tap gives a stronger stream. Liquid pressure depends on the height of the water column above the tap. The ground-floor tap is much lower down, so there is a taller column of water above it, which means more pressure. The third-floor tap, being closer to the tank, has a shorter column above it and so less pressure. More height above means more water pressure.
Air pushes too — atmospheric pressure
Here is something hard to believe at first: the air around you is pushing on you right now, from every side. We do not feel it, but it is real.
The Earth is wrapped in a thick blanket of air called the atmosphere. It is made mostly of nitrogen and oxygen, with small amounts of other gases, and it stretches many kilometres up. All that air has weight, and like any liquid, it presses on everything below and around it. The push of this air is called atmospheric pressure.
How do we know air really pushes? Try lifting an upside-down paper plate that is covered by a sheet of chart paper, using a stick. With a small folded sheet it lifts easily. But cover it with a large unfolded sheet — same weight of paper — and now it is much harder to lift. The only thing that changed is the area of the covering sheet. More area, more effort. That extra effort is the air pushing down on the sheet. More area means more air force on it. Since force per area is pressure, this proves air exerts pressure on things.
The best demonstration is a rubber sucker. Press one firmly onto a smooth wall and it sticks — and it is surprisingly hard to pull off. Why? Figure 6.3 below shows what is happening.
When you press the sucker, you squeeze almost all the air out from under its cup. So there is very little air left inside to push back out. But the air outside, all around the sucker, is still at full atmospheric pressure, pushing in hard from every side. The outside push is much stronger than the weak inside push, so the air pins the sucker tight against the wall. To pull it off, your hand must beat that whole pressure difference.
So how strong is atmospheric pressure? Astonishingly strong. The air pushing down on a square the size of a small window pane (about 15 cm by 15 cm) presses with a force equal to the weight of a 225 kg load — over 2000 newtons. That is like a small motorbike resting on that little patch.
Then why are we not crushed flat? Because the pressure inside our body — from the fluids and gases moving through our tissues — is also equal to the atmospheric pressure outside. The inside push and the outside push are perfectly balanced, so we feel nothing at all. It is like a tug-of-war where both teams pull equally hard: nothing moves.
Why winds blow
Now we reach the heart of the chapter. We know air pushes. We also know our master rule: air moves from high pressure to low pressure. Put these together and you understand wind.
First, let us prove the rule simply. Take an inflated balloon (high pressure inside) and let go of its neck. Air rushes out — from the high pressure inside to the low pressure outside. A punctured cycle tube does the same. Or join an inflated balloon to a flat one with a straw: air flows from the full one (high pressure) into the empty one (low pressure) until both reach the same pressure and the flow stops. Air always moves from high to low pressure, until the two are equal.
So wind is simply air on the move from high pressure to low pressure. But what creates these differences in the first place? The Sun. Here is the step-by-step mechanism — and Figure 6.4 below shows it.
When the Sun heats a patch of ground, the air just above that hot patch warms up. Warm air spreads out and becomes lighter than the cooler air around it. Being lighter, it rises up, like a hot-air balloon. As that warm air leaves, it leaves behind a region with less air pressing down — a low pressure area. Nearby, the cooler, heavier air is at higher pressure. Following the master rule, this cool air rushes sideways into the low pressure area to fill the gap. That sideways rush of cool air, near the ground, is the wind we feel.
This also explains the breezes near a sea, which you met in Class 7. By day, land heats faster than water; warm air rises over the land, making low pressure, so cool air blows in from the sea — a sea breeze. By night, the water stays warmer than the land; now low pressure forms over the sea, so the wind blows out from the land — a land breeze. Both are just air obeying the same rule, driven by pressure differences.
And the bigger the pressure difference, the faster the air rushes. That is why winds are stronger on some days than others — it depends on how big the pressure gap is.
On a hot summer afternoon, a sudden strong breeze blows from the river toward the open field beside it. Explain, step by step, why this wind forms and which way it blows.
- Recall the rule. Air always moves from a high pressure area to a low pressure area. So I need to find where low pressure forms.
- See what the Sun does. The open field (land) heats up much faster than the river water on a hot afternoon. So the air above the field gets warmer than the air above the river.
- Warm air rises. The warm air over the field becomes lighter and rises up. As it leaves, it creates a low pressure area over the field.
- Find the high pressure side. The air over the cooler river stays denser and heavier, so it is at higher pressure compared with the field.
- Apply the rule. Air moves from high pressure (over the river) to low pressure (over the field). So the wind blows from the river toward the field. The strong heat makes a big pressure difference, which is why the breeze is strong.
High-speed winds lower the air pressure
There is one more twist that explains how roofs get blown off. Fast-moving air has lower pressure than still air.
You can feel this. Hang two balloons a few centimetres apart and blow hard into the gap between them. You might expect them to push apart. Instead they swing toward each other! Blowing made the air between them move fast, which lowered the pressure there. The normal, higher air pressure on the outer sides then pushed the balloons together. Fast air means low pressure.
Now picture a strong wind racing over the roof of a house. The fast wind above the roof has low pressure. But the still air trapped inside the house is at normal, higher pressure. So there is a bigger push from below than from above. If the wind is fierce and the roof is weak, this difference can lift the roof clean off — pushing it up and away. That is why during a high-wind storm it is safer to keep doors and windows open: the wind then blows through the house too, so the pressure inside and over the roof stay closer, and the roof is far less likely to be lifted off.
How a thunderstorm forms
A storm is just strong winds with heavy rain. In hot, humid places like much of India, storms are common. Here is how one builds, step by step.
The Sun heats the land, so warm, moist air (air carrying a lot of water vapour) rises, leaving low pressure. Cooler air rushes in, warms, and rises in turn — a never-ending circulation of wind. As the moist air climbs high, it cools. Cool air cannot hold as much water vapour, so the vapour condenses into tiny water droplets, forming clouds. The droplets join into bigger, heavier drops that finally fall as rain, hail, or snow. Strong rising and falling winds inside the cloud make this a turbulent, powerful system — a storm.
Then comes the electricity. High up, where it is freezing, some droplets turn into ice particles. Strong winds rush up and down inside the cloud, making these ice particles and water droplets rub against one another. You learnt earlier that rubbing two things together makes them gain electric charge. Here, the rubbing builds up static charge inside the cloud. The lighter, positively charged ice particles gather at the top of the cloud; the heavier, negatively charged droplets sink to the bottom. So the cloud ends up with separated charges — positive up high, negative down low.
Now, air normally does not let charges flow through it — it is an insulator. But when the charge build-up becomes very large, the air’s insulating power breaks down. Suddenly the charges leap across in a huge spark. That bright flash is lightning. The lightning can jump within one cloud, between two clouds, or between a cloud and the ground (when the negative cloud-bottom makes the ground below it positive). The lightning heats the air around it so fiercely that the air expands with a bang — the loud thunder we hear. A storm with lightning and thunder is a thunderstorm.
| Storm | Thunderstorm | Cyclone | |
|---|---|---|---|
| What it is | Strong winds with heavy rain | A storm that also has lightning and thunder | A giant spinning system of clouds, winds and rain |
| Main cause | Warm moist air rising, cooling and condensing | Charges build up from ice and droplets rubbing, then leap | Very low pressure over warm ocean, plus Earth’s rotation |
| Extra feature | Rain, sometimes hail | Bright lightning flash and a loud thunder bang | A calm low-pressure centre called the eye |
| Where common | Hot, humid regions like much of India | During the rainy season in warm regions | Over warm ocean waters, moving toward coasts |
How a cyclone forms
A cyclone is the biggest storm of all — a vast, spinning system born over warm seas. Let us build it step by step.
Over a warm ocean, the sea heats the air above it. This warm, moist air rises, and its water vapour condenses into raindrops. Here is the clever part that makes a cyclone so powerful: when water vapour condenses back into liquid, it releases heat (the same heat it had soaked up to become vapour in the first place). This released heat warms the rising air even more, so it rises even faster and higher. That makes the pressure at the surface drop even lower. Air from all around rushes in toward this very low pressure — and as it rushes in, the spinning of the Earth makes the whole inrushing system curve and spin. Round and round it goes, building into a huge whirl of cloud, wind, and rain. That spinning system is a cyclone. Figure 6.5 below shows a slice through it.
At the very centre sits the eye of the cyclone — the spot of lowest pressure. Strangely, inside the eye the wind is calm. But the ring of cloud just around the eye has the fiercest winds and heaviest rain of the whole storm. So a sudden calm does not mean the cyclone has passed — the worst may be the wall of storm right behind the eye.
A cyclone moving from ocean toward land brings far faster winds than an ordinary thunderstorm. The 2020 Amphan cyclone reached peak winds of 270 km/h. Once a cyclone moves over land, it loses its supply of warm moist sea air and slowly weakens — but not before it can do enormous damage. Its winds push a wall of seawater onto the coast, 3 to 12 metres high, flooding low areas. Heavy rain overflows rivers and can trigger landslides. Salty seawater spoils drinking water and farmland. Fallen trees block roads, and power can be lost for days.
Staying safe
Knowing the science is not enough — you must know how to stay safe. Here is exactly what the chapter teaches.
During lightning, electricity seeks the tallest, easiest path to the ground, so:
- Stay away from tall objects like single trees, poles, and towers.
- Find a low-lying open area and crouch down, keeping your contact with the ground small. Do not lie down flat.
- Do not use an umbrella with a metal rod.
- If you are in water, get out of it.
- Inside a closed bus or car you are comparatively safe.
Buildings are protected by a lightning conductor — a metal rod fixed along a tall building, with its pointed top above the highest point and its other end buried deep in the ground. It gives the lightning’s charge an easy, safe path straight into the earth, so the building is spared.
During a cyclone:
- Stay updated with weather reports, alerts, and warnings from the India Meteorological Department (IMD). Thanks to weather satellites, cyclones can now be tracked and their path predicted, which saves many lives.
- If you live in a cyclone-prone area, keep an emergency kit of essentials ready.
- When a cyclone strikes, quickly move to a nearby designated cyclone shelter.
Common Mistakes
Read these once, and you will sidestep the traps students fall into.
A heavier object always presses with more pressure than a lighter one.
In daily life 'heavy' and 'presses hard' feel like the same thing, so it seems obvious that more weight must always mean more pressure.
Pressure depends on force divided by area, not on weight alone. A heavy object spread over a very large area (like a camel on wide feet, or a person lying flat on sand) can give less pressure than a light object on a tiny point (like a thumbtack). You must always consider the area, not just the weight.
A liquid presses only downward, on the bottom of its container.
We usually watch water settle and press down on the floor of a glass, and gravity pulls everything down, so it feels like the push must only be downward.
A liquid presses in all directions at once — downward, and also sideways on the walls. That is why water spurts out of a hole made in the side of a bottle, and why a dam wall must be thick at the bottom to hold back the sideways water pressure.
The eye at the centre of a cyclone is the most dangerous, stormiest part.
It sits right in the middle of the cyclone, so it seems it should be where the storm is strongest of all.
The eye is actually the calmest part, with the lowest pressure and almost no wind. The fiercest winds and heaviest rain are in the ring of cloud just around the eye. A sudden calm can be the eye passing over — the wall of storm right behind it may be the most violent part.
Quick Check
Try each one. They each test a single idea from the chapter.
Two bags weigh exactly the same. One has broad straps, the other has narrow straps. Why does the broad-strap bag feel more comfortable on the shoulder?
In which direction does air always move?
Why is the rubber sucker held tightly against a smooth wall?
What is the eye of a cyclone?
Practice Problems
Try each one yourself first. Then tap to see the full solution.
Easy
Write the formula for pressure and state its SI unit.
The formula for pressure is:
Pressure = Force / Area
It tells us how much force acts on each unit of area.
The SI unit of force is the newton (N) and the SI unit of area is the square metre (m²). So the SI unit of pressure is newton per square metre (N/m²), which is given the special name pascal (Pa).
So 1 Pa = 1 N/m².
Why are overhead water tanks placed high up on rooftops?
The pressure of a liquid depends on the height of its column. The taller the column of water above a tap, the greater the water pressure at that tap.
By placing the tank high on the roof, there is a tall column of water above every tap in the house. This gives high water pressure, so water flows out of the taps in a strong stream. A tank kept low would give a tall column only for the lowest taps, and a weak flow elsewhere.
Medium
An elephant stands on four feet. The area covered by one foot is 0.25 m², and the elephant's weight is 20000 N. Calculate the pressure it exerts on the ground.
First find the total area touching the ground. The elephant stands on four feet:
Total area = 4 × 0.25 m² = 1 m²
Now use the pressure formula. The whole weight (force) acts over this total area:
Pressure = Force / Area = 20000 N / 1 m² = 20000 N/m²
So the elephant exerts a pressure of 20000 Pa (20000 N/m²) on the ground.
(Notice we used the area of all four feet together, because the weight is shared over all of them.)
Boat A has a base area of 7 m² and carries 5 people. Boat B has a base area of 3.5 m² and carries 3 people. Each person weighs 700 N. Which boat's base feels more pressure, and by how much? (Ignore the weight of the boats themselves.)
Find the force (total weight of people) on each boat, then divide by its base area.
Boat A: Force = 5 × 700 N = 3500 N. Area = 7 m². Pressure = 3500 / 7 = 500 N/m²
Boat B: Force = 3 × 700 N = 2100 N. Area = 3.5 m². Pressure = 2100 / 3.5 = 600 N/m²
Compare: Boat B has the greater pressure on its base.
Difference = 600 − 500 = 100 N/m²
So Boat B experiences more pressure on its base, by 100 N/m² (100 Pa). Even though Boat A carries more people, its much larger base spreads the force out more, giving lower pressure.
Challenge
Explain, step by step, how an ordinary storm over a warm ocean can grow into a cyclone. Why does the rising air keep speeding up?
Let us build it up step by step:
-
Warm sea heats the air. The warm ocean heats the air just above it. This warm, moist air becomes lighter and rises.
-
Vapour condenses and rain forms. As the moist air rises and cools, its water vapour condenses into raindrops, forming clouds.
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Condensation releases heat — the key step. When water vapour condenses back to liquid, it gives back the heat it had taken in to become vapour. This released heat warms the rising air even more.
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The air rises faster and faster. Because it is now even warmer, the air rises even higher and faster. This keeps the surface pressure dropping lower and lower. So the rising does not slow down — it feeds itself, getting stronger.
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Air rushes in from all sides. This very low pressure pulls in air from all around. As more air rushes in, it too warms, rises, and adds to the system.
-
The Earth’s rotation makes it spin. The spinning Earth twists the inrushing air, so the whole system rotates. This spinning mass of cloud, wind, and rain, around a very low pressure centre, is a cyclone.
So the rising air keeps speeding up because condensation keeps releasing heat into it — a feedback that makes a cyclone so powerful.
Figure 6.4's idea in reverse: trees along a sea coast on a summer afternoon are bent so they lean away from the sea, toward the inland side. Which side is the land — the sea side or the inland side? Explain using pressure.
The trees lean away from the sea, which means the wind is blowing from the sea toward the land. We need to work out why the wind blows that way on a summer afternoon.
- On a hot afternoon, the land heats up faster than the sea water.
- So the air above the land becomes warm and light, and rises, creating a low pressure area over the land.
- The air over the cooler sea stays denser, at higher pressure.
- Air moves from high pressure to low pressure, so the wind blows from the sea toward the land — a sea breeze.
This matches the bent trees: the wind comes off the sea and pushes the trees over toward the inland side. So the land is the side the trees lean toward (the inland side), and the sea is the side the trees lean away from. The pressure difference between the cool sea (high) and the hot land (low) drives the breeze.
Summary
Here is everything you can now explain to a friend:
- Pressure = Force / Area. It is how hard a force lands on each unit of area. Its SI unit is the pascal (Pa), which is one newton per square metre (N/m²).
- For the same force, a smaller area gives more pressure — that is why a sharp nail pierces, a knife’s edge cuts, and broad straps feel comfortable.
- Liquids exert pressure in all directions, and the pressure depends on the height of the liquid column, not on how much it holds. This is why overhead tanks are placed high.
- The air around us is the atmosphere, and it presses on everything as atmospheric pressure. It does not crush us because the pressure inside our body balances it.
- Air moves from high pressure to low pressure. When warm air rises and leaves low pressure behind, cooler air rushes in — and that moving air is wind.
- Fast-moving air has lower pressure. This is why high-speed winds can lift weak roofs (keep doors and windows open in a storm).
- A storm is strong wind with heavy rain. Rubbing ice and droplets build up charge in clouds; when it leaps, we get lightning and thunder — a thunderstorm.
- A cyclone is a giant spinning storm over warm seas, with a calm low-pressure eye at its centre. Condensation releasing heat makes it grow stronger.
- Stay safe: during lightning, crouch low away from tall objects and avoid metal; during cyclones, follow IMD warnings, keep an emergency kit, and move to a cyclone shelter.
What’s Next
You have just seen how air and water push on things, and how moving air becomes wind and storms. But what is air made of, really? Why can you squeeze a balloon but not a brick? Why does a smell spread across a whole room?
To answer these, we need to zoom right in and look at the tiny particles that everything is made of. In the next chapter, Chapter 7 — Particulate Nature of Matter, you will discover that all matter — solid, liquid, and gas — is built from countless tiny particles, always on the move. Get ready to see the world up close!
Frequently Asked Questions
What is pressure and how is it different from force?
Force is the total push or pull on something. Pressure is how much of that force lands on each bit of area. Pressure equals force divided by area. So the same force feels very different depending on how big or small the area is. A sharp point gives a tiny area, so even a small force makes a huge pressure and pierces easily.
Why does a bag with broad straps feel lighter than one with narrow straps?
Both bags can weigh the same, so the force on your shoulder is the same. But broad straps spread that force over a larger area, while narrow straps press it into a small area. Since pressure equals force divided by area, the broad straps give less pressure on your shoulder, so the bag feels more comfortable even though its weight has not changed.
What is atmospheric pressure and why does it not crush us?
Atmospheric pressure is the push of the thick layer of air around the Earth. This air has weight and presses on everything from all sides. It does not crush us because the pressure inside our body, caused by the fluids and gases in our tissues, is equal to the air pressure outside. The two balance each other, so we feel nothing.
Why do winds blow?
Winds blow because of differences in air pressure. When the Sun heats a patch of ground, the warm air there becomes lighter and rises up. This leaves a low pressure area behind. Cooler, heavier air from a nearby high pressure area rushes sideways to fill the gap. That moving air near the ground is what we feel as wind, and it always moves from high pressure to low pressure.
What is the eye of a cyclone?
The eye of a cyclone is the calm centre of the storm. It is the region of lowest pressure, right in the middle of the spinning system of clouds, winds, and rain. Inside the eye the wind is calm, but just around it the storm is at its strongest, with very fast winds and heavy rain. So a calm eye does not mean the cyclone is over.