Pressure, Winds, Storms, and Cyclones – Class 8 Science Chapter 6 Notes & NCERT PDF

Class 8ScienceChapter 6NCERT book: Curiosity - Science Class 8

Notes and a simple summary of Chapter 6 of the NCERT Class 8 Science book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 6: Pressure, Winds, Storms, and Cyclones

Probe and Ponder

Why are winds stronger on some days than others?

Why are water tanks usually placed at a height?

Can air pressure really crush us?

What causes storms and cyclones?

Introduction

Fallen leaves swirl, trees bend, doors slam and clothes flutter when wind blows. The force exerted by wind creates wind pressure. This chapter explores the link between force and pressure and how pressure differences drive winds, thunderstorms and cyclones.

Key Concepts

6.1 Pressure

Megha and Pawan carry equally heavy bags to a picnic, but Pawan's shoulders hurt. His bag has narrow straps; Megha's has broad straps. The narrow straps spread the same weight over a smaller area.

Pressure = Force ÷ Area (considering forces acting perpendicular to the surface). The SI unit is newton/metre² (N/m²), also called a pascal (Pa).

Broad handles on buckets and a round cloth under head-loads reduce pressure by increasing area.

A nail is easier to drive in by its pointed end, and an apple is easier to cut with the sharp edge of a knife — a smaller area gives higher pressure.

Pressure in liquids

Activity 6.1: Two pipes of different diameters, each with a balloon at the bottom, are filled with water to the same height. Both balloons bulge equally — so it is not the weight of water but the pressure of the water column that matters.

Adding more water makes the balloon bulge more: liquid pressure increases with the height of the column. That is why overhead tanks are placed at a height — to give a good stream of water from taps.

Activity 6.2: Water spurts out of holes made around the sides of a bottle. Liquids exert pressure on the walls too — in fact, in all directions.

Dams have a broad base to withstand the very large horizontal water pressure near the bottom.

6.2 Pressure exerted by air

The envelope of air around the Earth is the atmosphere (nitrogen, oxygen, argon, carbon dioxide and other gases).

Activity 6.3: An inverted paper plate covered with a folded chart paper is easy to lift by a stick; covered with an unfolded (larger) sheet, it is harder — air exerts a force that increases with area, i.e. air exerts pressure.

The pressure exerted by the air around us is called atmospheric pressure.

A balloon inflates in all directions because air exerts pressure in all directions.

Activity 6.4 — the sucker: pressing a rubber sucker pushes out air, lowering the pressure inside; the higher outside pressure holds it to the surface.

How large is it? The force of the air column on an area of 15 cm × 15 cm is nearly equal to the force of gravity on 225 kg (2250 N). We are not crushed because the pressure inside our bodies is equal to atmospheric pressure.

The practical unit of air pressure is the millibar (mb) = 100 Pa; the hectopascal (hPa) also equals 100 Pa.

6.3 Formation of wind

Activity 6.5: An inflated balloon joined by a straw to an uninflated one pushes air into it until both are almost the same size. Air moves from high pressure to low pressure, and stops when the pressures become equal.

Sea breeze (day): land heats faster, air above it rises and creates low pressure; air blows from the sea (high pressure) to the land.

Land breeze (night): the sea is warmer, so low pressure forms over the sea and wind blows from land to sea.

The greater the pressure difference, the faster the air moves.

6.4 High-speed winds lower air pressure

Activity 6.6: Blowing between two hanging balloons makes them move towards each other — the fast-moving air creates low pressure between them. Blowing harder brings them together faster.

High-speed winds over a house create low pressure above the roof. If the difference is large and the roof is weak, it can be blown away. Keeping doors and windows open during such storms lets air pass through and reduces the pressure difference.

6.5 Storms, thunderstorms and lightning

Heated land makes warm, moist air rise (low pressure); cooler air rushes in, gets heated and rises — a continuous wind circulation. Rising air cools, moisture condenses into clouds, and drops fall as rain, hail or snow.

Strong winds accompanied by rain are called a storm.

At great heights, droplets turn into ice particles. Strong upward and downward winds rub ice particles and water droplets, creating static charges. Lighter, positively charged ice particles move to the upper part of the cloud; heavier, negatively charged water droplets stay in the lower part. The ground below becomes positively charged.

Normally air is an insulator, but when charge build-up is very large, this breaks down and a sudden flow of charges produces a flash — lightning (within a cloud, between clouds, or between cloud and ground).

Lightning heats the air, which expands and produces thunder. A storm with lightning and thunder is a thunderstorm.

Local names: Kalboishakhi (West Bengal, Bihar, Jharkhand), Bordoisila (Assam), mango showers (Kerala, Karnataka, Tamil Nadu). They come before the monsoon and help kharif crops, mangoes and coffee.

Lightning safety: stay away from tall objects; find a low-lying open area, crouch down and minimise contact with the ground; do not lie flat; avoid umbrellas with metal rods; get out of water; a bus or car is comparatively safer.

A lightning conductor is a metal rod along a building, pointed end above the highest point and other end buried deep in the ground; it gives charges an easy path into the ground.

6.6 Cyclone

Over warm ocean water, warm moist air rises. When water vapour condenses, heat is released, warming the rising air further and creating even lower pressure. Surrounding air rushes in, and Earth's rotation makes it spin.

This spinning system of clouds, winds and rain around a very low-pressure centre is a cyclone.

The eye of the cyclone is the region of lowest pressure at the centre — the wind there is calm, while the surrounding region has strong winds and heavy rain.

Over land the supply of moist air is cut off, so it weakens, but it can leave destruction lasting months or years.

Cyclone Amphan (2020) had peak wind speeds of 270 km/h. A cyclone can push a wall of sea water 3–12 metres high onto the coast (storm surge), flood areas, trigger landslides, contaminate drinking water, make soil salty, block roads and cause long power outages.

Staying safe: follow warnings from the India Meteorological Department (IMD); satellites help track cyclones; keep an emergency kit; move to a designated cyclone shelter.

Worked Examples

Pressure on cardboard: Force 100 N on area 2 m² → Pressure = 100 ÷ 2 = 50 N/m² (50 Pa).

Elephant (exercise 4): weight 20000 N on 4 feet of 0.25 m² each → area = 4 × 0.25 = 1 m² → Pressure = 20000 ÷ 1 = 20000 Pa.

Two boats (exercise 5): Boat A: 5 × 700 N = 3500 N on 7 m² → 500 Pa. Boat B: 3 × 700 N = 2100 N on 3.5 m² → 600 Pa. Boat B experiences more pressure, by 100 Pa.

Atmospheric pressure: 2250 N on 15 cm × 15 cm = 0.0225 m² → about 100000 Pa (1000 mb).

Key Terms

Pressure (दाब) — force per unit area

Pascal, Pa (पास्कल) — N/m²

Liquid pressure (द्रव दाब)

Atmosphere (वायुमंडल)

Atmospheric pressure (वायुमंडलीय दाब)

Millibar (मिलीबार) — 100 Pa

Wind (पवन)

Sea breeze / land breeze (समुद्री समीर / स्थल समीर)

Storm (तूफ़ान)

Thunderstorm (तड़ित झंझा)

Lightning (तड़ित / बिजली)

Lightning conductor (तड़ित चालक)

Cyclone (चक्रवात)

Eye of the cyclone (चक्रवात की आँख)

Real-life Connections

School bags with broad straps, sharp knives and pointed nails all use the idea of pressure.

Your building's water tank is on the roof for good tap pressure.

Holes in banners and hoardings let wind pass through, reducing the pressure difference that could tear them.

Common Misconceptions

"A wider pipe filled to the same height gives more pressure at the bottom." — Pressure depends on the height of the liquid column, not the amount of water.

"Liquids push only downwards." — They exert pressure in all directions.

"The eye of a cyclone is the most violent part." — The eye is calm; the strong winds are around it.

"Lying flat on the ground is safest in lightning." — The book says crouch, do not lie flat.

Keep the Curiosity Alive — Hints

Connected vessels P, Q, R: water settles to equal levels in all three.

A sucker sticks to a smooth surface but not a rough one.

To get more pressure on the ground floor, increase the height H of the tank.

Two vessels with water at the same level: equal pressure at the bottom; the force depends on the bottom area.

True/False: air flows high → low pressure (T); liquids press only at the bottom (F); weather is stormy at the eye (F); a car is safer in a thunderstorm (T).

A boy standing sinks more in sand than when lying down — his weight acts on a smaller area.

💡 Key Learning Points

✓Pressure = Force ÷ Area; its SI unit is N/m², called the pascal (Pa). Smaller area means higher pressure.
✓Liquid pressure increases with the height of the column and acts in all directions — hence overhead tanks and broad dam bases.
✓Air exerts atmospheric pressure in all directions; air moves from high to low pressure, creating winds, sea breeze and land breeze.
✓Rubbing of ice particles and water droplets in clouds separates charges; when air's insulation breaks down, lightning and thunder occur.
✓Cyclones form over warm oceans as low pressure deepens and Earth's rotation spins the air; the calm eye has the lowest pressure, and IMD warnings help keep people safe.

👨‍🏫 Teaching Tips

→Start with Megha and Pawan's bags: let students hold a heavy bag by a thin string and by a broad strap and describe the difference.
→Compare driving a nail with its head vs its point (under supervision) to show that smaller area gives more pressure.
→Set up Activity 6.1 with two pipes and balloons, then keep adding water so students see the bulge grow with height.
→Do the paper plate and chart paper lift (Activity 6.3) — it makes the invisible atmospheric pressure feel real.
→Work through the elephant and two-boat problems on the board step by step, writing units at every step.
→Blow between two hanging balloons and ask students to predict first — most expect them to move apart.
→Use a local weather report or IMD bulletin to discuss thunderstorms and cyclone warnings in your region.
→Practise the lightning safety crouch position and list the dos and don'ts as a class poster.

📋 Assessment Questions

Can the student define pressure, state its SI unit and solve problems like the elephant and boats?
Can the student explain why overhead tanks are high and dam bases are broad?
Can the student explain with an activity that air moves from high pressure to low pressure?
Can the student describe how charges separate in a cloud and cause lightning and thunder?
Can the student explain how a storm becomes a cyclone and describe the eye?

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← Previous: Chapter 5Exploring Forces Next: Chapter 7 →Particulate Nature of Matter