Pressure, Winds, Storms, and Cyclones Class 8: Pressure, Air Pressure, Lightning and Cyclones

What Pressure, Winds, Storms, and Cyclones is about

Key concepts

Pressure: force per unit area

NCERT section 6.1 Pressure

Two bags of the same weight feel different: the one with narrow straps hurts because its weight acts on a smaller area of the shoulders. So we use pressure, the force acting per unit area (here we consider forces acting perpendicular to the surface):

$P=\frac{F}{A}$

• SI unit: newton per square metre (N/m²), called the pascal (Pa).
• 100 N on a cardboard of area 2 m² gives $\frac{100}{2}=50$ Pa.

Same force, smaller area, more pressure: a pointed nail goes in easily; a sharp knife cuts an apple easily (Table 6.1).

Same force, larger area, less pressure: broad straps and broad bucket handles feel comfortable; people carrying loads on their heads place a round cloth under them; lying on sand makes you sink less than standing.

Worked example

A school bag weighing 60 N rests on the shoulders through two straps. Find the pressure if the total strap area is (a) 0.0024 m² (narrow straps), (b) 0.006 m² (broad straps). Which is more comfortable?

Try it first, then show the solutionSolution

1. (a) $P=\frac{60}{0.0024}=25000$ Pa.
2. (b) $P=\frac{60}{0.006}=10000$ Pa.
3. Broad straps give less than half the pressure, so they are more comfortable.

Answer: 25,000 Pa and 10,000 Pa; broad straps.

Pressure in liquids

NCERT section 6.1 Pressure

Activity 6.1: two pipes of different widths, each with a balloon tied at the bottom, are filled with water to the same height. Both balloons bulge equally, although the wide pipe holds more (heavier) water. So it is the height of the water column, not the total weight, that sets the pressure at the bottom. Adding more water to one pipe makes its balloon bulge more: liquid pressure increases with the height (depth) of the column.

Activity 6.2: a bottle with four holes at the same height near the bottom squirts water out of all four holes: liquids press on the sides too. In fact, liquids exert pressure in all directions.

Applications:

• Overhead tanks are placed high so that the water column above the taps is tall and the taps get a strong flow; a lower floor (more height below the tank) gets a stronger stream.
• Water spurts out of holes and leaky joints in pipes because of the pressure on the walls.
• Dams have a much broader base than top, because water pressure on the wall is greatest near the bottom.

Worked example

An overhead tank is on the roof of a three-storey building. Will a tap on the first floor or the second floor give a stronger stream? Explain.

Try it first, then show the solutionSolution

1. Liquid pressure depends on the height of the water column above the tap.
2. The first-floor tap is further below the tank, so the column above it is taller.
3. So the first-floor tap has higher pressure and a stronger stream.

Answer: The first-floor tap, because the water column above it is taller.

Air pressure (atmospheric pressure)

NCERT section 6.2 Pressure Exerted by Air

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

Activity 6.3: an inverted paper plate with a stick is covered first with a folded chart sheet, then with the same sheet unfolded. It is harder to lift with the unfolded (larger) sheet, although the sheet's weight is the same: air presses down on the sheet, and the force grows with the area. So air exerts pressure: the atmospheric pressure.

• Air in a balloon presses on its walls in all directions, so it expands all round.
• Activity 6.4, the sucker: pressing a rubber sucker on a smooth surface pushes out most of the air inside; the higher air pressure outside holds it on, and you must pull hard to remove it. (On a rough surface, air leaks in and it does not stick.)
• Size of atmospheric pressure: the air presses on an area of 15 cm by 15 cm with a force of about 2,250 N, like the weight of a 225 kg mass. We are not crushed because the pressure inside our bodies (from fluids and gases in tissues) balances it.
• Practical units for air pressure: millibar (mb) and hectopascal (hPa); $1\text{ mb}=1\text{ hPa}=100$ Pa.

Worked example

The air presses on an area of 15 cm by 15 cm with a force of about 2,250 N. Find the atmospheric pressure in pascal and in millibar.

Try it first, then show the solutionSolution

1. Area $=0.15\times 0.15=0.0225$ m².
2. $P=\frac{2250}{0.0225}=100000$ Pa.
3. In millibar: $100000÷100=1000$ mb.

Answer: About 1,00,000 Pa, that is about 1,000 mb.

How winds form

NCERT section 6.3 Formation of Wind

Activity 6.5: an inflated balloon and an empty balloon are joined by a straw. Air flows from the inflated balloon (higher pressure) to the empty one (lower pressure) until both are about the same size and the pressures are equal; then the flow stops.

So air moves from a region of high pressure to a region of low pressure; moving air is wind. The greater the pressure difference, the faster the air moves. A punctured tyre tube and a balloon left open show the same thing.

Sea and land breezes:

• By day, land heats faster than the sea; air over land warms, becomes lighter and rises, leaving low pressure over land; cooler air from the sea (high pressure) blows in: a sea breeze.
• At night, the sea is warmer; low pressure forms over the sea and air blows from land to sea: a land breeze.

Worked example

On a summer afternoon at the coast, which way does the breeze blow? Explain using pressure.

Try it first, then show the solutionSolution

1. In the afternoon the land is hotter than the sea.
2. Air over the land warms, rises and leaves low pressure; the air over the sea is cooler with higher pressure.
3. Air moves from high to low pressure, so the breeze blows from the sea to the land (a sea breeze).

Answer: From sea to land (sea breeze).

High-speed winds lower the air pressure

NCERT section 6.4 High-Speed Winds Result in Lowering of Air Pressure

Activity 6.6: two inflated balloons hang 6 to 10 cm apart. Blowing air into the gap makes them move towards each other, faster when you blow harder. Fast-moving air between them has lower pressure, and the higher pressure of the still air outside pushes them together.

So high-speed winds are accompanied by reduced air pressure.

Roofs in storms: a fast wind over a roof lowers the pressure above it, while the air inside the house is still at higher pressure. If the difference is large and the roof is weak, the roof is pushed up and blown off. Keeping doors and windows open lets wind pass through, reducing the pressure difference and helping keep the roof in place.

Similarly, blowing over a strip of paper held below your lips makes it rise.

Worked example

During a storm with very strong winds, a tin roof is lifted off a closed shed. Explain why, and how the owner could have reduced the risk.

Try it first, then show the solutionSolution

1. Fast wind over the roof lowers the air pressure above it.
2. The air inside the closed shed stays at higher pressure, so there is a net upward push on the roof.
3. Opening doors and windows lets air flow through, reducing the pressure difference; fixing the roof firmly also helps.

Answer: Low pressure above, higher below; open windows to reduce the difference.

Storms, thunderstorms and lightning

NCERT section 6.5 Storms, Thunderstorms, and Lightning

How storms form: heated land warms the moist air above it; this air rises, creating low pressure; cooler air from high-pressure surroundings moves in, warms and rises too, keeping a circulation going. Rising air expands and cools; its water vapour condenses into droplets, forming clouds; droplets merge and fall as rain, hail or snow. Strong winds with rain make a storm; storms are frequent in hot, humid, tropical places like India.

Charges in clouds: warm air may rise so high that droplets freeze into ice. Strong up and down winds rub water droplets and ice particles together, building up static charges (as rubbing does in Chapter 5). Lighter positively charged ice particles collect at the top of the cloud and heavier negatively charged droplets at the bottom; the negative cloud base makes the ground, trees and buildings below positively charged.

Lightning: air normally insulates, but when the charge build-up is very large, air's insulation breaks down and charges flow suddenly, giving a bright flash: lightning, within a cloud, between clouds, or between a cloud and the ground. It heats the air rapidly; the air expands with a loud sound, thunder. A storm with lightning and thunder is a thunderstorm.

Local thunderstorms before the monsoon: *Kalboishakhi* (West Bengal, Bihar, Jharkhand), *Bordoisila* (Assam), and 'mango showers' (Kerala, Karnataka, Tamil Nadu).

Worked example

Explain in order how charges separate in a thunder cloud and how this leads to lightning and thunder.

Try it first, then show the solutionSolution

1. Strong up and down winds rub water droplets and ice particles, charging them.
2. Light positive ice particles rise to the top; heavy negative droplets gather at the bottom; the ground below becomes positive.
3. When the build-up is very large, air stops insulating and charges rush across: lightning.
4. Lightning heats the air suddenly; the air expands violently, making the sound of thunder.

Answer: Rubbing separates charges; a huge build-up breaks down air's insulation (lightning); heated air expands (thunder).

Lightning safety and lightning conductors

NCERT section 6.5 Storms, Thunderstorms, and Lightning

Lightning can start fires, damage buildings and cause severe burns or death.

During lightning:

• stay away from tall objects such as isolated trees and poles;
• in the open, find a low-lying area, crouch down and keep contact with the ground small; do not lie flat;
• do not use an umbrella with a metal rod;
• get out of water;
• inside a bus or car you are comparatively safe.

Lightning conductor: a metal rod fixed along a building's wall during construction, with a pointed top above the highest point of the building and the other end buried deep in the ground. It gives the charges an easy path into the ground, protecting the building.

Why lightning needs insulating air: if air and clouds conducted well, charges would leak away gradually and the huge build-up needed for lightning would not occur.

Worked example

You are walking in an open field when a thunderstorm begins. A tall, lonely tree is nearby. What should you do and why?

Try it first, then show the solutionSolution

1. Do not shelter under the tall tree: lightning tends to strike tall objects.
2. Move to a low-lying open area, crouch down with feet together and keep contact with the ground small; do not lie flat.
3. Put away any metal-rod umbrella, and move into a car or bus or building if one is near.

Answer: Avoid the tree; crouch low in a low-lying area; seek a vehicle or building.

Cyclones

NCERT section 6.6 Cyclone

Cyclones are huge storms that form over warm ocean water:
1. Warm, moist air over the sea rises.
2. Its water vapour condenses into rain, releasing heat (the heat taken up during evaporation).
3. This heat warms the rising air further, so it rises more, making the pressure even lower.
4. Air rushes in from surrounding regions and also rises; Earth's rotation makes the moving air spin.
5. Repeating this builds a very low-pressure centre with high-speed winds revolving around it: a spinning system of clouds, winds and rain.

The centre, the eye, has the lowest pressure and is calm; around it are the strongest winds and heaviest rain. Over the sea a cyclone grows stronger than ordinary thunderstorms; over land it loses its source of moist air and weakens gradually.

Damage: very strong winds (cyclone Amphan in 2020 had peak winds of about 270 km/h); a storm surge, a wall of sea water 3 to 12 m high, flooding coasts; heavy rain causing floods and landslides; salt water spoiling drinking water and farmland; fallen trees blocking roads; long power cuts.

Protection: follow warnings from the India Meteorological Department (IMD); satellites track cyclones and predict their paths; keep an emergency kit; move quickly to a designated cyclone shelter.

Worked example

On a weather map, the pressure at the centre of a cyclone is 994 mb and at the outer edge 1008 mb. Find the difference in pascal, say which way the air moves, and describe the weather at the centre.

Try it first, then show the solutionSolution

1. Difference $=1008-994=14$ mb $=14\times 100=1400$ Pa.
2. Air moves from high pressure (edge) towards low pressure (centre), spinning because of Earth's rotation.
3. At the eye (centre) the wind is calm; the strongest winds and heaviest rain surround it.

Answer: 1,400 Pa; air moves inwards towards the low-pressure eye, which is calm.

Science in real life: Pressure, Winds, Storms, and Cyclones

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Pressure: Force per Unit Area, Liquid Pressure and Atmospheric Pressure
• Winds, Thunderstorms, Lightning and Cyclones

Pressure, Winds, Storms, and Cyclones: formulas and key facts

Pressure

$P=\frac{F}{A}$

Unit of pressure

$1\text{ Pa}=1{\text{ N/m}}^2$

Air pressure units

$1\text{ mb}=1\text{ hPa}=100\text{ Pa}$

Force from pressure

$F=P\times A$

Liquid pressure

$\text{increases with the height (depth) of the liquid column}$

Wind

$\text{air flows from high pressure}\to \text{low pressure}$

Fast air

$\text{higher wind speed}\Rightarrow \text{lower pressure}$

Atmospheric pressure (approx.)

$\approx 1,00,000\text{ Pa}\approx 1000\text{ mb}$

Pressure, Winds, Storms, and Cyclones: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Pressure, Winds, Storms, and Cyclones

The question patterns to expect, and the method that works for each.

Tricks to remember Pressure, Winds, Storms, and Cyclones

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Small area, big pressure

  Sharp knife, pointed nail: same push, smaller area, bigger pressure.

  Use it when: Pressure situations.

• Height, not width

  Liquid pressure cares how tall the column is, not how wide.

  Use it when: Activity 6.1 and tanks.

• High to low, like water downhill

  Air 'flows downhill' from high pressure to low pressure.

  Use it when: Wind formation.

• Fast air, low pressure

  Blow between balloons and they hug: fast air is low-pressure air.

  Use it when: Roofs and storms.

• Plus on top, minus below

  Thundercloud: light positive ice up top, heavy negative drops below.

  Use it when: Lightning diagram.

• Calm eye, angry wall

  The cyclone's eye is calm; the ring around it is fierce.

  Use it when: Cyclone structure.

Pressure, Winds, Storms, and Cyclones: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Pressure, Winds, Storms, and Cyclones

The slips examiners see most often, and what to write instead.

• ✗Mistake: Not converting cm² to m²

  Areas given in cm² are used directly.

  Wrong working

  Force 20 N on 1 cm²: $P=20$ Pa.

  Right working

  $1{\text{ cm}}^2=0.0001$ m², so $P=\frac{20}{0.0001}=2,00,000$ Pa.

  ✓Do this: Convert areas to m² before dividing.

  Usually costs: Answer mark.

• ✗Mistake: Thinking more water gives more pressure at the same height

  Weight of water is confused with pressure.

  Wrong working

  The wide pipe has more water, so its balloon bulges more.

  Right working

  At the same height both balloons bulge equally; pressure depends on height, not the amount of water.

  ✓Do this: Look at the column height.

  Usually costs: 1 mark.

• ✗Mistake: Saying wind blows from low to high pressure

  Direction is reversed.

  Wrong working

  Air rushes from the low-pressure land to the high-pressure sea by day.

  Right working

  By day air blows from the sea (high pressure) to the land (low pressure).

  ✓Do this: Air always moves from high to low pressure.

  Usually costs: 1 mark.

• ✗Mistake: Describing the eye as the stormiest part

  The centre is assumed to be the worst.

  Wrong working

  At the eye of the cyclone, winds are strongest.

  Right working

  The eye is calm with the lowest pressure; the strongest winds and rain are around it.

  ✓Do this: Eye: calm and lowest pressure.

  Usually costs: 1 mark in true/false items.

• ✗Mistake: Unsafe lightning advice

  Shelter is confused with height.

  Wrong working

  Stand under a tall tree or lie flat on the ground during lightning.

  Right working

  Avoid tall objects, crouch low in a low-lying area without lying flat, or get into a car or building.

  ✓Do this: Learn the safety list exactly.

  Usually costs: 1-2 marks.

NCERT exercise guide: Pressure, Winds, Storms, and Cyclones

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Pressure, Winds, Storms, and Cyclones

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   1 millibar equals
   (a) 1 Pa
   (b) 10 Pa
   (c) 100 Pa
   (d) 1000 Pa

   Hint

   Same as hPa.

   Answer

   (c) 100 Pa

2. Q2 · 1 mark · MCQExam level

   At the eye of a cyclone the weather is
   (a) stormy with heavy rain
   (b) calm
   (c) always snowy
   (d) very windy

   Hint

   Lowest pressure.

   Answer

   (b) calm

3. Q3 · 1 mark · Very short answerExam level

   Why are knives sharpened?

   Hint

   Smaller area.

   Answer

   A sharp edge has a very small area, so the same force produces a much larger pressure, making cutting easy.

4. Q4 · 2 marks · Very short answerExam level

   Find the pressure when a force of 250 N acts on an area of 5 m².

   Hint

   Divide.

   Answer

   $\frac{250}{5}=50$ Pa

5. Q5 · 2 marks · Very short answerHarder exam level

   Why does water come out of all four holes at the same height near the bottom of a bottle?

   Hint

   Direction of liquid pressure.

   Answer

   Water exerts pressure on the walls of the container in all directions; at the same depth the pressure is the same, so water spurts out of all the holes.

6. Q6 · 2 marks · Very short answerHarder exam level

   A pressure of 1,000 Pa acts on a window of area 2 m². What force acts on the window?

   Hint

   $F=P\times A$.

   Answer

   $1000\times 2=2000$ N

7. Q7 · 3 marks · Short answerHarder exam level

   Using a punctured cycle tube or a balloon, explain how you would show a younger student that air moves from a region of higher pressure to one of lower pressure.

   Hint

   Air rushing out.

   Answer

   Inflate a balloon (or use a pumped-up tube) and let its mouth open near the student's hand: air rushes out because the pressure inside is higher than outside, and it stops when the pressures become equal. This shows air moving from high to low pressure.

8. Q8 · 2 marks · Very short answerHarder exam level

   Why does a rubber sucker stick to a smooth tile but not to a rough wall?

   Hint

   Air leaking in.

   Answer

   On a smooth tile pressing pushes out most of the air inside, so outside air pressure holds it on. On a rough wall air leaks back in through the gaps, so the pressures equalise and it falls.

9. Q9 · 2 marks · Very short answerHarder exam level

   Why are local pre-monsoon thunderstorms like mango showers useful?

   Hint

   Crops.

   Answer

   They bring rain before the monsoon, helping kharif crops, ripening mangoes and growing coffee plants in some regions.

10. Q10 · 5 marks · Long answerHarder exam level

    A girl weighing 450 N wears flat sandals with a total contact area of 0.03 m², and later heels with a total contact area of 0.009 m². (a) Find the pressure in each case. (b) How many times greater is the second? (c) Which is more likely to sink into soft mud and why? (d) Suggest a footwear choice for a wet field trip.

    Hint

    $P=\frac{F}{A}$.

    Answer

    (a) $\frac{450}{0.03}=15000$ Pa; $\frac{450}{0.009}=50000$ Pa. (b) $\frac{50000}{15000}\approx 3.3$ times. (c) The heels, because the same weight acts on a smaller area, giving more pressure. (d) Broad, flat shoes or boots that spread the weight.

11. Q11 · 3 marks · Short answerHOTS / competency

    Two inflated balloons hang 8 cm apart. Predict what happens if you blow hard between them and what happens if you blow on the outside of one balloon. Explain both.

    Hint

    Fast air has low pressure.

    Answer

    Blowing between them lowers the pressure in the gap, so the higher outside pressure pushes them together. Blowing on the outside of one lowers the pressure there, so that balloon is pushed outwards (away from the other).

12. Q12 · 3 marks · Short answerHOTS / competency

    During a thunderstorm, why should you not take shelter under an isolated tree, and why is it safer to crouch rather than lie flat?

    Hint

    Tall objects and ground contact.

    Answer

    Lightning tends to strike tall objects, so a lone tree is dangerous. Crouching keeps you low while touching the ground as little as possible; lying flat increases contact with the ground, where the current from a nearby strike can spread.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Define pressure.Show answer

   Force acting per unit area.

2. 2.What is the SI unit of pressure?Show answer

   Pascal (Pa), equal to N/m².

3. 3.On what does the pressure of a liquid at the bottom of a column depend?Show answer

   On the height (depth) of the liquid column.

4. 4.Why is the base of a dam broad?Show answer

   To withstand the large water pressure near the bottom.

5. 5.Why are we not crushed by atmospheric pressure?Show answer

   The pressure inside our bodies balances the atmospheric pressure.

6. 6.How is 1 millibar related to the pascal?Show answer

   1 mb = 100 Pa (same as 1 hPa).

7. 7.In which direction does air move?Show answer

   From a region of high pressure to a region of low pressure.

8. 8.What happens to air pressure where wind speed is high?Show answer

   It is reduced.

9. 9.What is the eye of a cyclone?Show answer

   The calm central region of lowest pressure.

10. 10.What is a lightning conductor?Show answer

    A pointed metal rod on a building, buried in the ground at its lower end, giving charges an easy path to the ground.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you calculate pressure in pascal with areas in cm²?
• ☐Can you explain why broad straps and sharp knives work?
• ☐Can you explain why both balloons bulge equally in Activity 6.1?
• ☐Can you explain why taps lower down get a stronger flow?
• ☐Can you show that air exerts pressure with a paper plate or a sucker?
• ☐Can you explain sea and land breezes with pressure?
• ☐Can you explain why balloons move together when you blow between them?
• ☐Can you describe charge separation in a cloud?
• ☐Can you list lightning safety rules?
• ☐Can you explain how a cyclone forms and why its eye is calm?

Pressure, Winds, Storms, and Cyclones: frequently asked questions