Class 8 › Science › Chapter 6
Pressure, Winds, Storms, and Cyclones
Pressure as force per unit area, pressure due to liquids and air, how pressure differences create winds, and how this leads to storms, thunderstorms, lightning and cyclones.
1. Pressure
Pressure
$$\text{Pressure} = \dfrac{\text{Force}}{\text{Area}}$$
SI unit
newton per square metre (N/m²) = pascal (Pa)
- Here only forces acting perpendicular to the surface are considered.
- Example: force 100 N on area 2 m² gives pressure = 100 ÷ 2 = 50 N/m² (50 Pa).
- Same force, smaller area → greater pressure; larger area → smaller pressure.
Reduce pressure
Increase area
Broad bag straps; broad bucket handles; a round cloth under a load carried on the head; the broad base of a dam; a lying boy sinks less in sand than a standing boy.
Increase pressure
Decrease area
The pointed end of a nail drives in easily; the sharp edge of a knife cuts an apple more easily than the blunt edge.
2. Pressure Exerted by Liquids
Balloon-and-pipe activity
Two pipes of different widths, same water height, rubber balloons on the ends: both balloons bulge equally, though the weight of water differs. So it is the height of the water column, not the amount, that matters.
Adding more water (greater height) makes the balloon bulge more.
- Pressure of a liquid depends on the height of its column (not on the shape or width of the container).
- Overhead tanks are placed high so the water pressure at the taps is greater, giving a good stream. A tap on a lower floor (greater depth below the tank) gets a stronger stream.
- Side holes in a bottle: water spurts from all holes, so liquids exert pressure on the walls too. Liquids exert pressure in all directions (also why water spurts from leaking pipes).
- Dam: the base is broader because horizontal water pressure is very large near the bottom.
3. Pressure Exerted by Air
Atmosphere: the envelope of air around the Earth (nitrogen, oxygen, argon, CO₂ and others), extending many kilometres above the surface. Atmospheric pressure: the pressure exerted by this air on all objects.
- Paper-plate activity: a plate covered with unfolded chart paper is harder to lift than one covered with the folded sheet. The weight is the same, so air exerts a force that grows with the area, i.e. air exerts pressure.
- Air exerts pressure in all directions: an inflated balloon expands in all directions; if its mouth is open, air escapes from high pressure inside to low pressure outside.
- Rubber sucker: pressing it pushes out air and lowers the pressure inside, so the higher outside air pressure holds it on a flat smooth surface.
- How large is it? The atmosphere's force on an area of 15 cm × 15 cm is about 2250 N, equal to the force of gravity on a 225 kg object.
- Why we are not crushed: the pressure inside our body (due to fluids and gases in tissues and organs) equals the atmospheric pressure, so they balance.
- Units: practical unit is millibar (mb) = 100 Pa. Hectopascal (hPa) = 100 Pa, so 1 mb = 1 hPa.
4. Formation of Wind
Rule
Air moves from a region of high pressure to a region of low pressure. This moving air is wind.
- Two-balloon straw activity: air flows from the inflated (higher pressure) balloon to the other until the pressures are equal, then flow stops and both are about the same size.
- The greater the pressure difference, the faster the wind.
- Warm air is lighter, so it rises and leaves a low-pressure area; cooler air from high-pressure surroundings rushes in.
Daytime: land heats faster, air rises, so wind blows from sea to land.
| Sea breeze | Land breeze | |
|---|---|---|
| When | Day | Night |
| Warmer surface | Land (air above is warm, lighter, rises: low pressure) | Sea (low pressure above the sea) |
| Direction | Sea to land | Land to sea |
5. High-Speed Winds Lower Air Pressure
- Two hanging balloons: blowing between them makes them move towards each other because blowing creates a low-pressure region between them and the higher pressure outside pushes them inward. Blowing harder brings them together faster.
- Conclusion: high-speed winds are accompanied by reduced air pressure.
- Roofs in storms: fast wind over a roof lowers the pressure above it, so the higher pressure below can lift or blow away a weak roof.
- Safety: keeping doors and windows open during a storm reduces the pressure difference between inside and above the roof and helps prevent roofs being blown off.
6. Storms, Thunderstorms and Lightning
How a thunderstorm forms
1. Heated land warms moist air, which is lighter and rises, creating low pressure; cooler air flows in and also gets heated: continuous wind circulation.
2. Rising air expands and cools; moisture condenses into droplets and forms clouds; droplets merge and fall as rain, hail or snow. Strong winds with rain = storm (frequent in hot, humid, tropical regions like India).
3. High up, cold temperatures turn droplets into ice particles. Strong up and down winds make droplets and ice particles rub, producing static charges (as learnt in Exploring Forces).
4. Lighter positively charged ice particles go to the top of the cloud; heavier negatively charged droplets stay at the bottom: charge separation. The negative cloud base makes the ground and objects (trees, buildings) below positively charged.
5. Air is normally an insulator. When the charge build-up becomes very large, air's insulating property breaks down and charge flows suddenly: lightning (bright flash). It can be within a cloud, between clouds, or between cloud and ground.
6. Lightning rapidly heats the air, which expands and makes the loud sound called thunder. A storm with lightning and thunder is a thunderstorm.
- Requirements: moisture and strong winds.
- Local thunderstorms in India: Kalboishakhi (West Bengal, Bihar, Jharkhand) and Bordoisila (Assam) come before the monsoon and help kharif crops; mango showers (Kerala, Karnataka, Tamil Nadu) help mangoes ripen; local storms in Karnataka help coffee plants.
7. Lightning Safety
Do
Stay safe
Stay away from tall objects; go to a low-lying open area and crouch down, minimising contact with the ground; get out of water; a bus or car is comparatively safer.
Do not
Avoid
Do not lie flat on the ground; avoid an umbrella with a metal rod.
- Dangers: lightning can start fires, damage buildings and cause severe burns or death.
- Lightning conductor: a metal rod fixed along the building's wall, with its pointed end above the highest point and the other end buried deep in the ground. It gives charges an easy path into the ground and protects the building.
8. Cyclones
Cyclone: a large spinning system of clouds, winds and rain formed over warm ocean waters, with a very low-pressure centre and high-speed winds revolving around it.
Formation
1. Warm ocean water heats the air above it; warm, moist air rises.
2. Rising vapour condenses into raindrops, releasing heat (the heat water took in during evaporation). This warms the rising air further, so it rises even higher and the pressure falls even lower.
3. Air from surrounding regions rushes in and also rises. Earth's rotation makes the moving air spin. The cycle repeats and a very low-pressure area with fast winds develops.
- Eye of the cyclone: the centre, with the lowest pressure; the wind there is calm, while the surrounding region has strong winds and heavy rain.
- Over the ocean it gains strength; on reaching land the moist air supply is cut off and it gradually weakens.
- Impact: Amphan (2020) had peak winds of 270 km/h. A storm surge (wall of water 3 to 12 m high) floods coasts and beyond; heavy rain causes floods and landslides; seawater contaminates drinking water and makes soil less fertile; fallen trees block roads; power cuts last for days.
- Protection: follow IMD (India Meteorological Department) alerts and warnings; satellites help track cyclones and predict their path; keep an emergency kit ready; move to a designated cyclone shelter.
9. Quick Revision
| Concept | Key point |
|---|---|
| Pressure | Force ÷ area; unit Pa (N/m²) |
| Liquid pressure | Depends on height of the liquid column; acts in all directions |
| Atmospheric pressure | Exerted by air in all directions; balanced by pressure inside our bodies |
| Wind | Air moving from high to low pressure |
| Fast wind | Accompanied by low pressure |
| Lightning | Charge separation in clouds, then breakdown of air's insulation |
| Cyclone | Spinning system over warm ocean, lowest pressure and calm wind in the eye |
Exam tip
Pressure numericals
Find the total area in contact before dividing (e.g. an elephant on four feet: area = 4 × area of one foot). Pressure = weight ÷ total area.
Remember
Cause chain
Warm air rises, low pressure, cool air rushes in, clouds, rain, charge separation, lightning, thunder.
Common mistakes:
• Saying liquid pressure depends on the width or amount of liquid (it depends on the height of the column).
• Saying liquids exert pressure only at the bottom (also on the sides, in all directions).
• Forgetting to use the total contact area when finding pressure.
• Saying fast wind has higher pressure (it has lower pressure).
• Saying the eye of a cyclone is stormy (it is calm, with the lowest pressure).
• Mixing up the charges: lighter ice particles are positive (top); heavier droplets are negative (bottom).
• Lying flat on the ground during lightning (crouch low instead).
• Saying liquid pressure depends on the width or amount of liquid (it depends on the height of the column).
• Saying liquids exert pressure only at the bottom (also on the sides, in all directions).
• Forgetting to use the total contact area when finding pressure.
• Saying fast wind has higher pressure (it has lower pressure).
• Saying the eye of a cyclone is stormy (it is calm, with the lowest pressure).
• Mixing up the charges: lighter ice particles are positive (top); heavier droplets are negative (bottom).
• Lying flat on the ground during lightning (crouch low instead).