Chapter 6 • Class 8 Science
⛈️ Pressure, Winds,
Storms & Cyclones
Storms & Cyclones
Curiosity — Textbook of Science for Grade 8
📋 Table of Contents
6.1 What is Pressure?
@edugrown
Fig: Narrow strap hurts
more than broad strap!
more than broad strap!
Same weight bag — narrow strap hurts more than broad strap. Why?
- Narrow strap → force on smaller area → more pressure 😣
- Broad strap → force spread on larger area → less pressure 😊
- More area = less pressure for same force
- Less area = more pressure for same force
📌 Formula
Pressure = Force ÷ Area
SI Unit: Newton/metre² = N/m² also called Pascal (Pa)
Example: 100 N force on 2 m² area → Pressure = 50 N/m²
Example: 100 N force on 2 m² area → Pressure = 50 N/m²
Pressure in Daily Life!
Knife's sharp edge → less area → more pressure → easy to cut!Nail's pointed end → less area → more pressure → easy to drive in!
Knife's blunt edge → more area → less pressure → hard to cut!
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6.2 Liquid Pressure
@edugrown
Fig: Equal water heights →
equal balloon bulge
equal balloon bulge
Liquids exert pressure in all directions — bottom, sides, and walls!
- Pressure depends on height of liquid column (not width)
- More height → more pressure at bottom
- Same height → same pressure (regardless of pipe diameter!)
- Liquids also push on walls of containers
📌 Key Concept
Why Overhead Water Tanks are Placed High?
More height → more pressure → stronger water flow from taps! 🚰
Person on ground floor gets more pressure than person on top floor.
More height → more pressure → stronger water flow from taps! 🚰
Person on ground floor gets more pressure than person on top floor.
@edugrown
Fig: Water shoots from
holes in all directions
holes in all directions
Make holes in bottle sides at same height → water shoots out equally from all holes!
- Proves liquids push on sides of container too
- Liquids exert pressure in ALL directions
- Holes at greater depth → water shoots out faster (more pressure)
Why is Dam Base Broader?
Water in dam exerts more pressure near the bottom (more depth). A broader base can withstand this large horizontal pressure. That's why dams are always wider at the base! 💪
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6.2 Pressure Exerted by Air
Air surrounds Earth — this envelope of air = Atmosphere. It contains nitrogen, oxygen, argon, CO₂ etc. and extends many km above Earth.
- Air exerts pressure on all objects around us
- Balloon inflates → air inside pushes outward in all directions
- Sucker sticks to wall → atmospheric pressure outside > pressure inside
- Air pressure over 15×15 cm area = weight of 225 kg object! 😲
- We're not crushed because body pressure equals atmospheric pressure
📌 Key Definition
Atmospheric Pressure → The pressure exerted by air around us on all objects.
Units: N/m² = Pa | Practical unit: millibar (mb) or hectopascal (hPa) = 100 Pa
Units: N/m² = Pa | Practical unit: millibar (mb) or hectopascal (hPa) = 100 Pa
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6.3 Formation of Wind
@edugrown
Fig: Air moves from
high → low pressure
high → low pressure
Key Rule: Air always moves from High Pressure → Low Pressure region!
- Balloon air escapes → from high (inside) to low pressure (outside)
- Punctured cycle tube → same reason!
- Greater pressure difference → faster air movement = stronger wind!
🌊 Sea Breeze vs Land Breeze
| Breeze | Time | What Happens? | Wind Direction |
|---|---|---|---|
| 🌊 Sea Breeze | Daytime | Land heats faster → warm air rises → low pressure on land | Sea → Land |
| 🏔️ Land Breeze | Night | Sea is warmer → warm air rises → low pressure over sea | Land → Sea |
📌 Core Concept
Wind = Air moving from a region of High Pressure to Low Pressure.
Bigger the pressure difference → Stronger the wind! 💨
Bigger the pressure difference → Stronger the wind! 💨
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6.4 High-Speed Winds → Lower Air Pressure
- Blow air between two hanging balloons → balloons come closer!
- Why? Blowing creates low pressure area between balloons
- Higher surrounding pressure pushes them together
- High-speed winds = Lower air pressure in that region!
@edugrown
Fig: High speed wind
blows roof away!
blows roof away!
@edugrown
Fig: Open doors/windows
— roof stays safe!
— roof stays safe!
High-speed wind over roof → low pressure above roof → high pressure inside house pushes roof UP → roof may get blown away!
Safety Tip: Keep doors & windows OPEN during storms! Wind passes through house → pressure equalizes → roof stays safe! ✅
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6.5 Storms, Thunderstorms & Lightning
🌧️ How Storm Forms
@edugrown
Fig: Warm air rising
creates storm
creates storm
1Land heats up → warm moist air rises → low pressure area
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2Cool air from high pressure areas rushes in → gets heated → rises again
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3Rising air cools → water vapour condenses → forms clouds & rain
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4Strong winds + rain = STORM!
⚡ How Lightning Forms
@edugrown
Fig: Charge separation
in storm clouds
in storm clouds
1Strong winds blow ice particles & water droplets → rubbing → static charges build up
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2+ve (lighter ice) → top of cloud | −ve (heavier drops) → bottom
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3Charge build-up breaks air's insulation → sudden discharge = Lightning ⚡
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4Lightning heats air rapidly → air expands → loud sound = Thunder 💥
📌 Key Definitions
Lightning = Sudden flow of charges → bright flash of light
Thunder = Sound produced when lightning rapidly heats air
Thunderstorm = Storm + Lightning + Thunder together ⛈️
Thunder = Sound produced when lightning rapidly heats air
Thunderstorm = Storm + Lightning + Thunder together ⛈️
⚡ Lightning Safety Rules!
- Stay away from tall objects (trees, poles)
- Find a low-lying open area and crouch down
- Do NOT use metallic umbrella
- Get out of water immediately
- Inside a bus or car = comparatively safer
- Do NOT lie flat on ground
Lightning Conductor!
A metallic rod on buildings with one pointed end higher than the building and other end buried in ground. Provides easy path for charges to flow safely into ground. Protects the building! ⚡→🏠✅
Local Thunderstorms in India!
Kalboishakhi (West Bengal, Bihar, Jharkhand) | Bordoisila (Assam) | Mango Showers (Kerala, Karnataka, Tamil Nadu) — help ripen mangoes! ☔🥭
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6.6 Cyclone
Cyclones form over warm ocean waters. They are large, spinning storms with very low pressure at the centre.
🌀 How Cyclone Forms — Step by Step
1Warm ocean water heats air → warm moist air rises → low pressure
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2Water vapour condenses → releases heat → air rises even more → pressure drops further
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3Air from surrounding areas rushes in → Earth's rotation makes it spin 🌀
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4Creates very low pressure centre with high-speed winds revolving around it = CYCLONE!
@edugrown
Fig: Cyclone structure
@edugrown
Fig: Eye of cyclone
📌 Key Concept
Eye of the Cyclone = Centre of cyclone where pressure is LOWEST.
At the eye → wind is calm! Surrounding region → extremely strong winds + heavy rain.
At the eye → wind is calm! Surrounding region → extremely strong winds + heavy rain.
💥 Damage Caused by Cyclones
- Wind speeds up to 270 km/h (Amphan cyclone, 2020)
- Wall of water 3–12 metres high → coastal flooding
- Rivers overflow → landslides
- Seawater contaminates drinking water, destroys farmland
- Power outages for days, roads blocked
🛡️ Cyclone Safety Tips!
- Stay updated with IMD (India Meteorological Department) alerts
- Keep emergency kit ready
- Move to nearest cyclone shelter quickly
- Satellites help track & predict cyclone path today! 🛰️
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📸 Quick Revision Snapshots
Pressure = Force ÷ Area. Unit = N/m² = Pascal (Pa). More area → less pressure.
Liquids exert pressure in all directions. Pressure increases with depth. Overhead tanks placed high for more pressure.
Atmospheric Pressure = pressure exerted by air on all objects. Air exerts pressure in all directions.
Wind = air moving from High Pressure → Low Pressure. Greater difference = stronger wind.
High-speed winds → lower air pressure. Open doors during storms to prevent roof being blown away.
Storm = strong winds + heavy rain. Moisture + strong winds needed for thunderstorm formation.
Lightning = sudden discharge of static charges in clouds. Thunder = sound from rapidly heated air.
Cyclone = large spinning storm over warm oceans with very low pressure at Eye (centre). IMD monitors them.
✨ Notes by @edugrown — Happy Learning! ✨