Pressure, Winds, Storms, and Cyclones
Chapter Notes — Science Class 8 (Old NCERT)
1. Introduction
Why can a gentle breeze one day become a raging storm the next?
Every time wind slams a door, swirls leaves or bends trees, it shows the invisible role of air pressure and moving air. Understanding what causes pressure and how changes in pressure create winds, storms and cyclones helps us explain many weather events and their effects on daily life.
In this chapter, you will learn how pressure and air movement shape weather, from gentle breezes to destructive cyclones, and why these processes matter.
2. What Is Pressure?
Pressure is defined as the force applied per unit area.
Formula: Pressure = Force / Area
Only the component of force acting perpendicular to the surface, called the normal force, is considered when calculating pressure.
Everyday Examples of Pressure
Carrying bags: Two bags of the same weight feel different when their straps have different widths. Narrow straps concentrate force on a smaller area, producing higher pressure and discomfort. Broad straps spread force over a larger area, producing lower pressure.
Bucket handles: A broad handle is easier on the hand because it reduces pressure for the same force.
Carrying loads on head: A round piece of cloth placed under a load increases the area of contact, reducing pressure and making load carrying comfortable.
Nails: Using the pointed end of a nail is easier because the small area of the tip produces greater pressure, allowing the nail to pierce wood or wall with less effort.
Knife's edge: A sharp knife cuts easily because its thin edge has a smaller area, producing greater pressure for the same force. A blunt knife has a larger contact area and is less effective.
Overhead water tanks: Placing a water tank at a height increases the pressure in water pipes due to the height of the water column.
3. Units of Pressure
SI unit of force: newton (N)
SI unit of area: square metre (m²)
SI unit of pressure: newton per square metre (N/m²), also called the pascal (Pa).
Worked Examples
4. Pressure Exerted by Liquids
Water Column Activity: Height Dependence
Suppose two pipes of different diameters have balloons tied to their lower ends. If both pipes are filled with water up to the same height, both balloons bulge equally, despite differing quantities and weights of water.
Key Principle: Liquid pressure at a point depends on the height of the liquid column above that point, provided liquid density, temperature, and gravity remain constant.
Effect of Height: A taller water column increases pressure at the bottom, causing balloons to bulge more. This is why overhead water tanks are elevated.
Pressure on Container Walls & Base
When small holes are made near the bottom of a water bottle, water squirts sideways.
Sideways Pressure: Liquids exert pressure not only downwards but also sideways on container walls.
Direction: Liquid pressure acts in all directions at a given depth.
Why Are Dams Broad at the Base?
Depth Relationship: Water pressure increases with depth because the height of the water column above becomes greater.
Dam Design: Dams are built broad and thick at the base to withstand the immense horizontal pressure exerted by deep water.
5. Pressure Exerted by Air
What Is Atmospheric Pressure?
The layer of air surrounding Earth is called the atmosphere (composed mainly of nitrogen and oxygen). Atmospheric pressure is the pressure produced by the weight of the air column above a surface, along with the motion of air molecules.
Does Air Exert Pressure?
Chart Paper Activity: Lifting an inverted paper plate is harder when covered by an unfolded chart paper (large area) than a folded one (small area), proving that air force increases with surface area.
Balloons: Blowing air into a balloon inflates it as air pushes outwards in all directions. Opening the mouth allows high-pressure air to rush out until pressure equals external air.
Rubber Suckers: When pressed against a smooth surface, air beneath is removed. External atmospheric pressure holds the sucker firmly against the surface.
How Strong Is Atmospheric Pressure?
Sea Level Pressure: About 101,325 Pa (1,013 hPa / 1,013 mb).
Force on Area: On an area of 15 cm × 15 cm (0.0225 m²), atmospheric force is ~2,250 N (equivalent to ~225 kg mass).
Why We Aren't Crushed: Fluids and gases inside our body exert outward pressure that balances the external atmospheric pressure.
6. Formation of Wind
Wind is the movement of air caused by pressure differences. Air always moves from high-pressure regions to low-pressure regions. The larger the pressure difference, the stronger the wind.
Sea Breeze vs. Land Breeze
7. High-Speed Winds and Lowered Pressure
Fast-moving air creates a region of reduced air pressure. Surrounding higher-pressure air pushes toward the low-pressure zone.
Balloons Experiment: Blowing air between two suspended inflated balloons lowers pressure between them; higher outer pressure pushes them together.
Roof Lifting: Strong winds blowing over roofs reduce pressure above. High pressure inside the building can lift or blow off weak roofs.
8. Storms, Thunderstorms and Lightning
Formation of Storms
Heating & Rising: Sunlight warms land and moist air. Warm, moist air rises, leaving low pressure near the surface.
Circulation & Cloud Formation: Cooler air moves in and warms up. As rising air cools at high altitudes, water vapour condenses into cloud droplets.
Precipitation & Storm: Heavy droplets fall as rain, hail, or snow accompanied by strong winds.
Thunderstorms & Lightning Mechanism
Drafts & Charging: Updrafts and downdrafts collide ice particles and droplets. Friction charges particles: positive charges rise to cloud top, negative charges settle at bottom.
Lightning: Negative cloud base induces positive ground charge. When electric field breaks air insulation, a massive spark passes — lightning.
Thunder: Lightning rapidly heats air, causing violent expansion and a loud acoustic wave (thunder).
Regional Names & Safety
Regional Names (India): Kalboishakhi (West Bengal, Bihar, Jharkhand), Bordoisila (Assam), Mango Showers (South-West states). Beneficial for crops like mangoes and coffee.
Safety Precautions: Avoid tall isolated trees/poles; crouch low (do not lie flat); avoid metallic umbrellas; exit water bodies immediately; stay inside vehicles.
Lightning Conductor: Pointed metallic rod on building roofs wired to ground, safely discharging lightning strikes into Earth.
9. Cyclone
A cyclone is a large rotating storm system formed over warm ocean waters, characterized by very low surface pressure, intense winds, and torrential rain.
How Cyclones Form & Structure
Warm Ocean Fuel: Moist air over warm oceans rises, creating a low-pressure area.
Latent Heat Release: Condensation releases latent heat, further warming air, lowering pressure, and accelerating air inflow.
Coriolis Effect: Earth's rotation causes inflowing winds to spin into a spiral structure.
Eye & Eyewall: The Eye is the calm, low-pressure center. The Eyewall surrounding it contains the strongest winds and heaviest rain.
Landfall: Moving over land cuts off warm moist air fuel, causing the cyclone to weaken.
Hazards & Safety
Storm Surge: High water wall (3–12 metres) pushed ashore, causing severe coastal flooding.
Other Hazards: Heavy flooding, inland landslides, seawater contamination of freshwater/soil, destruction of infrastructure.
Safety Measures: Follow India Meteorological Department (IMD) warnings, maintain emergency kit, evacuate to cyclone shelters.
10. Key Terms to Remember
Pressure: Force exerted per unit area.
SI Unit of Pressure: Newton per square metre (N/m²) or pascal (Pa).
Pressure in Fluids: Liquids and gases exert pressure on container walls and in all directions at a given depth.
Atmospheric Pressure: Pressure exerted by the weight of the air atmosphere surrounding Earth.
Cause of Winds: Pressure differences driving air from high-pressure to low-pressure regions.
Thermal Air Movement: Warm air rises (low pressure); cool air sinks/inflows (high pressure).
Thunderstorm Prerequisites: Moisture, rising warm air, and powerful vertical updrafts/downdrafts.
Cloud Electrification: Frictional collisions between ice crystals and water droplets in clouds.
Lightning: Sudden electric discharge between charged cloud layers or cloud and ground.
Lightning Conductor: Roof-mounted metallic rod that provides a grounded path for lightning current.
Cyclone: Low-pressure rotating oceanic storm with eye, eyewall, and spiral wind bands.
IMD: India Meteorological Department — official national agency monitoring storms and issuing weather advisories.
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