Chapter 6 — Pressure, Winds, Storms, and Cyclones

Chapter 6 is about pressure, winds, storms, and cyclones. It explains how pressure (force per unit area) works in solids, liquids, and air; how air pressure differences create wind; and how winds escalate into thunderstorms, lightning, and cyclones.

Pressure is defined as force per unit area. Its SI unit is newton per metre² (N/m²), which is also called a pascal (Pa). The practical unit of air pressure is the millibar (mb), equal to 100 Pa.

The weight of the bag is the same in both cases, but broad straps spread that force over a larger area of the shoulder, reducing the pressure felt. Narrow straps concentrate the same force on a smaller area, creating higher pressure that causes discomfort.

No. Liquids exert pressure at the bottom of a container, on its sides, and in all directions. Activity 6.2 in the chapter shows water spurting through holes made on the sides of a bottle, confirming that liquids press on container walls as well.

The pressure exerted by a liquid increases with the height of its column. Placing a water tank at a height increases the water column above the taps, which raises the pressure and produces a good stream of water from the taps.

The force exerted by the atmospheric air column over a 15 cm × 15 cm area equals the gravity on a 225 kg object. We are not crushed because the pressure inside our bodies (caused by the movement of fluids and gases in tissues and organs) is equal to the atmospheric pressure, so the inward and outward forces balance each other.

Air moves from regions of high air pressure to regions of low air pressure. When land heats up faster than the sea during the day, the air above land becomes warmer and lighter, rises, and creates a low-pressure area. Cooler high-pressure air from the sea blows in to replace it, generating a sea breeze. The reverse happens at night, producing a land breeze.

High-speed winds are accompanied by a reduced air pressure. When these winds blow over a roof, the air pressure above the roof becomes lower than the air pressure inside the house. If this pressure difference is large enough and the roof is weak, it can be blown away. Keeping doors and windows open during storms lets wind pass through the house and reduces the pressure difference, helping to prevent roof damage.

Strong winds in storm clouds blow upwards and downwards, rubbing ice particles and water droplets against each other. This builds up static electric charges: lighter positively charged ice particles rise to the upper part of the cloud and heavier negatively charged water droplets occupy the lower part. When the charge build-up is very large, the insulating property of air breaks down and a sudden flow of charges produces a bright flash called lightning. It can occur within a cloud, between clouds, or between a cloud and the ground.

Lightning rapidly heats the air around it, causing that air to expand suddenly. This rapid expansion produces a loud sound known as thunder. Because light travels faster than sound, we see the lightning flash before we hear the thunder.

Cyclones form over warm ocean waters. As ocean water heats up, warm moist air rises, creating low pressure. When the water vapour condenses to form raindrops, heat is released back into the atmosphere, warming the ascending air further and lowering the pressure even more. Surrounding air rushes in and rises, and Earth's rotation causes the moving air to spin. This creates a very low-pressure centre with high-speed rotating winds, clouds, and rain — a cyclone. The system weakens once it moves over land and loses its moisture supply.

The eye of a cyclone is the region of lowest pressure at its centre. Winds at the eye are calm, but the area surrounding the eye experiences strong winds and heavy rainfall.

Strong cyclone winds can push ocean water towards the shore, creating a storm surge up to 3–12 metres high that floods coastal and inland areas. Heavy rainfall can cause rivers to overflow and trigger landslides. Seawater flooding farmland can make soil less fertile. Roads may be blocked by fallen trees and debris, and power outages can last for days. For example, Cyclone Amphan in 2020 had peak wind speeds of 270 km/h.

A lightning conductor is a metallic rod installed along the walls of a building. Its pointed end is kept above the highest point of the building and the other end is buried deep in the ground. The rod provides an easy path for electric charges from lightning to flow safely into the ground, protecting the building.

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