Chapter 6

Pressure, Winds, Storms, and Cyclones

Fallen leaves swirl in the air, doors slam, windows rattle, clothes flutter in a strong wind — all effects of the force wind exerts, which creates wind pressure. This chapter explores the relationship between force and pressure, and how they shape powerful natural events like thunderstorms and cyclones.

6.1 Pressure

Two people carrying equally heavy bags can feel very differently loaded: a bag with narrow straps digs into the shoulder, while one with broad straps feels comfortable, even though both weigh the same. The weight of the narrow-strap bag acts on a smaller area of the shoulder, while the broad-strap bag's weight spreads over a larger area. Since the area over which a force acts matters, we define pressure as force per unit area: Pressure = Force ÷ Area. The same reasoning explains why a bucket with a broad handle is easier to lift than one with a narrow handle, and why people carrying head-loads often place a round cloth pad underneath — both increase the area the force acts over, reducing pressure.

The SI unit of force is the newton and of area is metre², so the SI unit of pressure is newton/metre² (N/m²), also called a pascal, denoted Pa. A force of 100 N applied over an area of 2 m² gives a pressure of 100 N ÷ 2 m² = 50 N/m². This is also why it's easier to drive a nail in point-first than head-first, and easier to cut with a knife's sharp edge than its blunt one: a smaller area concentrates the same force into a much higher pressure.

Overhead water tanks are always placed at a height: filling two pipes of different width with water to the same level produces the SAME size of balloon-bulge at an outlet at the bottom of each, regardless of the pipes' different diameters — showing that a liquid's pressure depends on the height of its column, not the amount of liquid or the width of its container. Raising the water column higher increases the bulge further, confirming that pressure increases with column height; this is exactly why tanks are elevated, to increase the pressure (and so the flow) at the taps below. Liquids also press on the sides of their container, not just the bottom — a bottle with holes punched around its base leaks water from every hole once unsealed, showing that liquids exert pressure in all directions, not only downward.

6.2 Pressure Exerted by Air

The envelope of air surrounding the Earth is called the atmosphere, made mostly of nitrogen and oxygen with smaller amounts of argon, carbon dioxide, and other gases, extending many kilometres above the surface. Covering an inverted paper plate with a larger sheet of chart paper makes it noticeably harder to lift with a stick than covering it with a small folded sheet, even though the sheet's weight barely changes — showing that air exerts a force on the covering sheet that grows with the area covered. Since force per unit area is pressure, this means air exerts pressure on every object around us; this is called atmospheric pressure. Blowing air into a balloon inflates it in every direction, confirming air pressure acts in all directions, not just one.

Pressing a rubber sucker onto a smooth surface pushes out most of the air between the cup and the surface, lowering the pressure inside the sucker below the pressure of the air outside; the outside air pressure is then what holds the sucker firmly in place, and pulling it off requires enough force to overcome that pressure difference. Atmospheric pressure is surprisingly large: the force exerted by the air column over just a 15 cm × 15 cm patch is nearly equal to the weight of a 225 kg mass (about 2250 N) — we aren't crushed by it because the pressure inside our own bodies, maintained by the movement of fluids and gases through our tissues and organs, balances the pressure pushing in from outside.

6.3 Formation of Wind

Wind blows strongly on some days and stays calm on others. Connecting an inflated balloon to an uninflated one with a straw shows air flowing from the inflated (higher-pressure) balloon into the uninflated (lower-pressure) one, until both reach nearly the same size and the same pressure — air moves from a region of high pressure to a region of low pressure, and flow stops once the pressure difference disappears. This is exactly the mechanism behind sea and land breezes (studied in Grade 7): during the day, land heats faster than water, so the warmer air above the land rises, creating a low-pressure area that pulls in air from the higher-pressure sea, producing a sea breeze; at night, the sea stays warmer than the land, reversing the pressure difference and producing a land breeze instead.

6.4 High-Speed Winds Result in Lowering of Air Pressure

Blowing air into the narrow gap between two hanging balloons makes them swing together rather than apart, and blowing harder pulls them together faster — because the moving air between them creates a region of lower pressure there, and the higher pressure surrounding the balloons pushes them inward. The same effect explains why high-speed winds during a storm can tear roofs off houses: fast wind passing over a roof lowers the pressure just above it, and if the pressure difference between the air below the roof and above it becomes large enough, the roof can be blown away. This is why it's safer to keep doors and windows open during high-speed winds: letting the wind move through the house reduces the pressure difference between inside and above the roof.

6.5 Storms, Thunderstorms, and Lightning

When land heats up, the warm, moist air above it rises, creating a low-pressure area; cooler air from surrounding higher-pressure regions rushes in to take its place, gets heated in turn, and rises too, setting up a continuous cycle of wind circulation. As the rising air expands, it cools, and its moisture condenses into water droplets, forming clouds; these droplets merge into heavier drops that fall as rain, hail, or snow. Strong winds accompanied by rain make up a storm, more frequent in hot, humid, tropical regions like India. Under the right conditions, warm air rises to heights cold enough to turn water droplets into ice particles.

Strong winds blowing upwards and downwards rub the resulting ice particles and water droplets against each other, and — just as rubbing two objects together charges them, as covered in the "Exploring Forces" chapter — this builds up static electric charge inside the clouds. Lighter, positively charged ice particles rise to occupy the upper part of the cloud, while heavier, negatively charged water droplets settle in the lower part, creating a charge separation; as the negative lower part drifts closer to the ground, it makes the ground and nearby objects (trees, buildings) become positively charged. Air is normally an electrical insulator that keeps opposite charges apart, but once the built-up charge is large enough, the air's insulating property breaks down and a sudden flash of built-up charge occurs — called lightning. Lightning rapidly heats the surrounding air, making it expand suddenly and produce a loud sound called thunder. A storm accompanied by lightning and thunder is a thunderstorm.

Lightning can ignite fires, damage buildings, and cause severe burns or death. During lightning, the safest response is to stay away from tall objects, find a low-lying open area and crouch down (never lie flat), avoid an umbrella with a metallic rod, get out of any water you're in, and know that being inside a bus or car is comparatively safer.

6.6 Cyclone

Cyclones are large storms that form over warm ocean waters. As the ocean heats up, warm moist air rises above it, and its water vapour condenses into raindrops; since evaporation absorbs heat, condensation releases that heat back into the atmosphere, warming the ascending air further and making it rise even more, creating an even lower pressure. Air from surrounding regions rushes in and starts rising too, and the Earth's rotation causes this moving air to spin — repeating this cycle builds a very low-pressure area with high-speed winds revolving around it: a spinning system of clouds, winds, and rain called a cyclone.

The region of lowest pressure, at the very centre of a cyclone, is the eye of the cyclone — calm at its centre, even though the surrounding region experiences strong winds and heavy rainfall. A cyclone generates higher wind speeds as it moves from ocean to land than an ordinary thunderstorm does, but once it reaches land and loses its supply of moist ocean air, it gradually weakens.

Even while weakening over land, a cyclone can leave a trail of destruction taking months or years to repair; the 2020 Amphan cyclone had peak wind speeds of 270 km/h. Strong cyclone winds can push a wall of ocean water 3–12 metres high onto the shore, flooding coastal areas and areas far from the sea; the accompanying heavy rainfall can overflow rivers and trigger landslides. Seawater rushing inland can contaminate drinking water and reduce soil fertility, hurting crops; fallen trees and debris can block roads and delay help, and power outages during a cyclone can last for days. Staying updated through India Meteorological Department (IMD) weather reports and alerts, tracking cyclones via weather satellites, keeping an emergency kit ready, and moving quickly to a designated cyclone shelter when needed are all important protective measures in cyclone-prone areas.