Pressure, Winds, Storms, and Cyclones

1. What this chapter is about

You have seen fallen leaves swirl up, doors slam, windows rattle and clothes flutter when a strong wind blows. The wind clearly pushes on things — it exerts a force. This chapter builds from that everyday idea to something powerful: how the simple relationship between force, area and pressure explains thunderstorms and cyclones. The journey is:

• 6.1 Pressure — force spread over an area.
• 6.2 Pressure exerted by air — the atmosphere pushes on everything.
• 6.3 Formation of wind — air moving from high to low pressure.
• 6.4 High-speed winds lower air pressure — roofs, hoardings.
• 6.5 Storms, thunderstorms and lightning.
• 6.6 Cyclone — and how to stay safe.

2. Pressure — force per unit area (6.1)

Megha and Pawan carry equally heavy bags, yet Pawan's bag with narrow straps hurts his shoulders while Megha's bag with broad straps feels comfortable. The weight (force) is the same, but the narrow straps press on a smaller area, while broad straps spread the same weight over a larger area. So what matters is not just the force, but the force shared over each unit of area. We call this quantity pressure:

(At this stage we only consider forces acting perpendicular to the surface.) Smaller area → higher pressure; larger area → lower pressure, for the same force. This single idea explains a lot of daily life:

• Buckets / loads: a broad handle or a round cloth pad under a head-load spreads the weight over a larger area, reducing pressure on the hand or head — so it feels lighter to carry.
• Sharp vs blunt: the pointed end of a nail and the sharp edge of a knife have tiny areas, so they produce very high pressure and cut or pierce easily; the head of the nail or blunt edge has a large area and low pressure.

3. Unit of pressure and a worked value

The SI unit of force is the newton (N) and of area is the metre² (m²). Therefore the SI unit of pressure is newton/metre² (N/m²), also called the pascal (Pa).

A step further: in weather science the pascal is small, so we use the millibar (mb) and the hectopascal (hPa), both equal to 100 Pa.

4. Liquids exert pressure — and it grows with height

Do liquids also exert pressure? Activity 6.1 says yes. Two transparent pipes of different diameters have rubber balloons tied at the bottom and are filled with water to the same level. Both balloons bulge to the same extent — even though the pipes hold different amounts of water. So the weight of water is not what causes the bulge; it is the pressure of the water column, which depends on the height of the column, not its width.

Pour more water into one pipe (Fig. 6.6) and the balloon bulges more. Conclusion: liquid pressure increases as the height of the column increases.

• Overhead water tanks (Fig. 6.4) are placed at a height so the tall water column gives a high pressure and a good, strong stream from the taps. A tap on a lower floor (with a taller column above it) gets a more powerful stream than a tap on a higher floor.
• Activity 6.2 — a bottle with four holes at the same height near the bottom: when filled with water, it squirts out of all sides, showing liquids exert pressure not only at the bottom but on the side walls too — in all directions.
• Ever heard of… a dam (Fig. 6.8): the base of a dam is made much broader than the top because water pressure acts horizontally on the walls and is very large near the bottom (greatest height of water above it). The broad base withstands this large pressure.

5. Air exerts pressure — atmospheric pressure (6.2)

The envelope of air surrounding the Earth is the atmosphere (mostly nitrogen and oxygen, with argon, carbon dioxide and other gases), extending many kilometres up. Does this air push on things?

• Activity 6.3 — an inverted paper plate on a stick is covered first with a folded chart paper, then with an unfolded (larger-area) sheet. Lifting the plate is harder with the larger sheet, even though the sheet's weight is unchanged. Air pushes down on the sheet, and the push (force) increases with area — force per unit area is pressure, so air exerts pressure. The pressure exerted by air around us is called atmospheric pressure.
• Activity 6.4 (the sucker): pressing a rubber sucker on a smooth surface pushes most of the air out, lowering the pressure inside. The higher outside air pressure holds it firmly to the surface, so it is hard to pull off.

Why aren't we crushed? The atmospheric air column over an area of $15\,\text{cm}\times15\,\text{cm}$ presses with a force nearly equal to the weight of a $225\,\text{kg}$ mass (about $2250\,\text{N}$). We are not crushed because the pressure inside our body (from fluids and gases in our tissues) is equal to the atmospheric pressure, balancing the outside push.

6. Formation of wind (6.3)

When an inflated balloon's mouth is left open, air rushes out; when a tube is punctured, air escapes and it collapses. In both cases air moves from a region of high pressure to a region of low pressure. Activity 6.5 proves it: connect an inflated balloon (high pressure) to an uninflated one (low pressure) through a straw — air flows from the inflated to the uninflated balloon until both are the same size (pressures equal). So:

The bigger the pressure difference, the faster the air moves — that is why winds are stronger on some days than others.

Sea breeze and land breeze are everyday examples. During the day, land heats up faster than water; the warm air over land rises (low pressure), and cooler air from over the sea blows in to take its place — a sea breeze (sea → land). At night, water stays warmer than land; low pressure forms over the sea, so wind blows from land to sea — a land breeze (land → sea). Both arise from pressure differences over land and sea.

7. High-speed winds lower air pressure (6.4)

Activity 6.6 — hang two inflated balloons with a small gap and blow air between them. Instead of pushing apart, they move towards each other. The fast-moving air between them lowers the pressure there, and the higher pressure outside pushes the balloons together. Blow harder and they come together faster. Conclusion: high-speed winds are accompanied by reduced air pressure.

• Roofs blown off (Fig. 6.14): high-speed wind over a roof lowers the pressure above it; the higher pressure below pushes the roof up, and a weak roof gets blown away. Keeping doors and windows open during such storms lets wind flow through, reducing the inside-vs-top pressure difference and protecting the roof.
• Holes in banners and hoardings: holes let wind pass through, reducing the pressure difference across the banner so it is less likely to be torn or knocked down.

8. Storms, thunderstorms and lightning (6.5)

When land gets heated, warm moist air (being lighter) rises, creating low pressure; cooler air from surrounding high-pressure areas rushes in, gets heated and rises too — a continuous wind circulation. As the rising air expands and cools, its moisture condenses into water droplets, forming clouds. The droplets merge into heavier drops that fall as rain, hail or snow. Strong winds accompanied by rain make a storm. (Storms are more frequent in hot, humid, tropical regions like India. The two key requirements for thunderstorms are moisture and strong winds.)

How lightning forms: strong winds blowing up and down (Fig. 6.15) rub water droplets and ice particles against each other, building up static electric charges in the cloud. Lighter, positively charged ice particles gather at the top of the cloud; heavier, negatively charged water droplets gather at the bottom — this is charge separation. The negative lower part also makes the ground and nearby objects (trees, buildings) positively charged. Normally air is an insulator, but when the charge build-up becomes very large, the air's insulating property breaks down; a sudden flow of charges produces a bright flash called lightning. Lightning rapidly heats the surrounding air, which expands and produces a loud sound — thunder. A storm with lightning and thunder is a thunderstorm. Lightning can occur within a cloud, between clouds, or between a cloud and the ground.

Ever heard of… Local pre-monsoon thunderstorms have regional names: Kalboishakhi (West Bengal, Bihar, Jharkhand), Bordoisila (Assam), and mango showers in Kerala, Karnataka and Tamil Nadu, which help ripen mangoes; Karnataka storms also help coffee plants grow.

9. Lightning safety + lightning conductor

Lightning can ignite fires, damage buildings and cause severe burns or death. NCERT precautions during lightning:

• Stay away from tall objects (lone trees, poles).
• Find a low-lying open area and crouch down, minimising contact with the ground. Do not lie down flat.
• Avoid an umbrella with a metallic rod.
• If you are in water, get out of it.
• Inside a bus or car, you are comparatively safer.

Ever heard of… a lightning conductor (Fig. 6.18): a metallic rod fixed along a building's wall, its pointed top kept above the highest point of the building and its other end buried deep in the ground. It provides an easy path to safely carry electric charges into the earth, protecting the building.

10. Cyclone (6.6)

Cyclones are large storms that form over warm ocean waters. The cycle:

• Warm ocean water heats the air above it; the warm, moist air rises.
• As it rises and cools, water vapour condenses into raindrops, releasing heat (the heat taken up earlier during evaporation).
• This released heat warms the ascending air further, so it rises even more, creating an even lower pressure below.
• Air from surrounding regions rushes in and also starts rising; the Earth's rotation makes the moving air spin.
• Repeated, this builds a very low-pressure area with high-speed winds, clouds and rain spinning around it — a cyclone.

The centre, of lowest pressure, is the eye of the cyclone — calm here, but the surrounding region has strong winds and heavy rain. A cyclone produces much higher wind speeds than ordinary thunderstorms. Once it moves over land, its source of moist air is cut off and it gradually loses strength — but still leaves a trail of destruction taking months or years to repair. For example, the Amphan cyclone (2020) had peak winds of about 270 km/h.

Damage caused: strong winds push ocean water ashore as a wall of water (storm surge) 3–12 metres high, flooding coastal and inland areas; heavy rain overflows rivers and triggers landslides; seawater rushing inland contaminates drinking water and makes farm soil salty and less fertile; roads get blocked by debris; power outages last for days.

Safety during cyclones: stay updated on weather reports, alerts and warnings from the India Meteorological Department (IMD); weather satellites let us track cyclones and predict their path. If you live in a cyclone-prone area, keep an emergency kit ready and, during a cyclone, quickly move to a designated cyclone shelter.

11. The chain of events (NCERT "Let us wrap up")

• Warm air rises → creates a low-pressure area.
• Cool air rushes in to fill the low-pressure area.
• Rising warm air cools → water vapour condenses → clouds form.
• Bigger drops fall as rain, hail or snow.
• Strong up-and-down winds create positive and negative charges in clouds.
• When charges meet → lightning (within a cloud, between clouds, or cloud-to-ground).
• Under certain conditions, storms develop into cyclones.

12. Common mistakes to avoid

• Mixing up force and pressure — for the same force, smaller area means more pressure (not less).
• Thinking liquid pressure depends on the amount/width of water — it depends on the height of the column.
• Saying air moves from low to high pressure — it always moves high → low.
• Forgetting the eye of a cyclone is calm (lowest pressure), while the region around it is violent.
• During lightning, lying down flat or sheltering under a lone tall tree — both are dangerous; crouch low in the open instead.
• Confusing units: 1 mb = 1 hPa = 100 Pa (not 1 Pa).

13. Quick revision checklist

• Pressure = Force ÷ Area; unit N/m² = pascal (Pa).
• Smaller area → higher pressure (sharp knife, pointed nail).
• Liquids and gases push in all directions; liquid pressure rises with column height.
• Air pressure = atmospheric pressure; balanced by pressure inside our body.
• Wind = air flowing high → low pressure; bigger difference → faster wind.
• High-speed wind lowers nearby pressure (roofs, hoardings, balloons-together).
• Thunderstorm needs moisture + strong winds; charge separation → lightning → thunder.
• Cyclone = spinning low-pressure storm over warm seas; calm eye at centre; tracked by IMD.