Pressure, Winds, Storms and Cyclones

Push a drawing pin into a soft board with your thumb and it slides in easily. Now turn the pin round, press the flat head against the board with exactly the same push, and nothing happens. The force from your thumb has not changed at all. What has changed is the area over which that force acts. That single observation is the whole idea of pressure.

When you push or pull on a surface, the force acting at right angles to that surface is often called the thrust. The effect of a thrust does not depend only on how large it is; it also depends on how much surface it is spread over. Pressure is defined as the thrust acting on unit area:

Pressure = Force (thrust) / Area

In the SI system, force is measured in newton (N) and area in square metre (m²), so pressure comes out in newton per square metre (N/m²). This unit is given a special name, the pascal, written Pa. So a pressure of 1 Pa is what you get when a force of 1 N is spread evenly over an area of 1 m².

Read the relationship slowly, because everything else in this chapter follows from it. If the force stays the same and the area becomes smaller, the pressure becomes larger. If the same force is spread over a bigger area, the pressure becomes smaller. A long list of everyday objects has been designed around exactly this.

Small area on purpose, to get high pressure. The tip of a nail, the point of a pin, the edge of a knife, the tip of an injection needle and the thin blades of ice skates are all made sharp or narrow. A blunt knife and a sharp knife may be pressed on a vegetable with the same force from your hand, but the sharp one squeezes that force on to a tiny strip of area, so the pressure there is enormous and the vegetable gives way. Blunt the edge and the same push is now shared over a much wider strip, the pressure drops, and the vegetable simply gets squashed instead of cut.

Large area on purpose, to get low pressure. School bags are given broad, padded straps rather than thin strings, because the same weight spread across a wider patch of your shoulder presses far less painfully. Tractors and heavy earth-moving machines run on very wide tyres so that they do not sink into soft soil. The foundation of a building is made much wider than the wall it carries, so that the soil underneath is not crushed. A camel crosses soft sand on broad, flat feet, and an elephant of enormous weight stands on wide soles that spread its weight out.

A simple calculation. Suppose a box presses down on the floor with a force of 300 N. If it rests on a face of area 0.5 m², the pressure is 300 / 0.5 = 600 Pa. Tip the very same box on to a face of area 0.1 m² and the pressure becomes 300 / 0.1 = 3000 Pa, which is five times as much. Nothing about the box or its weight changed. Only the area did.

One more everyday check: stand on both feet, then lift one foot. Your weight has not changed, but it is now carried on half the area, so the pressure on the floor has doubled. On a soft mattress or wet sand you can actually see the difference in how deep the mark goes.

A solid block presses downwards on whatever it rests on, and that is all it does. Liquids behave in a far more interesting way. A liquid presses on the bottom of its container, it presses outwards on the side walls, and it presses on any object placed inside it. It pushes in every direction, not only downwards.

An activity you can do at home. Take an empty plastic bottle and make three small holes in a vertical line down one side, one near the top, one in the middle and one near the bottom. Cover the holes with tape, fill the bottle with water and stand it at the edge of a table with a tray below. Now peel off the tape. Water comes out of all three holes, which by itself already proves that the water is pushing sideways against the wall of the bottle. But look carefully at how far each jet travels before it falls. The jet from the lowest hole shoots out the farthest, the middle jet travels less far, and the top jet dribbles almost straight down. The deeper the hole, the harder the water is being pushed out of it.

The reason is easy to picture. At any point inside a liquid, all the liquid lying above that point is pressing down on it. The deeper the point, the taller that column of liquid above it, and so the greater the pressure at that point.

Two further results follow, and both are worth remembering:

• At a given depth, the pressure of a liquid is the same in all directions — downwards, upwards and sideways. That is why the water pushes sideways out of a hole in the wall just as readily as it pushes down on the base.
• The pressure at a given depth depends on the depth and on the liquid itself, because a denser liquid presses harder. It does not depend on the shape of the vessel or on how wide it is.

You can watch the second point in action using a set of containers of different shapes and widths that are all joined together at the bottom. Pour water into any one of them and the water settles at the same level in every container, however differently shaped they are. This is why a transparent tube fixed on the outside of a closed water tank shows you the level inside, and why masons use a long, water-filled clear tube to check that two points at opposite ends of a room are at the same height.

Liquid pressure explains a great deal of practical engineering. A dam is built far thicker at the bottom than at the top, because the water pressure that the wall must hold back is greatest at the base. Taps on the ground floor of a building run stronger than taps on the top floor, and water tanks are placed on the roof so that there is a good depth of water above every tap in the house. A diver going deep into the sea needs a specially built suit, because at great depth the pressure of the water pressing in from all sides would otherwise be more than the body could bear.

Gases have neither a fixed shape nor a fixed volume. Release a gas into a container and it spreads out until it occupies the whole of it. Its tiny particles are always moving about at speed, and they keep striking the walls of the container from the inside. Each collision is a tiny push, and the huge number of such pushes every second adds up to a steady force on every part of the wall. That is gas pressure, and like liquid pressure it acts in all directions.

Blow air into a balloon and watch what happens to its shape. It does not stretch only at the top, or only where you are blowing. It swells outwards on every side and becomes round. The trapped air is pushing outwards equally in all directions, and the rubber keeps stretching until its inward pull balances that outward push. Let go of the neck and the air rushes out, because the pressure inside the balloon is higher than the pressure of the air outside it, and air always moves from higher pressure to lower pressure.

The same effect makes a football firm enough to kick, keeps a cycle tyre hard enough to roll easily over a rough road, and holds up an air mattress under a sleeping person. In each case the object holds its shape because the air inside presses outwards on the whole inner surface. Puncture the tyre and the air escapes, the inside pressure falls until it matches the air outside, and the tyre goes flat.

A gas can also be compressed, that is, squeezed into a smaller space. Take a syringe without its needle, pull the piston back so that it fills with air, then block the nozzle firmly with your finger and push the piston in. It moves in a short way and then becomes very hard to push, and if you release it the piston springs back out. Forcing the same amount of air into a smaller volume raises its pressure. Now empty the syringe, fill it with water, block the nozzle and push again. This time the piston hardly moves at all, because liquids can barely be compressed. That contrast between gases and liquids is worth remembering.

High gas pressure can do useful work, and it can also be dangerous. A pressure cooker traps the steam produced inside it, and the raised pressure allows the water in the cooker to become hotter than it otherwise would, so food cooks faster and less fuel is used. Its weight valve and its separate safety valve exist to let steam escape before the pressure inside climbs too high. An over-inflated balloon or an over-inflated tyre bursts because the pressure inside grows larger than the material can hold. Cooking gas and medical oxygen are stored in thick steel cylinders for the same reason, and such cylinders should never be kept near a flame or in direct sunlight, because warming the gas raises its pressure further.

Air is matter, and like all matter it has weight. The Earth is wrapped in a layer of air called the atmosphere, which stretches many kilometres above our heads. All of that air is being pulled down by gravity, and it presses on every surface underneath it. The pressure exerted by the air of the atmosphere is called atmospheric pressure.

The size of it is surprising. At sea level, the atmosphere presses with roughly 100000 pascal, which is about one lakh newton on every square metre of surface. On the palm of one hand that already adds up to a considerable push. Why then are we not flattened by it? Because the fluids inside our body press outwards with very nearly the same pressure. The inside push and the outside push balance each other, so we feel nothing at all and go about our day unaware of it.

Four simple demonstrations make atmospheric pressure visible.

• The rubber sucker. Press a rubber sucker firmly against a smooth, clean tile and most of the air behind it is squeezed out. Atmospheric pressure now pushes on the outer face of the sucker with almost nothing to balance it from behind, so the sucker clings hard enough to hold a hook and a hanging towel. Lift one edge to let air in behind it, and it falls off at once.
• The inverted glass. Fill a glass to the brim with water, slide a stiff card across the mouth, hold the card in place and turn the glass upside down over a sink. Now take your hand away. The card stays put and the water does not fall out. The air below is pushing up on the card hard enough to support the whole column of water above it.
• Drinking through a straw. You do not really suck the drink upwards. What you do is draw air out of the straw, lowering the air pressure inside it. The atmosphere, pressing down on the surface of the drink in the glass, then pushes the liquid up the straw and into your mouth.
• The crushed can. Boil a little water in a thin metal can so that the steam drives most of the air out, close the lid tightly and then pour cold water over the can. The steam inside condenses back to a few drops of water, the pressure inside falls sharply, and the atmosphere outside crushes the can inwards with a loud crumple. Nothing pushed the can from the inside. The outside push simply stopped being balanced.

There is also a famous old experiment in which two metal bowls were joined rim to rim and the air was pumped out of the space between them. Teams of horses harnessed on either side and pulled in opposite directions could not drag them apart, yet the moment air was let back in, the two bowls came away easily.

Pressure falls as you go higher. The higher you climb, the less air there is left above you, so the atmospheric pressure is lower. This is why your ears feel blocked and then pop on a winding hill road, why a sealed packet of chips carried from the plains puffs up in the mountains, and why water boils at a lower temperature high up so that rice and dal take longer to cook. Climbers going very high carry oxygen cylinders, because the thin air up there supplies too little oxygen for hard physical work.

Atmospheric pressure is measured with an instrument called a barometer. In a mercury barometer, the atmosphere supports a column of mercury inside a closed tube, and at sea level that column stands about 76 cm high. Weather scientists watch barometer readings closely, because a marked fall in pressure over a region generally means unsettled, stormy weather is on its way. That link between falling pressure and storms is exactly what the rest of this chapter is about.

Wind is simply air on the move. Air flows from a region where the pressure is higher towards a region where the pressure is lower, in exactly the way air rushes out of a punctured tyre into the lower-pressure room around it. So to explain any wind at all, you only have to find out what created the pressure difference. Almost every time, the answer is uneven heating.

When a parcel of air is heated it expands. The same amount of air now spreads through a larger volume, so it becomes less dense than the cooler air around it. Notice what that does and does not mean: the warm air has not lost any weight, but a given volume of it now weighs less than the same volume of the cool air beside it, so the denser cool air pushes in underneath and lifts it. The warm air therefore rises, and cooler, denser air from the surroundings moves in sideways to take its place. That sideways movement is the wind you feel. You can watch the pattern with a simple test: hold a lighted incense stick near the top of a slightly open door of a warm room, and then near the bottom of the same door. The smoke drifts in opposite directions in the two positions, showing warm air leaving above and cooler air flowing in below.

The bigger the pressure difference between two places, the faster air rushes between them. A small difference gives a pleasant breeze. A very large difference gives the violent winds of a storm.

Land and water heat unequally. Land warms up quickly in sunshine, and it also cools down quickly once the Sun sets. Water takes far longer to warm up and far longer to cool down. This difference, repeated every single day along every coast, produces two familiar winds.

• Sea breeze. During the day the land becomes hotter than the sea. The air above the land is heated, it expands and rises, and the pressure over the land falls. Cooler air from over the sea then flows in towards the land. That is why a coastal town gets a refreshing wind blowing in from the sea in the afternoon.
• Land breeze. At night the land cools down faster than the sea, so now the sea is the warmer surface. The air rises over the sea instead, and the wind blows the other way, from the land out towards the sea. Fishermen have used this daily reversal for centuries, sailing out on the land breeze and returning on the sea breeze.

The same idea works on a far bigger scale and over a whole season. In summer the huge landmass of India heats up much more than the ocean around it. Air rises over the heated land, pressure there falls, and moisture-laden winds are drawn in from the sea. These are the monsoon winds, and the rain they bring is what our farming depends on. In winter the situation reverses, the land is now cooler than the sea, and the winds blow outwards from the land towards the ocean.

On the largest scale of all, the region near the equator receives the Sun's heat most directly through the year. The air there is strongly heated and rises, and cooler air from regions far to the north and far to the south flows in towards the equator to replace it. Because the Earth is spinning, these enormous moving masses of air do not travel in straight lines. They are bent sideways, which is why the great wind belts of the world blow along slanting paths instead of running straight north to south.

A thunderstorm needs two ingredients that Indian summers supply generously: heat and moisture. On a hot, humid day the ground warms the layer of air just above it. That warm, moist air rises rapidly in a strong upward current. As it climbs it cools, and the water vapour it carries condenses into tiny droplets. That is how the tall, dark, heavy-topped clouds you see gathering in the afternoon sky are built.

Condensation is the key step, because when water vapour condenses it releases heat into the air around it. That extra warmth pushes the air higher still, so the cloud grows taller and the upward currents grow stronger. High up inside the cloud the temperature is below freezing, and ice particles form there. At the same time, heavier drops and ice fall back down through the rising air. The result is a cloud in which air, water drops and ice are rushing violently upwards and downwards at once, and this churning is what separates electric charges within the cloud.

When enough charge has built up, the air can no longer keep the separated charges apart. A large electric discharge suddenly jumps between one part of the cloud and another, or between the cloud and the ground. That flash is lightning. The discharge heats the narrow channel of air along its path so violently and so suddenly that the air expands explosively, and the shock of that expansion reaches our ears as the crack and long rumble of thunder. Lightning and thunder are two effects of one event.

You always see the flash before you hear the sound, because light travels enormously faster than sound. Sound covers roughly one kilometre in about three seconds, so if you count about six seconds between the flash and the thunder, the discharge was around two kilometres away. If that gap keeps shrinking, the storm is moving towards you and it is time to get indoors.

Staying safe in a thunderstorm. Lightning is genuinely dangerous, and the rules for avoiding it are simple and worth knowing by heart.

• Get inside a building if you possibly can, or into a car or bus with the windows rolled up. A closed vehicle is a safe place; an open one is not.
• Do not shelter under a single tall tree standing by itself in a field, and stay away from poles, towers, metal fences and machinery.
• Get off open ground, playgrounds and terraces, and get out of water. Do not swim or bathe, and do not stand in a puddle or a flooded field.
• If you are caught in the open with no shelter at all, do not lie flat. Crouch low with your feet together, your hands on your knees and your head tucked down, so that you are touching the ground over as small an area as possible.
• Indoors, do not bathe during the storm, avoid touching metal taps and pipes, and stay away from corded telephones and appliances plugged into the mains. Unplug what you reasonably can before the storm reaches you.
• Tall buildings are protected by a lightning conductor — a pointed metal rod fixed above the roof, joined by a thick metal strip to a plate buried in the moist earth below. It offers the discharge an easy path straight into the ground instead of through the walls of the building.

A cyclone is a storm system grown to an enormous size, and it is born over the warm ocean. The chain of events runs like this.

1. Strong sunshine heats the surface water of the sea, and a great deal of water evaporates. The air just above the sea becomes warm and heavily loaded with water vapour.
2. This warm, moist air rises. As it climbs it cools, and the vapour condenses into droplets, forming clouds.
3. Condensation releases heat, warming the air around it. The warmed air rises faster still, and the pressure near the sea surface drops further.
4. Air from all around rushes in towards this low-pressure centre. On the way it picks up more moisture from the sea, and it too rises.
5. The cycle feeds itself and keeps repeating. The winds spiral inwards faster and faster, and a huge whirling system of cloud and wind takes shape. That is a cyclone.

Seen from a weather satellite, a mature cyclone looks like a vast spiral of cloud with a clear hole punched through the middle. That calm centre is the eye of the cyclone. Inside the eye the winds are light, there is little or no rain, and the sky may even clear for a while; it is a region a few tens of kilometres across. Wrapped around the eye is a thick wall of towering cloud where the winds are strongest and the rain heaviest. This is exactly why the sudden calm that arrives in the middle of a cyclone is a trap. The far wall of the storm, with winds now blowing from the opposite direction, has still to pass over.

The same kind of storm is given different names in different parts of the world. It is called a cyclone in our region of the Indian Ocean, a hurricane over the Atlantic, and a typhoon over the western Pacific. India is affected on both of its coasts, and the coastal districts along the Bay of Bengal are struck most often.

A cyclone does its damage in three ways at once. The wind uproots trees, tears off roofs, and brings down electric poles and communication lines. The rain is torrential and floods low-lying land. Most dangerous of all is the storm surge, a mound of seawater driven on to the shore by the force of the wind. The surge drowns coastal villages and fields, and the salt it leaves behind ruins standing crops and spoils the wells and ponds that people drink from.

A tornado is a different kind of storm, smaller and much shorter-lived: a dark, funnel-shaped column of rapidly whirling air that hangs down from a storm cloud and may touch the ground, sucking up dust, water and light objects along its narrow track. Tornadoes are not common in India.

Warnings save lives. Weather satellites and radars watch developing storms continuously, and the India Meteorological Department issues alerts and warnings to coastal districts ahead of a cyclone striking. Getting that warning to people quickly, and getting people to act on it instead of waiting, is a large part of the reason cyclones cost far fewer lives today than they once did.

Cyclone safety measures.

• Listen to official warnings on radio, television or an official mobile alert, and act on them straight away. Do not rely on rumours or on forwarded messages, and do not spread them.
• Keep an emergency kit ready: drinking water, dry food, a torch, spare batteries, essential medicines and important papers in a waterproof bag.
• Move to a cyclone shelter or a strong pucca building when you are told to. Take your animals along if you can, and help neighbours who cannot move quickly.
• Store clean drinking water in advance, and after the storm drink only water you are sure is safe, because flooded wells and broken pipes are easily contaminated.
• Stay indoors during the storm, and do not go out when the wind suddenly drops, because the eye may be passing over and the winds will return.
• After the storm, keep away from broken electric wires, damaged buildings and flooded roads, and check on elderly neighbours and small children.