Pressure Winds Storms — Exercises

Atmospheric pressure is the force exerted by the weight of the column of air above a unit area of the Earth's surface. At sea level, it is about 1 atm ≈ 10⁵ Pa, equivalent to a 76 cm column of mercury in a Torricelli barometer. We do not feel crushed by it because the pressure inside our body is equal to the pressure outside, so the forces balance.

A barometer is an instrument used to measure atmospheric pressure. In Torricelli's mercury barometer, a glass tube closed at one end is filled with mercury and inverted into a dish of mercury. The height of the mercury column (about 76 cm at sea level) tells us the atmospheric pressure directly.
A modern aneroid barometer uses a small evacuated metal capsule that expands or contracts as pressure changes; a needle displays the reading on a dial. Barometers are used for weather forecasting — a rapidly falling reading often warns of an approaching storm. They are also used as altimeters in aircraft and on mountains.

Water tanks are placed on tall stands so that water can flow easily to taps on every floor of a building by the force of gravity alone — no pump is needed. The height of the water column produces a pressure at the taps equal to \( P = h \rho g \), where \(h\) is the height of water above the tap, \(\rho\) is water's density and \(g\) is acceleration due to gravity. The greater the height \(h\), the greater the pressure at the tap and the stronger the flow of water. A tank placed at ground level would give very weak (or zero) flow on upper floors.

Pressure = Force ÷ Area.
Boat A: Force = 2 × 700 = 1400 N; Pressure = 1400 ÷ 7 = 200 Pa.
Boat B: Force = 3 × 700 = 2100 N; Pressure = 2100 ÷ 3.5 = 600 Pa.
Boat B experiences more pressure on its base (600 Pa vs 200 Pa). Boat B has both a larger load and a smaller base area, so the force per unit area is much higher.

Boat A: Force = 5 × 700 = 3500 N; Pressure = 3500 ÷ 7 = 500 Pa.
Boat B: Force = 3 × 700 = 2100 N; Pressure = 2100 ÷ 3.5 = 600 Pa.
Boat B experiences the greater pressure.
Difference = 600 − 500 = 100 Pa more than boat A.
Although boat A carries more people, its base is twice as large, so the force is spread out more. Boat B packs more weight onto a smaller base, producing higher pressure per square metre.

No, lightning would not occur in its familiar form. Lightning happens because opposite charges build up and get separated inside a thundercloud — positive at the top, negative at the bottom. Since the intervening air is a very poor conductor (an insulator), the charges stay apart until the voltage becomes huge enough to punch through the air as a sudden spark.
If the clouds themselves were excellent conductors, charges could not accumulate — any charge would immediately flow away and neutralise. There would be no large voltage difference, no sudden discharge, and hence no lightning flash or thunder.

Both balloons will bulge, but not equally — balloon B (the one lower down) will bulge more than balloon A.
Reason: Pressure inside a liquid increases with depth according to \(P = h\rho g\). Balloon B is at a greater depth than balloon A, so the water pressure pushing outwards on B is larger. A bigger outward force stretches B's rubber more, so it bulges more. If the water level were below balloon A, then A would not bulge at all (only air at atmospheric pressure would touch it).

A storm becomes a cyclone through a self-feeding chain of events over a warm ocean:

1. Warm, moist air rises quickly over an ocean whose surface is above 26.5 °C, leaving behind a low-pressure region at the sea surface.
2. The rising air cools, water vapour condenses into clouds, releasing heat. The heat warms the air still more, which rises further and pulls up even more moist air.
3. Surrounding higher-pressure air rushes in to fill the low. As it moves, it is deflected by the Earth's rotation (the Coriolis effect) and begins to spiral.
4. This rotating column draws up enormous quantities of moist air, grows in size, forms an eye at the centre and a ferocious eyewall around it.
5. With wind speeds exceeding about 62 km/h the system is officially classified as a cyclonic storm; above 120 km/h it is a full severe cyclone.

Side B is the land.
On a summer afternoon the land heats up faster than the sea. The air above the land rises, creating a region of low pressure over the land. The cooler, higher-pressure air from over the sea rushes towards the land — this is the sea breeze.
The trees are being pushed in the direction of the wind, which is from A towards B. So the wind is coming from A (the sea) and moving towards B (the land). Therefore B is the land and A is the sea.

Activity — Collapsing plastic bottle:

1. Take an empty plastic bottle with a tight cap.
2. Suck out as much air as you can from the mouth of the bottle.
3. The bottle collapses inward.

Explanation: By sucking, you reduce the pressure inside the bottle. The outside atmospheric pressure is now higher, so air would flow inwards to equalise — but the sealed mouth blocks it. The stronger outside pressure instead pushes the bottle walls inwards. If you release the cap or your mouth, you clearly hear a sharp "whoosh" — air rushing from the high-pressure outside to the low-pressure inside — confirming that air flows from high to low pressure.

A thunderstorm is a weather disturbance marked by towering thunderclouds (cumulonimbus), heavy rain, strong gusty winds, lightning and thunder. It usually occurs on hot, humid days.
Formation:

1. The Sun heats the ground. Air above the ground becomes hot and, because water evaporates from soil and plants, also moist.
2. This warm, light, moist air rises rapidly.
3. As it rises, it cools. The water vapour condenses into tiny water droplets, forming clouds. Condensation releases latent heat.
4. The released heat warms the surrounding air, which rises further and draws up more moist air from below.
5. This self-feeding rise builds huge cumulonimbus clouds that can reach up to 10 km tall. Strong up- and down-drafts inside them separate electric charges.
6. When the voltage between differently charged regions becomes enormous, electricity jumps as lightning. The super-heated air expands suddenly and produces the sound we call thunder.

Inside a tall thundercloud, strong up- and down-moving air currents cause tiny water droplets and ice particles to rub and collide with each other. This friction transfers electrons between them, and the droplets become electrically charged.
Gradually the charges separate: positive charges accumulate at the top of the cloud and negative charges at the bottom. This creates a very large voltage difference between different parts of the cloud, or between the cloud and the ground.
When this voltage becomes so high that the air (normally an insulator) can no longer resist it, a sudden and massive electric discharge takes place — a bright spark streaks between clouds, or from the cloud to the ground. This spark is lightning. The channel of air along the spark is heated to about 30,000 °C and expands explosively, producing the shock wave we hear as thunder.

Large banners and hoardings present a huge flat surface to the wind. A strong wind blowing against this surface applies an enormous force (force = pressure × area). This force can rip the banner, tear it off its supports, or even topple the entire hoarding.
By cutting small holes in the banner, part of the wind passes through the holes instead of pushing against the whole area. This reduces the effective area on which the wind pushes, and hence reduces the total force. The banner is much less likely to tear or be blown away, especially during strong winds and storms.