NCERT Solutions Curiosity Chapter 6 Chapter opener — Probe and ponder

Because the difference in air pressure between two places is larger on some days than on others, and wind speed depends on that difference.

Wind is simply air flowing from a region of higher pressure to a region of lower pressure. When the Sun heats one stretch of land strongly, the air above it becomes warm and lighter, rises, and leaves a low-pressure region behind. Air from the surrounding high-pressure regions moves in to take its place. If the region has warmed only a little, the pressure difference is small and the air drifts in slowly — a calm day. If the heating is strong, or moist air is rising fast and condensing, the pressure at the centre falls much lower and air rushes in — a windy day.

So that the water column above the taps is tall, because the pressure a liquid exerts depends on the height of its column.

Activity 6.1 shows that the balloon bulges more as the water column above it gets taller, and that the width of the pipe makes no difference. The water in a tank behaves the same way: the greater the height of water above a tap, the greater the pressure at that tap. That pressure is what pushes the water out. A tank standing on the ground would give only a weak trickle; the same tank on the roof gives a good, strong stream.

The atmosphere presses on us with an enormous force, but it does not crush us, because the pressure inside our body balances it.

The book gives the figure: the air column standing over an area of just 15 cm × 15 cm pushes down with a force of about 2250 N — the same as the force of gravity on a 225 kg mass. Every patch of our skin carries a load like that. We feel nothing because the fluids and gases moving in our tissues and organs keep the pressure inside the body equal to the atmospheric pressure outside. The two pressures push against each other and cancel.

Storms and cyclones both begin with warm air rising and leaving a low-pressure region behind. A cyclone, however, also needs the Earth's rotation to make that inrushing air spin — so if the Earth stopped rotating, cyclones as we know them would not form.

How a storm forms: the ground (or the sea) heats up, the warm and moist air above it becomes lighter and rises, creating a low-pressure area. Cooler air from surrounding high-pressure areas flows in, gets heated and rises in turn, so a continuous wind circulation is set up. The rising air expands and cools, its moisture condenses into water droplets and clouds form; the droplets merge into heavier drops that fall as rain, hail or snow. Strong winds together with this rain make a storm.

How a cyclone forms: over warm ocean water the same rising continues, but each time water vapour condenses into raindrops it releases the heat that was taken up during evaporation. That heat warms the ascending air further, so it rises even more, and the pressure at the centre falls even lower. Air from all around rushes in and begins rising too. The Earth's rotation makes this moving air spin, and the cycle repeats until there is a very low-pressure centre with high-speed winds revolving around it — a cyclone.