NCERT Solutions: Chapter6 Pressure, Winds, Storms, and Cyclones (class 8 Science)

Winds are stronger on some days because the difference in air pressure between two regions is larger. Air moves from a region of high pressure to low pressure, and a greater difference in pressure causes stronger winds.

Water tanks are placed at a height so that water can flow down easily under the action of gravity. The height gives the water a pressure head, which provides adequate pressure at taps on lower floors without needing extra pumping. Placing the tank higher increases the water pressure and ensures a steady supply to all parts of a building.

No, air pressure cannot crush us because the pressure inside our bodies is nearly equal to the atmospheric pressure outside. The atmosphere pushes on our body from the outside, but our body fluids and gases push outward with the same amount of pressure, so the forces balance and we do not get crushed under normal Earth conditions.

Storms and cyclones are caused when warm, moist air rises and creates areas of low pressure. In cyclones, the rising air draws in surrounding air that begins to rotate because of the Earth's rotation (the Coriolis effect).

If the Earth stopped rotating, the Coriolis effect would disappear and air would not start rotating in the same way, so large rotating systems called cyclones would not form. Ordinary storms caused by rising warm air and local pressure differences could still occur, but they would not develop the spinning structure of cyclones.

Possible questions you might ask after exploring these ideas:

• Why don't we feel atmospheric pressure even though it is so high?
• How do buildings and bridges withstand strong winds during storms?
• Do animals sense changes in air pressure before storms?
• What scientific instruments are used to measure wind speed and pressure?
• How do disaster warning systems work for cyclones?

Yes, liquids exert pressure. Liquid pressure acts in all directions (downwards, sideways and upwards) and increases with depth. The pressure at a given depth depends on the height of the liquid column above that point and not on the total amount of liquid or the shape of the container.

If we increase the height of the water column, the bulge of the balloon will become bigger because the pressure at the point where water presses on the balloon increases with the height (depth) of the water above it. Greater water pressure pushes the balloon more and makes a larger bulge.

Air moves from a high-pressure region to a low-pressure region. Moving air is called wind. Thus, differences in air pressure are the main cause of wind formation.

Correct Answer: (d) equal in all three vessels

Explanation: The level of water in connected vessels (communicating vessels) will be the same regardless of shape, as liquid pressure depends on the height of the column, not the vessel's shape or width.

Correct Answer: (c) M will stick but N will not stick.

Explanation: The rubber sucker sticks due to atmospheric pressure creating a vacuum on a smooth surface. On a rough surface, air leaks in, preventing the vacuum and thus the sticking.

Correct Answer: (a) increase the height 'H' at which the tank is placed.

Explanation: Liquid pressure increases with the height of the water column. Raising the tank increases the pressure head, resulting in a stronger stream of water.

Correct Answer: (b) $P_A = P_B, F_A < F_B$

Explanation: Pressure at the bottom depends strictly on the depth/height of the water column ($P = \rho g h$), which is identical in both vessels, so $P_A = P_B$. Force equals pressure times area ($F = P \times A$); since vessel A has a narrower bottom (smaller base area), $F_A < F_B$.

(i) True [T] — Air flows from a region of higher pressure to a region of lower pressure, and this movement of air is called wind.

(ii) False [F] — Liquids exert pressure not only at the bottom but also on the sides and in all directions; pressure at a point depends on the depth of the liquid column above that point.

(iii) False [F] — The eye of a cyclone is a calm region with light winds and relatively clear conditions; the stormy weather is found in the surrounding eyewall.

(iv) True [T] — Being inside a closed metal car during a thunderstorm is relatively safe because the car's body provides a conducting path that guides lightning around the occupants to the ground (similar to a Faraday cage), provided occupants do not touch metal parts and the car is not convertible.

The boy in Figure (b) (standing vertically) will sink more into the sand.

Reason: The weight (force) of the boy is the same in both cases. In case (a), the weight is spread over a larger contact area (lying down), so the pressure on the sand is smaller. In case (b), the same weight acts over a much smaller area of his feet (standing), producing a much larger pressure ($P = \frac{\text{Force}}{\text{Area}}$). Larger pressure pushes the sand particles apart more and causes deeper sinking.

Given Data:
• Force (weight) of the elephant ($F$) = $20000\text{ N}$
• Area of one foot = $0.25\text{ m}^2$
• Total area covered by four feet ($A$) = $4 \times 0.25\text{ m}^2 = 1\text{ m}^2$

Calculation:
$$\text{Pressure } (P) = \frac{\text{Force}}{\text{Area}} = \frac{20000\text{ N}}{1\text{ m}^2} = 20000\text{ Pa (Pascal)}$$

Answer: Thus, the pressure exerted by the elephant on the ground is $20000\text{ Pa}$.

For Boat A:
• Total downward force = $5 \times 700\text{ N} = 3500\text{ N}$
• Base area = $7\text{ m}^2$
• $\text{Pressure on Boat A } (P_A) = \frac{3500\text{ N}}{7\text{ m}^2} = 500\text{ Pa}$

For Boat B:
• Total downward force = $3 \times 700\text{ N} = 2100\text{ N}$
• Base area = $3.5\text{ m}^2$
• $\text{Pressure on Boat B } (P_B) = \frac{2100\text{ N}}{3.5\text{ m}^2} = 600\text{ Pa}$

Comparison:
Boat B experiences more pressure on its base.
$$\text{Difference} = 600\text{ Pa} - 500\text{ Pa} = 100\text{ Pa}$$

Answer: Boat B experiences more pressure by $100\text{ Pa}$.

No. If air and clouds were good conductors of electricity, electric charges would flow away continuously instead of accumulating.

Lightning requires a massive separation and buildup of positive and negative charges (for example, between different regions of a cloud or between a cloud and the ground). If the medium were highly conducting, the charges would neutralize immediately as soon as they formed, and the huge potential difference needed to produce a sudden electrical discharge (lightning) would never develop. Hence, lightning would not occur.

When water is filled so that the water level rises above the side tube entry points, both balloons will bulge.

Because the entry points are positioned at the exact same height (depth), the hydrostatic water pressure acting at both openings is identical ($P = \rho g h$). Assuming both balloons are identical (same material, size, and elasticity), they will bulge equally. (If the balloons differed in elasticity, the one offering lower resistance to stretching would bulge more).

A cyclone is a vast rotating storm system with destructive winds and torrential rains. The sequential stages of its development are:

• Warm Ocean Water: Cyclones form over warm tropical sea waters (usually at surface temperatures exceeding $27^\circ\text{C}$). The warm sea heats the air directly above it and saturates it with water vapor.
• Air Rises (Low Pressure Created): The warm, moist air expands, becomes lighter, and rises rapidly, creating a severe low-pressure trough near the sea surface.
• Surrounding Air Rushes In: High-pressure cool air from surrounding areas rushes inward to fill the void, absorbs heat and moisture, and rises in turn, sustaining a continuous convective cycle.
• Spiral Rotation Begins: Due to the Earth's rotation (the Coriolis effect), the rushing winds are deflected into a spinning spiral.
• Maturity: The system forms a calm, cloudless central core called the eye, surrounded by a violent ring of storm clouds, winds, and torrential rain called the eyewall.

Side A is the land side (and Side B is the sea side).

Explanation: During a hot summer afternoon, land heats up much faster than sea water. The air over the land expands and rises, creating a localized low-pressure zone. Cooler, denser air from above the sea moves inland toward the land as a sea breeze (blowing from B toward A). The trees in the illustration are bent toward Side A by winds blowing from Side B, confirming that B is the sea and A is the land.

Materials Required: Two similar rubber balloons, a hollow drinking straw, and thread.

Procedure:

1. Attach one end of the straw tightly to the neck of an uninflated balloon using thread so that no air leaks out.
2. Inflate the second balloon with air, then insert the open end of the straw into its neck and seal it tightly with thread so air can only transfer through the straw.

Observation: Air automatically rushes from the inflated balloon through the straw into the uninflated balloon. The inflated balloon shrinks while the uninflated balloon expands until the pressures in both balloons equalize.

Conclusion: Air moves naturally from an area of higher pressure (inflated balloon) to an area of lower pressure (uninflated balloon).

A thunderstorm is a violent, localized weather disturbance characterized by lightning, acoustic thunder, gusty winds, and heavy rain, usually developing on hot, humid days.

Formation Stages:

• Rapid Upward Convection: Solar heating warms the ground, causing warm, moisture-laden air to rise rapidly.
• Condensation: The rising air cools at higher altitudes, causing water vapor to condense into cumulus clouds.
• Cumulonimbus Development: Continuous updrafts enlarge the cloud into towering cumulonimbus storm clouds.
• Charge Separation & Lightning: Violent friction and collisions between rising water droplets and falling ice crystals generate massive electrical charge separation, producing lightning discharges that superheat the air to create thunder.
• Precipitation & Downdrafts: Heavy raindrops fall accompanied by strong, cool downdraft winds.

During a thunderstorm, strong vertical updrafts carry water droplets upward into freezing zones, while cold downdrafts push ice crystals downward. Violent collisions between ice particles and water droplets cause electric charges to separate:

• The upper regions of the storm cloud accumulate a massive positive charge.
• The lower base of the cloud accumulates a heavy negative charge.
• The concentrated negative charge at the cloud base induces an opposite positive charge on tall structures, trees, and the ground below.

When the resulting potential difference becomes enormous, it overcomes the electrical insulating resistance of air. A massive streak of electrical discharge flashes through the air—this is lightning. The lightning discharge instantly heats the surrounding air channel up to nearly $30,000^\circ\text{C}$, causing an explosive expansion of air that produces the shockwave known as thunder.

Holes or slits are made in banners, billboards, and cloth hoardings to allow wind to pass freely through them.

If a hoarding is completely solid, strong winds cannot pass through, creating high pressure on the front face and low pressure behind it. This huge pressure differential exerts an immense wind load force that can tear the cloth or collapse the metallic supporting structure. Perforations equalize the pressure on both sides, dramatically lowering wind resistance and preventing structural damage.

Prediction: The hanging paper strip will rise upward when air is blown across its upper surface.

Observation: Blowing horizontally across the top of the paper strip causes it to lift instantly into a nearly horizontal position.

Scientific Interpretation: Blowing air across the top surface increases the velocity of the air, which lowers the air pressure above the strip (Bernoulli's principle). The atmospheric pressure beneath the paper remains higher and exerts an upward force, lifting the strip. This proves that high-speed wind is accompanied by reduced air pressure.