Activity 6.6 — High-Speed Winds Lower the Air Pressure — Pressure, Winds, Storms, and Cyclones

1. 14 marksCuriosity Grade 8, Chapter 6, page 89, Activity 6.6

   Describe Activity 6.6. What happens when you blow between the two hanging balloons, and what happens when you blow harder?

   Hint. Predict first — most people expect the opposite of what happens.

   The activity. Take two balloons of the same size, inflate both, tie strings to them and hang them from a stick with a gap of 6–10 cm between them (Fig. 6.13). Now blow air into the narrow space between them.

   What happens. When you blow between the two balloons, you observe that they move towards each other. Blow harder and blowing harder increases the speed at which the balloons approach each other.

   Why. This happens because when you blow air between the balloons, a low pressure area is created between them. The higher air pressure surrounding the balloon pushes them towards each other.

   The inference. High speed winds are accompanied by a reduced air pressure.

   ✦ This is the most counter-intuitive result in the chapter, and worth predicting before you do it. Almost everyone expects blowing between two objects to push them apart — after all, you are blasting air into the gap. They come together instead. The moving air in the gap is at a lower pressure than the still air on the outside, so the still air wins and squeezes them inward.

   Blowing harder makes it stronger, not weaker, which rules out the obvious alternative explanation. If your breath were simply pushing the balloons, harder blowing would push them further apart. It does the reverse — so the effect is genuinely about the speed of the air, not its push.

   Keep the gap in the range the chapter gives. Too wide and the effect is too weak to see; too narrow and the balloons touch anyway.

2. 24 marksCuriosity Grade 8, Chapter 6, page 90

   Why are the roofs of houses sometimes blown away during a storm with high-speed winds?

   Hint. Compare the pressure above the roof with the pressure below it.

   Because a pressure difference builds up across the roof, pushing it upwards.

   The chain of reasoning:

   1. When high-speed winds blow over houses, a low-pressure area is created over them, as high-speed winds are accompanied by a reduced pressure — the result of Activity 6.6.
   2. Therefore, the air pressure above the roofs of the houses is lower than the pressure below them. Inside the house the air is still, so its pressure stays high.
   3. The higher pressure inside pushes up on the roof, and the lowered pressure above no longer balances it.
   4. If the pressure difference is large and the roofs are weak, they may be blown away (Fig. 6.14a).

   ✦ The roof is pushed off from below, not pulled off from above. It looks as though the wind lifts the roof away, and it does not — the wind's job is only to reduce the pressure above it. The lifting is done by ordinary indoor air pressing upward, which was there all along and was previously balanced.

   This is Activity 6.6 at full scale. Two balloons were pushed together by still air because the air between them was moving. A roof is pushed up by still air because the air above it is moving.

   And it explains why storm damage is often worst on light roofs — tin sheets, thatch, loosely-fastened tiles. A heavy, well-anchored roof resists the same pressure difference.

3. 34 marksCuriosity Grade 8, Chapter 6, page 90

   Why is it safer to keep the doors and windows of a house open during a storm with high-speed winds?

   Hint. The aim is to get rid of the difference, not the wind.

   Because it lets the pressure inside the house drop too, so the difference across the roof is much smaller.

   The chapter's explanation. That is why it is safer to keep doors and windows of the houses open during storms with high-speed winds. When the same wind moves over the roofs, and through the houses, the pressure difference between inside of the houses and over the roofs is reduced to a large extent. This helps prevent the roofs from being blown off (Fig. 6.14b).

   	Doors and windows shut	Doors and windows open
   Pressure above the roof	Lowered by the fast wind	Lowered by the fast wind
   Pressure inside the house	Stays high — the air is trapped and still	Falls as the wind moves through
   Difference across the roof	Large	Small
   Risk to the roof	High — it may be blown away	Much lower

   ✦ The advice sounds wrong until you see what is being fixed. Shutting everything feels like the safe response to a storm, and against rain it is. But against the roof-lifting effect it is exactly the wrong move, because a sealed house preserves the high indoor pressure that does the lifting. Opening up does not reduce the wind; it removes the difference the wind would otherwise act on.

   Note the exact wording of the book: reduced to a large extent, not eliminated. This makes a roof much safer, not immune.

   This is a piece of practical safety advice from the chapter. Follow the book's version and, in a real storm, whatever your local authorities advise.

4. 44 marksCuriosity Grade 8, Chapter 6, pages 89-90

   Explain why holes are made in banners and hoardings.

   Hint. A banner is a roof turned on its side.

   So that high-speed wind can pass straight through, instead of building up a large pressure difference between the two faces.

   The reasoning. A banner or hoarding is a large flat sheet. When strong wind blows across it, the fast-moving air on one side has a reduced pressure — high speed winds are accompanied by a reduced air pressure — while the sheltered side keeps a higher pressure. The difference acts over the banner's whole area, and since pressure times area gives force, a large area means a very large force. The banner can be torn, or the hoarding's frame bent or brought down.

   What the holes do. They let air flow through from one side to the other, so the two faces come much closer to the same pressure. The difference is reduced, and so is the force on the structure.

   ✦ This is exactly the open-doors argument of the previous question, applied to a flat sheet instead of a house. In both cases the remedy is not to block the wind but to let it through, removing the pressure difference rather than trying to resist it.

   Structure	Problem	Remedy
   Roof in a storm	Low pressure above, high pressure inside	Open doors and windows
   Banner or hoarding	Low pressure on the windward face	Cut holes through it

   Notice the cost of the fix. The holes make the banner slightly harder to read and a little less complete — a small price for a structure that survives the first storm. That trade-off is the sort of judgement engineering is made of.

   This is exercise question 13.

5. 53 marksCuriosity Grade 8, Chapter 6, page 97, project 1

   Hold a strip of paper 18 cm long and 2 cm wide between your thumb and forefinger so that it hangs freely. Predict what you will observe if you blow over the paper, then do it and interpret the result.

   Hint. Write your prediction down before you blow — that is the point of the project.

   Predict first, and be honest about it. Most people predict that blowing over the top of a hanging strip will push it downwards, since you are blowing downwards over it. Write down whatever you actually expect, then test.

   What happens. The far end of the strip rises towards the horizontal while you blow, and drops again when you stop.

   Interpretation. Blowing over the top surface makes the air there move fast, and high speed winds are accompanied by a reduced air pressure. The air below the strip is still, so its pressure stays higher. The higher pressure underneath pushes the strip up.

   ✦ This is the third appearance of one idea, and it is worth lining them up.

   Situation	Fast air	Still air	Result
   Two hanging balloons	Between them	Outside them	They move together
   Roof in a storm	Above the roof	Inside the house	The roof lifts
   Paper strip	Over the top	Underneath	The strip rises

   Things to record in your report: your prediction before the test, what actually happened, whether blowing harder made a difference, and what happened if you blew under the strip instead. A prediction that turned out wrong is a good thing to report, not something to hide — being surprised is how you know you learnt something.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity, Textbook of Science for Grade 8, Chapter 6 'Pressure, Winds, Storms, and Cyclones', book pages 80-97 (hecu106.pdf, 18 pages, Reprint 2026-27), downloaded from ncert.nic.in and read page by page. Every activity number, figure number, quantity and quoted sentence below was checked against that PDF. THREE FIGURES WERE MEASURED, NOT EYEBALLED, because three exercise answers turn on them. Fig. 6.22 (exercise 1(iv)) was rendered at 900 dpi and the vessel walls located from the dark outlines: vessel A is 519 px wide and vessel B is 726 px, so B is 1.40x wider, while the water columns measure 660 px and 671 px - equal to within 1.7%, matching the question's premise of equal levels. Hence P_A = P_B but F_A < F_B, answer (b). Fig. 6.24 (exercise 7) was segmented by colour: the two balloons occupy identical vertical ranges (rows 1260-1403, centroids both at row 1334), so they are at exactly the same height and bulge equally. Fig. 6.25 (exercise 9) was measured by tracking the palm trunks: the leftmost trunk's centre moves from x=655 at the top to x=763 near the base, so the crown sits about 108 px LEFT of the base, and all four crowns stream leftwards - the wind blows from B to A. Since a summer afternoon gives a SEA BREEZE (sea to land), B is the sea and A is the land. Fig. 6.21 was also rendered and confirmed to show the three vessels JOINED BY TUBES near their bases, which is what makes answer (d) correct. The chapter's own atmospheric-pressure figure was checked rather than assumed: 2250 N over 15 cm x 15 cm = 0.0225 m^2 gives 1,00,000 Pa = 1000 hPa = 1000 mb, which sits squarely inside the 994-1008 mb range marked on Fig. 6.19, so the book's number is internally consistent. Two deliberate restraints. (1) The chapter names exactly ONE cyclone (Amphan 2020, peak winds 270 km/h) and one surge range (3-12 m), and gives no cyclone categories, no casualty figures, no monsoon rainfall percentages and no atmospheric composition percentages. None are supplied here; the research projects tell the student to cite IMD or an equivalent checked source and to report disagreement between sources rather than pick a number. (2) Lightning-safety advice is reproduced exactly as the book gives it, with nothing added, since an invented extra rule could put someone at risk.. Questions are referenced from the NCERT textbook for identification.