Activity 6.5 — How Wind Forms — Pressure, Winds, Storms, and Cyclones

1. 14 marksCuriosity Grade 8, Chapter 6, page 88, Activity 6.5

   Describe the setup of Activity 6.5 with the two balloons and the straw.

   Hint. One balloon starts inflated and the other empty — that asymmetry is the experiment.

   The setup. Take two similar balloons of thin rubber and a drinking straw.

   1. Insert one end of the straw into one balloon (left uninflated) and secure it with a rubber band or thread.
   2. Inflate the second balloon and hold its mouth closed with your fingers so no air escapes.
   3. Insert the free end of the straw into the neck of the inflated balloon and secure it, making sure no air leaks out as you do so.

   You now have one end of the straw inside an inflated balloon and the other inside an empty one (Fig. 6.12), with a sealed path between them.

   Then: Predict what would happen to the balloons — before you look. Then observe, and check your prediction.

   ✦ Why the sealing matters so much. If air leaks out at either join, it escapes to the room instead of travelling down the straw, and the experiment shows nothing. Every step of the instructions is about keeping the two balloons connected only to each other.

   Why you are asked to predict first. Writing down what you expect, before you see it, is what makes the observation worth something. If you decide afterwards what you expected, you will always find you were right. The chapter asks the same of you again in the first project at the end of the chapter.

2. 24 marksCuriosity Grade 8, Chapter 6, page 88

   What happens to the two balloons in Activity 6.5, and why? Why does the flow of air eventually stop?

   Hint. Watch until nothing more changes — the stopping is as informative as the starting.

   What happens. The inflated balloon gets smaller and the uninflated one gets bigger. The air pressure in the inflated balloon is higher than that in the uninflated balloon. As a result, some air moves from the inflated balloon to the uninflated balloon, resulting in changes in the size of both the balloons.

   Why it stops. The flow of air continues till the air pressure in the inflated balloon is higher than the air pressure in the uninflated balloon. The air flow stops when the pressure in both balloons becomes equal. At this stage, both balloons are almost of the same size.

   The conclusion. Air moves from a region of high air pressure to a region of low air pressure.

   ✦ The stopping is the more important half of the observation. That air moved tells you a flow happened. That it stopped when the pressures became equal tells you what was driving it: not the amount of air, not the size of the balloon, but the pressure difference. When the difference reached zero, so did the flow — even though there was still plenty of air in both balloons.

   And that is what a wind is. Air flowing because of a pressure difference, and flowing faster when the difference is larger. The chapter adds: If we could measure the speed of the escaping air in Activity 6.5, we would find that the speed of the air is higher if the pressure difference is higher.

3. 33 marksCuriosity Grade 8, Chapter 6, pages 86-88

   The chapter mentions an open balloon and a punctured bicycle tube before Activity 6.5. Explain both using the activity's conclusion.

   Hint. In each case, ask which side has the higher pressure.

   Both are the same phenomenon: air moving from high pressure to low pressure.

   Case	Where the pressure is higher	What happens
   Inflated balloon left open	Inside the stretched balloon	Air rushes out to the lower-pressure room until the balloon is limp
   Punctured bicycle tube	Inside the pumped-up tube	Air escapes through the puncture and the tube collapses
   Activity 6.5	Inside the inflated balloon	Air travels down the straw into the empty balloon until the pressures are equal

   The chapter groups them itself: In both of these cases, does air move from a high pressure region to a low pressure region? — and Activity 6.5 is set up to answer yes.

   ✦ What Activity 6.5 adds that the other two cannot show. When a balloon or a tyre empties into the room, you cannot see the far end of the journey — the escaping air vanishes into an enormous atmosphere whose pressure never visibly changes. Connecting two balloons makes both ends visible: you watch one shrink and the other grow, and you watch the flow stop at the moment they match. That is why the chapter builds the straw apparatus instead of simply letting a balloon go.

   Pumping a tyre is the same process run backwards — you use muscular force to push air into a region where the pressure is already higher, which is precisely why it takes effort, and why it gets harder as the tyre fills.

4. 45 marksCuriosity Grade 8, Chapter 6, pages 88-89

   Explain how a sea breeze and a land breeze are formed. Why do they blow in opposite directions?

   Hint. Land and water heat and cool at different rates. Follow the consequences.

   The key fact: land heats faster than water during the day, and cools faster at night. Everything else follows from air moving high pressure → low pressure.

   Sea breeze — during the day. As land gets heated faster than water during the day, the air above the land becomes warmer and lighter. Hence, it rises, creating an area of low pressure. The air from the high pressure region of the sea blows to the low pressure region which develops on the land, resulting in a sea breeze.

   Land breeze — at night. At night, the water is warmer than the land. Therefore, a low pressure area develops above the sea. As a result, wind blows from the land to the sea, giving rise to land breeze.

   	Day (sea breeze)	Night (land breeze)
   Warmer	Land	Sea
   Air rises over	Land	Sea
   Low pressure over	Land	Sea
   High pressure over	Sea	Land
   Wind blows	Sea → land	Land → sea

   ✦ Why they reverse. Nothing about the mechanism changes between day and night — air always flows from high pressure to low. What reverses is which side is warmer, and therefore which side has the low pressure. The chapter sums it up: the phenomenon of land breeze and sea breeze is mainly due to the pressure differences over the land and the sea.

   A breeze is named for where it comes from, not where it goes. A sea breeze blows from the sea onto the land. Getting that convention the wrong way round is the commonest error in exercise question 9.

   You met sea and land breezes in Curiosity, Grade 7. What is new here is the pressure explanation for them.

5. 53 marksCuriosity Grade 8, Chapter 6, page 89

   On what does the speed of a wind depend? Use this to explain why some days are calm and others windy.

   Hint. The chapter states the rule in a single sentence right after the breezes.

   The rule. If we could measure the speed of the escaping air in Activity 6.5, we would find that the speed of the air is higher if the pressure difference is higher.

   So wind speed is set by the size of the pressure difference between two regions — not by the amount of air, and not by the pressure itself.

   Pressure difference	Wind
   Very small	Calm, still air
   Moderate	A steady breeze
   Large	Strong winds — a storm
   Very large	high-speed winds revolving around a very low pressure area — a cyclone

   Applied to calm and windy days. On a day when the sun heats one region much more than its surroundings, a large pressure difference builds up and the wind is strong. On a day when heating is even — overcast, or uniformly warm — differences stay small and the air is nearly still.

   ✦ This answers the very first Probe-and-ponder question of the chapter, and it is worth noticing that the answer took nine pages to arrive at. You needed the idea of pressure (6.1), that air has pressure (6.2), and that air flows down a pressure difference (6.3) before the question could be answered at all.

   And it sets up the rest of the chapter. Sections 6.5 and 6.6 are about what happens when the pressure difference becomes very large indeed. Fig. 6.19 shows a drop from 1008 mb to 994 mb across a cyclone — and the Amphan cyclone of 2020 reached peak wind speeds of 270 km/h.

6. 64 marksCuriosity Grade 8, Chapter 6, pages 87-89

   Describe an activity to show that air flows from a region of high pressure to a region of low pressure.

   Hint. Give the method, the observation and the reasoning — not just the method.

   Activity: two balloons joined by a straw.

   Materials. Two similar balloons of thin rubber, a drinking straw, rubber bands or thread.

   Method.

   1. Fix one end of the straw into an uninflated balloon and tie it tightly.
   2. Inflate the second balloon and pinch its mouth shut so no air escapes.
   3. Fix the free end of the straw into the neck of the inflated balloon and tie it, keeping the air in.
   4. Release your grip and watch both balloons (Fig. 6.12).

   Observation. The inflated balloon shrinks and the empty one swells. After a while both are almost the same size and nothing more happens.

   Reasoning. The air in the inflated balloon was at a higher pressure than the air in the empty one. Air therefore travelled through the straw from the high-pressure balloon to the low-pressure one. The flow stopped when the two pressures became equal — which shows that it was the pressure difference, and nothing else, that drove the flow.

   Conclusion. Air moves from a region of high air pressure to a region of low air pressure.

   ✦ Make sure your answer includes the stopping. A description that ends with 'the air moved from one balloon to the other' shows only that air moved. It is the flow ceasing exactly when the pressures match that identifies the cause, and that is the part an examiner is looking for.

   This is exercise question 10.

7. 73 marksCuriosity Grade 8, Chapter 6, pages 86-89

   Trace the chain by which the sun's heating produces a wind. List the steps in order.

   Hint. Six steps, and each one is stated somewhere in sections 6.2 and 6.3.

   The chain:

   1. The sun heats one region of the Earth's surface more strongly than another — for example, land more than sea during the day.
   2. The air above the hotter region becomes warmer and lighter.
   3. That warm air rises, creating an area of low pressure.
   4. The cooler region beside it is left at a comparatively high pressure.
   5. Air moves from a region of high air pressure to a region of low air pressure — so air flows in from the cooler region.
   6. That moving air is the wind. Its speed depends on how large the pressure difference is.

   The Snapshot version: Differences in air pressure cause winds to blow. Warm air rises, creating a low-pressure area. Cooler air from surrounding higher-pressure regions moves in to take its place.

   ✦ Every later phenomenon in this chapter is this chain, driven harder.

   Phenomenon	The same chain, plus
   Sea and land breeze	Nothing extra — just land and sea heating at different rates
   Storm	Enough moisture that the rising air forms clouds and rain
   Thunderstorm	Strong up-and-down winds that charge the clouds
   Cyclone	Heat released by condensation driving it further, and Earth's rotation making it spin

   Learning the six steps once means you have the backbone of sections 6.3, 6.5 and 6.6 together, and exercise questions 8, 9 and 11 all start from it.

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.