Pressure, Winds, Storms, and Cyclones — Class 8 Science
Curiosity · Chapter 6
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6- Mathematics
- Science
- + 3 more
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
2- Mathematics
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
Chapter summary
Key ideas & terms
- Pressure
- Force acting per unit area, calculated as force divided by area; its SI unit is the pascal (Pa).
- Atmospheric pressure
- The pressure exerted by the air of the atmosphere on all objects around us.
- Wind
- Moving air that flows from a region of high air pressure to a region of low air pressure.
- Thunderstorm
- A storm accompanied by lightning and thunder, formed from rising warm moist air and charge separation in clouds.
- Lightning
- A bright flash produced when built-up electric charges flow suddenly within or between clouds or to the ground.
- Cyclone
- A large spinning system of clouds, winds, and rain around a low-pressure centre called the eye, formed over warm oceans.
Extra questions & answers
How is pressure defined and what is its SI unit?
Pressure is force per unit area, and its SI unit is the pascal (Pa), equal to one newton per square metre.
Why are broad straps on a bag more comfortable than narrow straps?
Broad straps spread the same weight over a larger area, reducing the pressure on the shoulders.
On what does the pressure exerted by a liquid column depend?
It increases with the height of the liquid column, which is why overhead tanks are placed at a height.
Do liquids exert pressure only at the bottom of a container?
No, liquids exert pressure in all directions, including on the side walls of the container.
Why does a rubber sucker stick to a smooth surface?
Pressing it pushes out the air inside, so the higher outside atmospheric pressure holds it against the surface.
In which direction does wind blow with respect to pressure?
Air moves from a region of high pressure to a region of low pressure.
Why can high-speed winds blow off weak roofs?
High-speed winds lower the air pressure above the roof, and the higher pressure below can push a weak roof upward.
How do charges separate in a thundercloud?
Strong up-and-down winds rub water droplets and ice particles, leaving positive charges in the upper part and negative charges in the lower part of the cloud.
What is the eye of a cyclone?
The central region of lowest pressure where the wind is calm, surrounded by strong winds and heavy rainfall.
Which department monitors cyclones in India?
The India Meteorological Department (IMD), which tracks cyclones and issues warnings.
Keep the curiosity alive
1. Choose the correct statement. (i) Look at Fig. 6.21 carefully. Vessel R is filled with water. When pouring of water is stopped, the level of water will be ____________________. (a) the highest in vessel P (b) the highest in vessel Q (c) the highest in vessel R (d) equal in all three vessels
(d) equal in all three vessels. The three vessels are interconnected (communicating vessels). For a liquid at rest, the pressure at the same horizontal level is the same, which depends only on the height of the liquid column. Therefore water flows until the height is the same in all vessels, irrespective of their shape or volume.
1. Choose the correct statement. (ii) A rubber sucker (M) is pressed on a flat smooth surface and an identical sucker (N) is pressed on a rough surface: (a) Both M and N will stick to their surfaces. (b) Both M and N will not stick to their surfaces. (c) M will stick but N will not stick. (d) M will not stick but N will stick.
(c) M will stick but N will not stick. When pressed on a smooth surface, most air is pushed out from under the sucker and the atmospheric pressure outside is much higher than inside, so it is held firmly. On a rough surface air leaks through the gaps and the pressure inside and outside remains almost equal, so no net force holds the sucker.
1. Choose the correct statement. (iii) A water tank is placed on the roof of a building at a height ‘H’. To get water with more pressure on the ground floor, one has to (a) increase the height ‘H’ at which the tank is placed. (b) decrease the height ‘H’ at which the tank is placed. (c) replace the tank with another tank of the same height that can hold more water. (d) replace the tank with another tank of the same height that can hold less water.
(a) increase the height ‘H’ at which the tank is placed. Liquid pressure at the bottom depends on the height of the liquid column
1. Choose the correct statement. (iv) Two vessels, A and B contain water up to the same level as shown in Fig. 6.22.
(b)
2. State whether the following statements are True [T] or False [F]. (i) Air flows from a region of higher pressure to a region of lower pressure. [ ]
True [T]. Activity 6.5 with the two balloons joined by a straw shows air moving from the inflated (high pressure) balloon to the uninflated (low pressure) balloon until pressures become equal. The same causes winds and sea-land breezes.
2. State whether the following statements are True [T] or False [F]. (ii) Liquids exert pressure only at the bottom of a container. [ ]
False [F]. Liquids exert pressure in all directions - at the bottom, on the side walls and on any object inside. Activity 6.2 with holes around the sides of a bottle shows water spurting sideways, proving pressure on the walls.
2. State whether the following statements are True [T] or False [F]. (iii) Weather is stormy at the eye of a cyclone. [ ]
False [F]. The eye is the centre of lowest pressure where air is calm. Strong winds, heavy rain and clouds surround the eye in the eyewall, while the eye itself is calm and often clear.
2. State whether the following statements are True [T] or False [F]. (iv) During a thunderstorm, it is safer to be in a car. [ ]
True [T]. A closed metal car acts like a shield and lightning current passes over the outside to the ground. It is much safer than being in open fields, under trees, in water or using an umbrella with a metal rod. One should still close windows and not touch metal parts.
3. Fig. 6.23 a shows a boy lying horizontally, and Fig. 6.23b shows the boy standing vertically on a loose sand bed. In which case does the boy sink more in sand? Give reasons.
The boy standing vertically (Fig. 6.23b) sinks more. Pressure
4. An elephant stands on four feet. If the area covered by one foot is
Total contact area
5. There are two boats, A and B. Boat A has a base area of
Force on boat A:
6. Would lightning occur if air and clouds were good conductors of electricity? Give reasons for your answer.
No, lightning as we see it would not occur. Lightning happens because air is a good insulator and clouds are poor conductors, so charge separated by rubbing of ice particles and water droplets due to strong up-down winds can accumulate until the air breaks down and there is a sudden spark. If air and clouds were good conductors, charges would leak away continuously in small currents as soon as they formed, no large charge separation could build up, and there would be no sudden flash and thunder.
7. What will happen to the two identical balloons A and B as shown in Fig. 6.24 when water is filled into the bottle up to a certain height. Will both the balloons bulge? If yes, will they bulge equally? Explain your answer.
Yes, both balloons will bulge because water exerts pressure in all directions and on the walls of the container. They will not bulge equally. The balloon that is lower (deeper below the water surface) bulges more than the upper one. Liquid pressure increases with depth
8. Explain how a storm becomes a cyclone.
A storm becomes a cyclone over warm oceans: 1. Warm moist air above
9. Fig. 6.25 shows trees along the sea coast in a summer afternoon. Identify which side is land— A or B. Explain your answer.
Side A is the land (and B is the sea). In a summer afternoon the land heats up faster than the sea, so warm air rises over the land forming low pressure, and cooler air blows in from the higher-pressure sea as a sea breeze — from the sea toward the land. Coastal trees are permanently shaped by this prevailing wind and lean in the direction the wind blows (downwind), i.e. toward the land. In Fig. 6.25 the trees lean toward A, so A is the land side.
10. Describe an activity to show that air flows from a region of high pressure to a region of low pressure.
Take two similar thin rubber balloons and a drinking straw. Fix one end of the straw into one balloon (uninflated) with a rubber band. Inflate the second balloon, hold its mouth closed, and insert the free end of the straw into it and seal. Now both balloons are connected. The inflated balloon has high pressure, the uninflated has low pressure. Observe that air moves through the straw into the smaller balloon, inflating it while the larger one shrinks, until both become equal in size and flow stops. This shows flow is from high to low pressure, and stops when pressures equalise. Speed is greater when pressure difference is greater.
11. What is a thunderstorm? Explain the process of its formation.
A thunderstorm is a storm with strong winds, rain, and thunder and lightning. Formation: 1. In hot humid regions land heats, warm moist air rises creating a low-pressure area and cooler surrounding air rushes in. 2. Rising air expands, cools and water vapour condenses to form clouds and then heavier raindrops (rain/hail/snow) with strong winds - this is a storm. 3. In humid tropical conditions warm air rises to great heights where water droplets freeze to ice particles. 4. Strong upward and downward winds rub ice particles and water droplets, separating electric charges (positive lighter ice goes to top, negative heavier droplets to bottom). When charges become large enough, they cause lightning.
12. Explain the process that causes lightning.
During a thunderstorm, strong up and down winds inside clouds rub light ice particles against heavier water droplets. Friction produces static charges: ice becomes positively charged at the top of cloud, water droplets negatively charged at the bottom. The negative cloud base induces positive charge on the ground, trees and buildings below. Air normally insulates, but when charge buildup is very large the air breaks down and opposite charges suddenly meet, producing a bright flash called lightning. This can be cloud-to-cloud, within a cloud, or cloud-to-ground. Sudden heating of air by lightning makes it expand explosively, producing thunder.
13. Explain why holes are made in banners and hoardings.
Banners and hoardings present a large area to the wind. High-speed wind blowing over them creates low pressure on one side and a large pressure difference can exert huge force and tear them or topple the supports. Holes allow air to pass through, reducing the net force/pressure difference between the two sides and letting wind escape, similar to keeping doors and windows open during a storm to balance pressure.
Discover, design, and debate
Hold a strip of paper,
Prediction: The paper will lift upward instead of being pushed down. Observation: When you blow steadily over the top surface, the hanging part of the strip rises up almost horizontal. Interpretation: Fast-moving air over the paper creates a low-pressure region above it (high-speed winds are accompanied by reduced pressure, as shown in Activity 6.6 with two balloons). The higher atmospheric pressure below pushes the paper up. This is the same Bernoulli effect that helps create lift on airplane wings and can blow off roofs during storms.
List three major cyclones which have occured in India in the last 20 years. List two major destruction caused by each of the cyclones. What measures were taken by the local government and communities to reduce the loss of life and destruction of property? Mention two suggestions you would like to propose to the local government.
Examples (any three recent IMD-tracked cyclones): 1. Cyclone Amphan (May 2020, West Bengal/Odisha) - peak winds about
Collect data on the strength of thunderstorms for various regions of India. Compare your findings and identify which regions are more prone to thunderstorms. Can you give reasons for your findings?
Data from India Meteorological Department shows thunderstorms are most frequent in eastern and north-eastern India (West Bengal, Assam, Jharkhand - Kalboishakhi/Bordoisila pre-monsoon storms), central India and the western coast including Kerala/Karnataka (mango showers). They are less frequent and less intense in dry northwestern desert regions like Rajasthan except during monsoon. Reasons: Thunderstorms need moisture and intense heating with strong convective winds. Eastern India gets high humidity from Bay of Bengal and strong summer heating, ideal for convective lifting. Coastal southern regions get moist sea air and heating before monsoon. Arid inland western regions have less moisture, so fewer strong thunderstorms despite heat.