Class 7 Science Notes Chapter 8 (Chapter 8) – Examplar Problems Book

Alright class, let's get straight into Chapter 8: Winds, Storms and Cyclones from your Science Exemplar book. This is an important chapter, not just for your class exams, but concepts related to weather phenomena frequently appear in various government exams. Pay close attention!

Chapter 8: Winds, Storms and Cyclones - Detailed Notes for Exam Preparation

Core Concepts:

1. Air Exerts Pressure:

   • The envelope of air surrounding the Earth is called the atmosphere.
   • This air has weight and exerts pressure on everything. This is called air pressure.
   • Example: We don't feel crushed because the pressure inside our bodies balances the atmospheric pressure outside.
   • Demonstration: Crushing a hot can by pouring cold water over it shows that outside air pressure is significant.

2. High-Speed Winds are Accompanied by Reduced Air Pressure (Bernoulli's Principle - Simplified):

   • When air moves fast, the pressure in that region decreases.
   • Examples:
     • Blowing over a strip of paper makes it rise (low pressure above, higher pressure below pushes it up).
     • Blowing between two suspended balloons makes them move towards each other (low pressure between them, higher pressure outside pushes them in).
     • Roofs of houses can be blown off during strong winds because the high-speed wind above the roof creates low pressure, while the higher pressure inside the house pushes the roof upwards.

3. Air Expands on Heating and Contracts on Cooling:

   • When air is heated, its particles move faster and farther apart, making it expand and occupy more volume.
   • Expanded air is less dense (lighter) than the surrounding cooler air, so warm air rises.
   • When air is cooled, its particles slow down and come closer, making it contract and occupy less volume.
   • Contracted air is denser (heavier) than the surrounding warmer air, so cool air sinks.
   • This phenomenon is crucial for creating convection currents in the air, leading to wind.

4. Wind Currents are Generated due to Uneven Heating on the Earth:

   • Wind: Moving air is called wind. Air always moves from a region of high pressure to a region of low pressure.
   • The greater the difference in pressure, the faster the air moves (stronger wind).
   • Uneven heating of the Earth's surface by the sun is the primary cause of pressure differences and thus, winds.
   • Two main situations of uneven heating:
     • Uneven heating between the Equator and the Poles:
       • The Equator receives direct sunlight and gets heated more intensely. Air here warms up, becomes lighter, rises, creating a low-pressure region.
       • The Poles receive slanting sunlight and are colder. Air here is cold, dense, sinks, creating high-pressure regions.
       • Wind blows from the high-pressure polar regions towards the low-pressure equatorial region. (Note: Earth's rotation complicates the exact direction, leading to trade winds, westerlies etc., but the basic principle is high to low pressure).
     • Uneven heating of Land and Water:
       • Sea Breeze (Daytime): Land heats up faster than the sea. Air above land gets hot, rises (low pressure). Cooler, denser air from the sea (high pressure) moves towards the land.
       • Land Breeze (Night-time): Land cools down faster than the sea. Air above the sea is now warmer, rises (low pressure). Cooler, denser air from the land (high pressure) moves towards the sea.
       • Monsoons: These are large-scale seasonal winds caused by the differential heating of landmasses (like the Indian subcontinent) and large water bodies (Indian Ocean, Arabian Sea, Bay of Bengal). Summer monsoons bring rain as moist air moves from sea to land.

5. Thunderstorms:

   • Develop in hot, humid tropical areas frequently.
   • Rising temperatures produce strong upward rising winds carrying water droplets.
   • Droplets freeze at higher altitudes and fall down.
   • The swift movement of falling water droplets along with rising air creates friction, charging clouds. This leads to lightning (electric discharge) and sound (thunder).
   • Precautions: Do not take shelter under isolated trees; avoid metal objects; stay indoors or in a car; avoid water bodies.

6. Cyclones:

   • A cyclone is a large-scale weather system characterized by a low-pressure centre (called the eye) surrounded by strong, spiralling winds, heavy rain, and thunderstorms.
   • Formation Conditions: Warm sea surface temperature (above 27°C), high humidity (moisture supply), atmospheric instability, and the Coriolis effect (due to Earth's rotation, which helps the spinning).
   • Structure:
     • Eye: Calm, clear centre with very low pressure.
     • Eyewall: A ring of towering thunderstorms surrounding the eye; region of strongest winds and heaviest rain.
     • Rainbands: Spiral bands of clouds, rain, and thunderstorms extending outwards.
   • Destruction: Caused by:
     • Strong Winds: Damage houses, trees, communication systems.
     • Storm Surge: Abnormal rise in sea level caused by the cyclone pushing water towards the coast; causes massive flooding in coastal areas.
     • Heavy Rainfall: Leads to inland flooding.
   • Different Names: Called Hurricanes in the Atlantic and Northeast Pacific, Typhoons in the Northwest Pacific, and Cyclones in the South Pacific and Indian Ocean.
   • Safety & Monitoring: Advanced warnings through satellites and radar help in timely evacuation. Follow government agency warnings, move to safer places (cyclone shelters), store drinking water and food, keep emergency kits ready.

7. Tornadoes:

   • A violently rotating column of air extending from a thunderstorm cloud to the ground. Appears as a dark, funnel-shaped cloud.
   • Much smaller than cyclones but can have extremely high wind speeds (up to 300 km/h or more).
   • Relatively short-lived and travel shorter distances compared to cyclones.
   • Cause intense localized destruction.
   • Safety: Take shelter in a low-lying area, basement, or an interior room away from windows. If outdoors, lie flat in a ditch.

Key Takeaways for Exams:

• Understand the relationship between wind speed and air pressure.
• Know how temperature differences cause air movement (convection).
• Remember the causes of global wind patterns (Equator-Pole heating) and local winds (Land/Sea breeze).
• Be clear about the formation, structure, and destructive effects of Thunderstorms, Cyclones, and Tornadoes.
• Focus on the safety measures associated with each phenomenon.
• Remember the different names for cyclones in different parts of the world.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on Chapter 8 to test your understanding:

1. Moving air is called:
   a) Pressure
   b) Wind
   c) Atmosphere
   d) Cyclone

2. High-speed winds blowing over a hut's roof can lift the roof off because:
   a) The wind pushes the roof upwards from below.

   • b) High-speed wind creates low pressure above the roof compared to higher pressure inside.
     c) The roof material is very light.
     d) The wind creates high pressure above the roof.

3. Air moves from a region of ______ pressure to a region of ______ pressure.
   a) Low, High
   b) Low, Low
   c) High, Low
   d) High, High

4. During the daytime near the coast, the wind generally blows:
   a) From land to sea (Land breeze)
   b) From sea to land (Sea breeze)
   c) Vertically upwards
   d) Does not blow

5. Which of the following is essential for the formation of a cyclone?
   a) Cold sea surface
   b) Dry air
   c) Low humidity
   d) Warm sea surface

6. The calm, low-pressure centre of a cyclone is called the:
   a) Eyewall
   b) Rainband
   c) Funnel
   d) Eye

7. Which phenomenon involves a dark, funnel-shaped cloud reaching from the sky to the ground?
   a) Thunderstorm
   b) Cyclone
   c) Tornado
   d) Monsoon

8. Warm air is _______ than cold air and it _______.
   a) Heavier, sinks
   b) Lighter, sinks
   c) Heavier, rises
   d) Lighter, rises

9. Uneven heating between the Earth's equator and poles generates:
   a) Land breezes
   b) Sea breezes
   c) Global wind circulation patterns
   d) Tornadoes

10. Which is NOT a recommended safety measure during a thunderstorm?
    a) Taking shelter under an isolated tall tree.
    b) Staying inside a car with windows shut.
    c) Avoiding contact with water.
    d) Unplugging electrical appliances.

Answers:

1. b) Wind
2. b) High-speed wind creates low pressure above the roof compared to higher pressure inside.
3. c) High, Low
4. b) From sea to land (Sea breeze)
5. d) Warm sea surface
6. d) Eye
7. c) Tornado
8. d) Lighter, rises
9. c) Global wind circulation patterns
10. a) Taking shelter under an isolated tall tree.

Study these notes thoroughly. Understanding the 'why' behind these phenomena is key. Let me know if any part needs further clarification!