Class 7 Notes

Class 7 Science Notes Chapter 8 (Winds; Storms and Cyclones) – Science Book

Philoid

Philoid

— 5 min read

Science
Okay, let's prepare detailed notes for Chapter 8: Winds, Storms and Cyclones from the NCERT Class 7 Science textbook, focusing on aspects relevant for government exam preparation.

NCERT Class 7 Science - Chapter 8: Winds, Storms and Cyclones

Objective: To understand the properties of air related to pressure and temperature, how winds are generated, and the formation and impact of storms like thunderstorms, cyclones, and tornadoes.

Key Concepts & Principles:

1. Air Exerts Pressure

  • Definition: The weight of the air column above a certain area exerts force on that area. This is known as Air Pressure or Atmospheric Pressure.
  • Demonstration: A crushed can experiment (heating water in a can, sealing it, and pouring cold water over it causes the can to crush due to higher outside pressure). Inflating balloons, bicycle tubes shows air exerts pressure.
  • Significance: Differences in air pressure are the primary cause of air movement (wind). Air moves from a region of High Pressure (HP) to a region of Low Pressure (LP).

2. High-Speed Winds are Accompanied by Reduced Air Pressure

  • Principle: When air moves at a high speed, the pressure it exerts decreases in that region. (Related to Bernoulli's Principle, though not explicitly named in Class 7).
  • Demonstrations:
    • Blowing over a strip of paper makes it rise (low pressure above, high pressure below).
    • Blowing between two suspended balloons makes them move closer (low pressure between them, higher pressure outside).
  • Real-world Application: Roofs of houses can be lifted off during strong winds because the high-speed wind above the roof creates low pressure, while the pressure inside the house remains relatively higher, pushing the roof upwards.

3. Air Expands on Heating and Contracts on Cooling

  • Heating: When air is heated, its molecules move faster and farther apart, causing it to expand and occupy more volume. Expanded air becomes less dense (lighter).
  • Cooling: When air is cooled, its molecules slow down and come closer, causing it to contract and occupy less volume. Contracted air becomes denser (heavier).
  • Convection: Warm air, being lighter, rises. Cooler, denser air sinks to take its place. This continuous movement sets up convection currents in the air.

4. Wind Generation: Uneven Heating of the Earth

  • Fundamental Cause: The Sun heats the Earth's surface unevenly.
  • Mechanism:
    • Areas receiving more direct sunlight (like the Equator) get heated more intensely. The air above these areas gets hot, expands, becomes lighter, and rises, creating a region of Low Pressure (LP).
    • Areas receiving less sunlight (like the Poles) remain cooler. The air above these areas is cold, dense, and sinks, creating a region of High Pressure (HP).
    • Wind flows from the high-pressure regions (poles) towards the low-pressure regions (equator) at the surface. At higher altitudes, the flow is reversed.
  • Specific Scenarios of Uneven Heating:
    • Uneven Heating between Equator and Poles: Causes large-scale global wind patterns. Air rises at the equator and moves towards the poles at high altitudes, cools, sinks around 30° latitudes and near the poles, and flows back towards the equator near the surface. (Rotation of the Earth also influences wind direction - Coriolis Effect, though not detailed in Class 7).
    • Uneven Heating of Land and Water: Land heats up and cools down faster than water.
      • Summer: Land heats up faster than the ocean. Air above land becomes hot, rises (LP). Cooler, moisture-laden air from the ocean (HP) flows towards the land. This causes Summer Monsoon winds, bringing rain.
      • Winter: Land cools down faster than the ocean. Air above land is cooler, denser (HP). Warmer air above the ocean rises (LP). Wind flows from the land towards the ocean. This causes Winter Monsoon winds, which are generally dry.
      • Sea Breeze (Day): Land heats faster -> LP over land -> Wind from sea to land.
      • Land Breeze (Night): Land cools faster -> HP over land -> Wind from land to sea.

5. Thunderstorms

  • Formation: Develop in hot, humid tropical areas frequently.
    • Rising temperatures cause strong upward rising winds carrying water vapour.
    • Vapour freezes at higher altitudes, forming water droplets that grow larger.
    • Heavy droplets fall downwards as rain.
    • Swift movement of falling water droplets and rising air creates friction, charging clouds. Electrical discharge between clouds or between clouds and the ground causes Lightning.
    • The rapid heating of air by lightning causes sudden expansion, creating sound waves heard as Thunder.
  • Characteristics: Accompanied by heavy rain, lightning, thunder, strong winds, and sometimes hail.
  • Safety Precautions:
    • Do not take shelter under isolated trees or in open fields/garages/sheds.
    • Avoid metal objects, umbrellas with metallic rods.
    • Stay indoors, preferably in a building or car (windows closed).
    • Avoid contact with water, electrical wires, and telephones (corded).

6. Cyclones

  • Definition: A large-scale weather system characterized by a low-pressure center (the 'eye') around which strong winds spiral inwards.
  • Formation Conditions:
    • Warm sea surface temperature (typically above 26.5°C).
    • Presence of moisture (water vapour).
    • Wind patterns that encourage rotation (influenced by Earth's rotation - Coriolis effect).
  • Formation Process:
    • Sun heats water -> Water vapour forms and rises.
    • As vapour rises, it cools, condenses into clouds, releasing heat.
    • This released heat warms the surrounding air, causing it to rise further, leading to a drop in pressure below.
    • More air rushes towards the low-pressure center.
    • Due to the Earth's rotation, this incoming air starts spiraling.
    • A cycle is established: rising warm moist air -> condensation -> heat release -> more air rises -> pressure drops further -> stronger winds spiral inwards.
  • Structure:
    • Eye: The calm, clear center with low pressure and light winds.
    • Eye Wall: A ring of dense clouds surrounding the eye, with the strongest winds and heaviest rainfall.
    • Rain Bands: Bands of clouds and rain spiraling outwards from the eye wall.
  • Destruction Caused By:
    • Strong Winds: Damage houses, trees, communication systems, cause loss of life and property.
    • Heavy Rains & Flooding: Torrential rains lead to severe flooding, especially inland.
    • Storm Surge: Abnormal rise in sea level caused by the cyclone pushing water towards the coast. This is often the most destructive element, causing massive coastal inundation.
  • Different Names:
    • Cyclone: Indian Ocean (Bay of Bengal, Arabian Sea), South Pacific.
    • Hurricane: Atlantic Ocean, Northeast Pacific Ocean.
    • Typhoon: Northwest Pacific Ocean.
  • Safety Measures:
    • Heed warnings from meteorological departments (like IMD - India Meteorological Department).
    • Move to safer places (cyclone shelters) well in advance.
    • Store drinking water, food, first-aid kit.
    • Avoid driving on waterlogged roads.
    • Keep emergency contact numbers handy.
    • Cooperate with rescue teams.

7. Tornadoes

  • Definition: A violently rotating, narrow, funnel-shaped column of air extending from the base of a thunderstorm cloud down to the ground.
  • Characteristics:
    • Appear as dark, funnel-shaped clouds.
    • Extremely high wind speeds (can exceed 300 km/h).
    • Relatively short duration and narrow path of destruction compared to cyclones.
    • Can form over land, often associated with severe thunderstorms (supercells). Cyclones can sometimes spawn tornadoes upon landfall.
    • Difficult to predict precisely.
  • Safety Measures:
    • Seek shelter in a low-lying area, basement, or an interior room away from windows.
    • If outdoors, lie flat in a ditch or low-lying area, protecting your head.

8. Advanced Technology & Forecasting

  • Satellites: Provide images of clouds, helping track the development and movement of cyclones and storms.
  • Radar: Detects rainfall intensity and movement (Doppler radar can also estimate wind speed and rotation within storms, aiding tornado warnings).
  • Anemometer: Measures wind speed.
  • Wind Vane: Shows wind direction.
  • Importance: Early warnings (Cyclone Alert/Watch, Cyclone Warning) issued 24-48 hours in advance allow for timely evacuation and preparedness, significantly reducing loss of life.

9. Effective Safety Measures - Summary

  • Individual: Be aware, heed warnings, prepare emergency kits, know safe shelter locations.
  • Community: Help neighbours, follow official instructions, participate in awareness programs.
  • Government: Accurate forecasting, efficient warning dissemination, construction of shelters, disaster management planning, and rescue operations.

Exam Relevance Highlights:

  • Understand the relationship: Heating -> Expansion -> Lighter Air -> Rises -> Low Pressure.
  • Understand the relationship: Cooling -> Contraction -> Denser Air -> Sinks -> High Pressure.
  • Know that wind flows from High Pressure to Low Pressure areas.
  • Remember the principle: High Speed Wind = Reduced Pressure.
  • Know the causes of wind generation (uneven heating: Equator/Poles, Land/Water).
  • Understand the formation basics of Thunderstorms, Cyclones, Tornadoes.
  • Know the structure of a Cyclone (Eye, Eye Wall).
  • Identify the destructive elements of Cyclones (Wind, Rain, Storm Surge - especially storm surge).
  • Know the different names for Cyclones globally.
  • Be familiar with safety measures for each type of storm.
  • Recognize the role of technology (Satellites, Radar) in forecasting.

This detailed breakdown covers the core concepts of the chapter with added structure and emphasis suitable for exam preparation. Remember to correlate these points with the diagrams and activities mentioned in the NCERT textbook for better understanding.

Read more