Notes For All Chapters – Geography Class 7
1. Introduction to Winds
- The movement of air is called wind.
- We can feel the wind but cannot see it.
- Winds are generated due to differences in air pressure in different regions.
- Air moves from high-pressure areas to low-pressure areas, creating winds.
- The greater the difference in air pressure, the higher the wind velocity.
2. Factors Affecting Winds
Air Pressure: Winds blow from areas of high pressure to low pressure.
Earth’s Rotation: The Earth’s rotation affects the direction of winds due to the Coriolis Effect.
- In the Northern Hemisphere, winds are deflected to the right.
- In the Southern Hemisphere, winds are deflected to the left.
Temperature Differences:
- Land heats up and cools down faster than water, creating pressure differences that influence winds.
3. Types of Winds
A. Planetary Winds
These are permanent winds that blow throughout the year from high-pressure belts to low-pressure belts.
Easterlies (Trade Winds):
- Blow from the east between 25° and 35° latitude towards the equator.
- In the Northern Hemisphere, they blow from north-east to south-west.
- In the Southern Hemisphere, they blow from south-east to north-west.
Westerlies:
- Blow from the west between 35° and 60° latitude.
- In the Northern Hemisphere, they blow from south-west to north-east.
- In the Southern Hemisphere, they blow from north-west to south-east.
Polar Winds:
- Blow from the polar high-pressure belts to the subpolar low-pressure belts (55° to 65° latitude).
- Generally, they blow from east to west.
B. Seasonal Winds (Monsoon Winds)
These winds change direction according to the season due to the heating and cooling of land and water.
Summer Monsoon (June to September):
- Winds blow from the sea to land, bringing heavy rainfall.
- These are known as South-West Monsoons.
Winter Monsoon (September to December):
- Winds blow from the land to sea, bringing dry weather.
- These are known as North-East Monsoons.
Monsoon winds are large-scale land and sea breezes.
C. Local Winds
Local winds blow for a short duration in specific regions.
Examples of Local Winds:
- Föhn & Chinook: Warm, dry winds in mountainous regions.
- Loo: Hot winds in North India during summer.
- Bora: Cold winds in Europe.
- Mistral: Cold winds in France.
4. Roaring Forties, Furious Fifties & Screeching Sixties
In the Southern Hemisphere, winds blow with high speed due to the absence of land obstacles.
These winds are named based on their latitudes:
- Roaring Forties (40° S): Very strong winds.
- Furious Fifties (50° S): Stormy winds.
- Screeching Sixties (60° S): Extremely fast and noisy winds.
5. Sea and Land Breezes
Sea Breeze (Daytime):
- Land heats up faster than the sea.
- The warm air over land rises, creating a low-pressure zone.
- Cool air from the sea moves towards the land.
Land Breeze (Nighttime):
- Land cools down faster than the sea.
- The cool air over land creates a high-pressure zone.
- Warm air from the sea moves towards the land.
6. Calm Zones (Doldrums & Horse Latitudes)
Doldrums (Equatorial Calm Zone):
- Located 5° north and 5° south of the equator.
- Characterized by low pressure and calm winds.
Horse Latitudes:
- Found between 25° and 35° north and south latitudes.
- These regions have high pressure and little wind movement.
7. Cyclones and Anticyclones
A. Cyclones (Low Pressure Systems)
- A cyclone is a weather system where a low-pressure area is surrounded by high-pressure areas.
- Winds move towards the low-pressure center in a circular motion.
- Direction of Cyclonic Winds:
- Northern Hemisphere: Move counterclockwise.
- Southern Hemisphere: Move clockwise.
- Effects of Cyclones:
- Heavy rainfall and floods.
- Strong winds causing destruction.
- Storm surges in coastal areas.
- Examples of Cyclonic Storms:
- Typhoons: In the Pacific Ocean.
- Hurricanes: In the Caribbean Sea.
B. Anticyclones (High Pressure Systems)
- An anticyclone is a weather system where air pressure is high at the center and low in surrounding areas.
- Winds move outward from the center.
- Direction of Anticyclonic Winds:
- Northern Hemisphere: Move clockwise.
- Southern Hemisphere: Move counterclockwise.
- Effects of Anticyclones:
- Clear skies and calm weather.
- Pleasant conditions with low wind speeds.
8. Effects of Earth’s Rotation on Winds (Coriolis Effect)
- The Coriolis Effect is caused by the Earth’s rotation and influences wind direction.
- It deflects moving air:
- Rightward in the Northern Hemisphere.
- Leftward in the Southern Hemisphere.
- This effect is responsible for cyclonic circulation and global wind patterns.
9. Naming of Cyclones
- Cyclones in different parts of the world have specific names.
- A list of names is prepared for each oceanic region.
- When a storm’s wind speed exceeds 60 km/h (33 knots), it is officially named.
10. Summary & Key Points
- Winds blow from high-pressure to low-pressure areas.
- Types of Winds:
- Planetary Winds (Easterlies, Westerlies, Polar Winds).
- Seasonal Winds (Monsoons).
- Local Winds (Föhn, Chinook, Loo).
- Roaring Forties, Furious Fifties, Screeching Sixties are high-velocity winds in the Southern Hemisphere.
- Sea & Land Breezes occur due to temperature differences between land and water.
- Cyclones bring strong winds and rain, while Anticyclones bring calm and clear weather.
- Earth’s Rotation (Coriolis Effect) influences wind direction.
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