TROPICAL CYCLONES

1. Meaning
Tropical cyclones are violent storms that originate over tropical oceans and move towards coastal areas. They are characterized by:
- Very strong winds (squalls)
- Torrential rainfall
- Storm surge
- Closed circulation around a low pressure centre
- Anti-clockwise rotation in Northern Hemisphere and clockwise in Southern Hemisphere due to Coriolis force
They usually occur between 5° to 30° latitudes.

2. Conditions Favourable for Formation
- Sea Surface Temperature above 27°C
- Warm water depth of at least 60–70 m
- Presence of Coriolis force (minimum 5° latitude)
- Low vertical wind shear
- Pre-existing low pressure or disturbance
- Upper air divergence
- High humidity in mid-troposphere
- Abundant latent heat of condensation

3. Role of Coriolis Force
- Zero at equator, hence no cyclones at equator
- 65 percent cyclones occur between 10° and 20° latitudes

4. Origin and Development
Early Stage:
- Warm moist air rises, condensation releases latent heat
- Pressure falls, more air rushes in
- Coriolis force causes spiralling
- Eye begins to form due to centrifugal force

Mature Stage:
- Well developed eye and eyewall
- Spiral rain bands
- Continuous supply of moisture from sea
- Multiple convection cells

Dissipating Stage:
- Occurs after landfall or over cold water
- Moisture supply cut off

5. Structure of Tropical Cyclone
Eye:
- Calm, clear, lowest pressure
- Diameter 30–60 km (can vary)

Eyewall:
- Most violent winds and rainfall
- Maximum wind speed zone

Spiral Rain Bands:
- Narrow bands of heavy rainfall
- Cause alternating calm and stormy weather

Vertical Structure:
- Inflow layer (0–3 km)
- Main storm region (3–7 km)
- Outflow layer (above 7 km, anticyclonic)

6. Categories (Based on Wind Speed)
- Category 1: 90–125 km/h
- Category 2: 125–164 km/h
- Category 3: 165–224 km/h
- Category 4: 225–279 km/h
- Category 5: Above 280 km/h

7. Breeding Grounds
- Caribbean (Hurricanes)
- Bay of Bengal and Arabian Sea (Cyclones)
- Western Pacific (Typhoons)
- South East Africa
- North West Australia

8. Path of Tropical Cyclones
- Move westward initially
- Turn northward around 20°
- Turn northeast around 25°
- Die around 30° latitude

9. Warning and Prediction
- Satellite monitoring
- Radars and aircraft
- Early warning helps in evacuation and disaster management
- India has coastal radar network

TORNADO

1. Meaning
- Violently rotating column of air extending from thunderstorm to ground
- Very low pressure at centre (~800 mb)

2. Conditions for Formation
- Wind shear
- Moisture
- Instability
- Lift

3. Origin
- Horizontal spinning air becomes vertical due to strong updraft
- Forms supercell thunderstorm
- Funnel cloud touches ground forming tornado

4. Distribution
- Most common in USA
- Second highest in Canada
- Bangladesh is most prone in Indian subcontinent
- Rare in polar regions
- Absent in Antarctica

5. Damage
- Due to very high wind speed and sudden pressure fall

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Subject: Geography

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