Cyclones: Tropical and Temperate — Formation and Impact
July 19, 2026
Cyclones: Tropical and Temperate — Formation and Impact
Introduction
Cyclones are large-scale atmospheric circulation systems characterized by low-pressure centres and rotating winds. They are classified as tropical cyclones (formed over warm tropical oceans) and temperate/extratropical cyclones (formed in mid-latitudes along fronts). Both types significantly influence weather and can cause devastating impacts.
Tropical Cyclones
Definition and Characteristics
Intense low-pressure system over warm tropical/subtropical oceans
Diameter: 100-2,000 km
Wind speed: Up to 300+ km/h
Central pressure: 900-990 mb (extremely low)
Structure:
Eye: Clear, calm, low-pressure centre (~30-60 km)
Eyewall: Ring of intense thunderstorms (highest winds and rainfall)
Rainbands: Spiral bands of convection extending outward
Naming by Region
Region
Name
Examples
North Atlantic
Hurricane
Hurricane Katrina
NW Pacific
Typhoon
Typhoon Haiyan
Indian Ocean / Bay of Bengal
Cyclone
Cyclone Amphan, Cyclone Fani
SW Pacific / Australia
Cyclone/Willy-Willy
Cyclone Debbie
Conditions for Formation
Sea surface temperature > 27°C (deep warm layer to 60m)
Low vertical wind shear (less than 10 m/s between surface and tropopause)
Upper-level divergence (outflow at top — convection not capped)
Stages of Development
Stage
Characteristics
Formation (Tropical Disturbance)
Cluster of thunderstorms over warm ocean
Tropical Depression
Organized circulation; winds 37-61 km/h
Tropical Storm
Closed isobars; winds 62-118 km/h; named
Severe Cyclonic Storm
119-165 km/h (Category 1-3)
Very Severe / Super Cyclone
>166 km/h (Category 4-5)
Structure of a Mature Cyclone
Eye: At centre — 20-50% of cyclones have clear eye; descending air, clear skies
Eyewall: Most destructive — intense updrafts; torrential rain; highest winds
Spiral Rainbands: Convective cells wrapping around; heavy rain
Outflow layer: Upper-level anticyclonic flow — disperses rising air
Movement and Lifecycle
Tracks: Typically westward (trade winds) then curve poleward (westerlies)
Speed: 10-30 km/h (slower over ocean; accelerates after recurvature)
Landfall: Weakening begins due to friction and cutoff from warm ocean
Duration: 3-15 days typically
Destruction Caused
Hazard
Description
Sustained high winds
Structural damage, flying debris
Storm surge
~10-15 km inland; sea water rise up to 10m (most lethal)
Heavy rainfall
200-1000 mm in 24h; floods, landslides
Tornadoes
Embedded in spiral bands
Coastal erosion
Shoreline damage, loss of habitat
Indian Cyclone Seasons
Bay of Bengal: May-June and October-November (pre/post-monsoon)
Arabian Sea: Less frequent (May-June, October)
Bay of Bengal cyclones: More deadly due to shallow water, high population density, funnel shape
Temperate (Extratropical) Cyclones
Definition and Characteristics
Form in mid-latitudes (30°-60°) along the polar front
Energy source: Temperature contrast between warm tropical and cold polar air masses
Diameter: Larger than tropical — up to 3,000-5,000 km
Structure: Associated with fronts (warm, cold, occluded)
Formation: Polar Front Theory
Stage
Description
Stationary front
Cold polar air and warm tropical air meet; no movement
Wave development
Disturbance on front creates wave (low pressure forms)
Open stage
Cold front moves faster than warm front; warm sector forms
Occlusion
Cold front catches up to warm front; warm sector lifted off surface
Mature stage
Intense low; widespread precipitation
Dissipation
Occlusion complete; cold air dominates; cyclone weakens
Weather Associated
Warm front: Prolonged light to moderate rain; temperature rise; cloud sequence (cirrus, cirrostratus, altostratus, nimbostratus)
Cold front: Heavy showers, thunderstorms, squalls; rapid temperature drop; clearing after passage
Warm sector: Between cold and warm fronts; relatively warm, humid, cloudy
Comparison: Tropical vs Temperate Cyclones
Feature
Tropical Cyclone
Temperate Cyclone
Formation
Tropical oceans (8-30°)
Mid-latitudes (30-65°)
Energy source
Latent heat from warm water
Temperature gradient (fronts)
Structure
Symmetrical, eye at centre
Asymmetrical; fronts
Size
100-2,000 km
1,000-5,000 km
Duration
Few days to 2 weeks
3-10 days
Wind speed
Higher (up to 300 km/h)
Lower (up to 150 km/h)
Rainfall
Intense (200-1000 mm)
Widespread, moderate
Season
Summer/autumn (warm SST)
Winter/spring (strong gradient)
India affected
Coastal (Bay of Bengal, Arabian Sea)
Western Himalayas, North India (western disturbances)
Western Disturbances (India-specific)
Extratropical cyclones originating in Mediterranean/Caspian Sea
Travel eastward; affect North/West India in winter (Nov-March)
Effects: Winter rainfall in North India; snow in Himalayas; crucial for Rabi crops
Interaction: Sometimes interact with tropical systems
Cyclone Forecasting and Mitigation
Forecasting Tools
Satellite imagery: INSAT series, Kalpana-1 — tracking cyclone development and movement
Radar: Doppler weather radar network for real-time monitoring
Models: Global and regional (IMD uses GFDL, IMD GFS, HWRF)
Alert system: Color-coded warnings (green → red) from India Meteorological Department (IMD)
Mitigation in India
NDMA guidelines: National Disaster Management Authority
Cyclone shelters: Multi-purpose cyclone shelters along east coast
Early warning: IMD bulletins every 3 hours during cyclone
Mangrove restoration: Natural barrier against storm surge (Sundarbans)
Infrastructure: Cyclone-resistant design for buildings
Coastal zone management: CRZ regulations, setback norms
Conclusion
Tropical and temperate cyclones are powerful atmospheric systems with distinct formation mechanisms and characteristics. While tropical cyclones pose a greater threat to India's coastal regions, temperate cyclones (western disturbances) provide crucial winter precipitation. Improved forecasting, early warning systems, and disaster preparedness have significantly reduced cyclone-related mortality, though economic damage remains substantial.
Practice Questions
Compare and contrast the formation and characteristics of tropical and temperate cyclones.
"Bay of Bengal is more prone to cyclones than the Arabian Sea." Explain why.
Discuss the role of western disturbances in India's climate. Why are they important for Rabi crops?