Context:
Cyclone Ditwah weakened on Sunday while moving northward parallel to the Tamil Nadu–Puducherry coast, causing torrential rains, loss of lives, crop damages, and widespread flooding. Sri Lanka also reported massive casualties due to the cyclone’s impact.
- Definition
A tropical cyclone is a rapidly rotating storm system with a low-pressure centre, a closed wind circulation, and intense convective activity, forming over warm tropical oceans.
Different regions use different names:
- Cyclone – Indian Ocean
- Hurricane – Atlantic/Eastern Pacific
- Typhoon – Western Pacific
- Willy-willy – Australia
- Conditions Favourable for Formation (UPSC favourite)
- Warm Ocean Temperature ≥ 26.5°C (to depth ~60 m) – provides latent heat.
- Coriolis Force – minimum 5° latitude from equator; no cyclones at the equator.
- Low Vertical Wind Shear – allows vertical cloud development.
- High Humidity in the mid-troposphere.
- Pre-existing low-pressure disturbance (easterly waves, ITCZ).
- Upper-level divergence – helps exhaust air from the top.
- Structure of a Tropical Cyclone
- Eye
- Calm, clear region at the centre.
- Lowest pressure.
- Diameter: 20–50 km.
- Eyewall
- Surrounds the eye.
- Most destructive winds & rainfall.
- Intense convection.
- Rainbands
- Spiral bands producing heavy rainfall.
- Warm Core
- The cyclone is a warm-core system, deriving energy from latent heat.
- Stages of Cyclone Development
- Tropical Disturbance
- Tropical Depression
- Tropical Storm
- Tropical Cyclone / Hurricane / Typhoon
India Meteorological Department (IMD) classification (wind speed):
- Depression
- Deep Depression
- Cyclonic Storm
- Severe Cyclonic Storm
- Very Severe Cyclonic Storm
- Extremely Severe Cyclonic Storm
- Super Cyclonic Storm (> 222 kmph)
- Movement (Track) of Tropical Cyclones
- Move westward initially due to trade winds.
- Later may recurve north or northeast due to westerlies.
- Influenced by:
- Coriolis force
- Pressure systems
- Sea surface temperature
- Subtropical ridge
- Why Tropical Cyclones Rarely Form on the West Coast of India?
- Lower sea surface temperature (due to cold currents, upwelling).
- Higher vertical wind shear in Arabian Sea.
- Less pre-existing disturbances compared to Bay of Bengal.
- Why Bay of Bengal Produces More Cyclones? (Very important for UPSC)
- Warmer sea temperatures.
- More inflow of moist winds from Pacific.
- Frequent easterly waves & remnants of typhoons.
- Higher humidity and conducive conditions.
- Destructive Impacts
- Storm Surge – major cause of deaths; water pushed inland.
- Heavy Rainfall & Flooding
- High Winds – infrastructure damage.
- Coastal erosion
- Loss of life and property