Tropical Cyclones

Notes

A tropical cyclone is an intense low-pressure system that forms over warm tropical oceans, with strong winds spiralling inwards (anticlockwise in the Northern Hemisphere, clockwise in the Southern). Regional names: cyclone (Indian Ocean), hurricane (Atlantic, north-east Pacific), typhoon (north-west Pacific), willy-willy (Australia).

Conditions for formation

Sea surface temperature above 26-27°C to a depth of about 60 m.
Coriolis force: needed to spin the system, so they rarely form within about 5° of the equator.
Weak vertical wind shear, a pre-existing low-pressure disturbance, high humidity in the mid-troposphere and upper-level divergence.

Structure: the eye (calm, clear and warm with sinking air, 10-50 km wide); the eyewall (the strongest winds and heaviest rain); spiral rainbands. They get energy from latent heat of condensation and weaken quickly over land or cold water, which cuts off moisture.

Hazards: violent winds, torrential rain, and the storm surge, the leading cause of deaths.

IMD classification (maximum sustained wind)

Low pressure area; Depression (17-27 knots); Deep Depression (28-33); Cyclonic Storm (34-47); Severe Cyclonic Storm (48-63); Very Severe (64-89); Extremely Severe (90-119); Super Cyclonic Storm (120 knots and above). The Saffir-Simpson scale (Category 1-5) is used in the Atlantic.

India

The Bay of Bengal gets far more cyclones than the Arabian Sea. Peaks: April-June and October-December.
IMD (New Delhi) is a Regional Specialised Meteorological Centre (RSMC) for the North Indian Ocean; names are assigned from a list agreed by the WMO/ESCAP Panel members (India, Bangladesh, Pakistan, Sri Lanka, Maldives, Myanmar, Oman, Thailand and others).
Notable storms: Odisha super cyclone (1999), Hudhud (2014), Fani (2019), Amphan (2020), Tauktae (2021), Biparjoy (2023), Michaung (2023).
Management: early warning, cyclone shelters, evacuation (NDMA, IMD), coastal zone and mangrove protection.

Key Points

Formed over warm seas (above 26-27°C) with the Coriolis effect, away from the equator.
Eye (calm), eyewall (strongest winds); anticlockwise in the NH.
Names: cyclone, hurricane, typhoon, willy-willy.
Storm surge is the main killer.
Bay of Bengal is more cyclone-prone than the Arabian Sea.

MCQs

Q1. Tropical cyclones rarely form near the equator because of:

(a) Low sea temperature
(b) The absence of the Coriolis force
(c) Strong winds
(d) High pressure
Ans: (b).

Q2. Tropical cyclones in the north-west Pacific are called:

(a) Hurricanes
(b) Typhoons
(c) Willy-willies
(d) Cyclones
Ans: (b).

Q3. The calm, clear centre of a tropical cyclone is called the:

(a) Eyewall
(b) Eye
(c) Rainband
(d) Front
Ans: (b).