Tropical cyclones and storm surges

Detail

1. What a tropical cyclone is

• A tropical cyclone is a very strong low-pressure system (an area where air pressure is lower than all around it). Winds spiral into its centre.
• It forms between 30°N and 30°S.
• It is 500-1,000 km wide and 12-14 km tall.

• It turns anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

• Source of energy is latent heat of condensation. Latent heat is the heat that is stored in water vapour and given out when the vapour turns back into water.

• Warm sea water evaporates, and the moist air rises.
• The rising air cools, and the vapour condenses into cloud droplets, which releases latent heat.

• This heat warms the air, so the air rises faster. Pressure at the surface drops further, and more moist air rushes in. The storm feeds itself.

• The storm loses strength over land. Once it makes landfall (crosses the coast), it is cut off from moisture. So its force falls with distance from the sea.

• The eye (Class 7 shows it with Cyclone Fani, 2019):
• The eye is the calm centre of the storm. Air sinks there, so the sky is clear or only partly cloudy, winds are light and pressure is at its lowest.
• The eyewall is a ring of tall clouds around the eye. It has the strongest winds and heaviest rain.
• When the eye passes over a place, there is a short calm. Then the winds return from the opposite direction.

2. Conditions needed for formation

Formation is covered in detail in the air-masses-fronts-cyclones note. In short:

Condition	Why it is needed
Steady supply of warm, moist air (warm sea surface)	Gives the latent heat that powers the storm
Strong Coriolis force (the turning force caused by Earth's spin, which bends moving air to the right in the Northern Hemisphere)	Makes the inflowing air spin. It is almost zero near the equator, so no cyclone forms within 0-5° of the equator
Unstable troposphere (lowest layer of the atmosphere, where air rises easily)	Lets tall storm clouds grow
No strong vertical wind shear (wind shear is a big change in wind speed or direction with height)	Strong shear tears the tall storm column apart before it can organise

3. Where and when cyclones form in India

• India gets cyclones from two seas: the Bay of Bengal and the Arabian Sea.
• October-November peak (post-monsoon): Bay of Bengal storms form between 16°-2°N (as printed in NCERT; read it as roughly 10°-16°N), west of 92°E.
• July (monsoon): they form near 18°N, west of 90°E, close to the Sundarbans delta (West Bengal and Bangladesh).
• The east coast is hit most. The coasts of Odisha, Andhra Pradesh, Tamil Nadu and West Bengal face the Bay. Storms there move west or north-west, straight onto the coast.
• IMD (India Meteorological Department) tracks each storm's formation, path, landfall and later life.

Feature	Bay of Bengal	Arabian Sea
Number of cyclones	About 4 times as many	Fewer
Most-hit coasts	Odisha, Andhra Pradesh, Tamil Nadu, West Bengal, Bangladesh, Myanmar	Gujarat, Maharashtra, Pakistan, Oman
Recent trend	Still the main cyclone zone	Storms getting stronger: Tauktae (2021), Biparjoy (2023)

4. IMD classification (North Indian Ocean)

IMD grades a storm by its maximum sustained surface wind. This is the average wind speed, not short gusts. [2]

Category	Wind (knots)	Wind (km/h)
Depression (D)	17-27	31-49
Deep Depression (DD)	28-33	50-61
Cyclonic Storm (CS)	34-47	62-88
Severe Cyclonic Storm (SCS)	48-63	89-117
Very Severe Cyclonic Storm (VSCS)	64-89	118-166
Extremely Severe Cyclonic Storm (ESCS)	90-119	167-221
Super Cyclonic Storm (SuCS)	≥120	≥222

• Source for the whole table: [2]
• Weaker than a depression is a Low Pressure Area. It has no fixed wind band. [2]

5. Naming of cyclones

• RSMC New Delhi names cyclones over the Bay of Bengal and the Arabian Sea. RSMC means Regional Specialised Meteorological Centre, and IMD runs it. It also sends cyclone and storm-surge warnings to 13 member countries of the WMO/ESCAP Panel on Tropical Cyclones (WMO: World Meteorological Organization; ESCAP: UN Economic and Social Commission for Asia and the Pacific). [6][7]
• The 13 countries are Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, UAE and Yemen. [7]
• Five of them (Iran, Qatar, Saudi Arabia, UAE, Yemen) were added after the Panel's 45th session at Muscat, Oman (September 2018). [7]
• The current list was adopted in April 2020. It has 169 names: 13 names from each of the 13 countries. The names are used in order. [6][7]

6. Storm surge

• Storm surge is an abnormal rise of sea level above the normal astronomical tide (the tide caused by the Moon and Sun). It is pushed onto the coast by a cyclone. It comes from the air, the sea and the land acting together:
• Wind: strong onshore winds pile water against the coast. NCERT gives an average of about 180 km/h on the coast.
• Pressure: very low pressure at the centre lets the sea surface bulge upward.

• Rain: heavy rain swells rivers, and the rivers cannot drain into a raised sea.

• What it does:

• floods settlements and farmland
• makes soil salty (salinisation), which harms crops for years
• knocks down buildings and other structures

• causes most cyclone deaths, more than the wind does

• Why some coasts suffer more: coasts that are shallow, gently sloping and funnel-shaped squeeze the water and push it higher and further inland. Examples are the head of the Bay of Bengal, the Sundarbans, Bangladesh and Odisha.

• Measured and forecast surges:
• Phailin (2013): 2-2.5 m above tide in low areas of Ganjam district, Odisha. Salt water reached about 1 km inland. [11]
• Fani (2019): a surge of about 1.5 m above tide was forecast for low-lying Odisha districts. [12]
• Amphan (2020): up to 4-5 m above tide was forecast for West Bengal and 2.5-3.5 m for Bangladesh. [9]

7. Mitigation and management

• Cyclones are easier to forecast than earthquakes. Satellites can follow a storm for days before landfall, so there is time to warn and move people.
• Ways to reduce damage:
• satellite monitoring and early warnings
• cyclone shelters
• embankments and dykes (raised earth walls that keep the sea out)
• reservoirs

• shelterbelts (rows of trees along the coast that slow the wind and the surge)

• Deaths stay high in India, Bangladesh and Myanmar. Many poor people live on low, crowded coasts in weak houses.

8. Case studies: from mass death to "zero casualty"

Cyclone	Year	Landfall	Key facts
Odisha super cyclone (BOB06)	29 Oct 1999	Odisha	About 10,000 dead (WMO: "over 10,000") [8]
Phailin	Oct 2013	Ganjam, Odisha	About 1 million evacuated; about 45 deaths, though it was as strong as the 1999 storm [8][10]
Hudhud	Oct 2014	Visakhapatnam, Andhra Pradesh	Severe damage to the city
Fani (ESCS)	3 May 2019	Near Puri, Odisha	Winds 180-190 km/h; peak 190-200 km/h with gusts to 220 km/h on 2 May; about 1 million evacuated; fewer than 50 deaths reported at first [8]
Amphan	20 May 2020	Between Digha (West Bengal) and Hatiya (Bangladesh), across the Sundarbans	Grew quickly into a super cyclonic storm, then hit the coast as a VSCS (winds 155-165 km/h, gusts 185 km/h); about 1 million evacuated in India and nearly 2 million in Bangladesh [9][13]

• The "zero casualty" model of OSDMA:
• Odisha set up the Odisha State Disaster Management Authority (OSDMA) in 1999, after the super cyclone.

• Its approach rests on accurate IMD warnings, shelters in every village, volunteers and early evacuation.

• Phailin became a global example. UNDRR (UN Office for Disaster Risk Reduction) chose its evacuation as a model before the Third UN World Conference on Disaster Risk Reduction (2015). That conference adopted the Sendai Framework. [10]

9. National Cyclone Risk Mitigation Project (NCRMP)

• What it is: a programme run by NDMA (National Disaster Management Authority) under the Ministry of Home Affairs, with World Bank loans. It aims to make coastal communities safer from cyclones. It covers 8 coastal states in two phases. [5]

Phase	States	Cost / timing
Phase I	Andhra Pradesh, Odisha	₹1,496.71 crore [4]; approved 2010 [5], work from 2011
Phase II	Gujarat, Goa, Karnataka, Kerala, Maharashtra, West Bengal	₹2,361.35 crore [3]; World Bank agreement signed August 2015 [14]

• What it builds:
• multipurpose cyclone shelters (MPCS): used as schools or community halls in normal times and as shelters during storms
• early warning dissemination systems
• evacuation and access roads, and bridges
• saline embankments

• underground power cabling [3]

• Phase I output: 219 MPCS in Andhra Pradesh and 316 MPCS in Odisha. [5]

Prelims Hooks

• No tropical cyclone forms within 0-5° of the equator, because the Coriolis force is too weak there.
• A tropical cyclone's energy comes from the latent heat of condensation. It weakens after landfall because its moisture supply is cut off.
• IMD's Super Cyclonic Storm threshold is ≥120 knots (≥222 km/h). The ESCS range is 90-119 knots (167-221 km/h). [2]
• RSMC New Delhi serves 13 countries. Iran, Qatar, Saudi Arabia, UAE and Yemen joined in 2018. The current 169-name list dates from April 2020. [7]
• In the eye of a cyclone, winds are calm, pressure is lowest and skies are clear. The strongest winds are in the eyewall, not in the eye.
• Storm surge, not wind, causes most cyclone deaths. It is worst on shallow, funnel-shaped coasts such as the head of the Bay of Bengal and the Sundarbans.
• Bay of Bengal cyclones peak in October-November. The Bay gets about 4 times as many cyclones as the Arabian Sea.
• NCRMP Phase I covered Andhra Pradesh and Odisha. Phase II covered Gujarat, Goa, Karnataka, Kerala, Maharashtra and West Bengal. Trap: Tamil Nadu is not in either phase. [3][4]
• Pairs: Fani (2019) — Puri, Odisha; Amphan (2020) — Sundarbans, West Bengal and Bangladesh; Phailin (2013) — Ganjam, Odisha. [8][9][11]

Mains Points

• Why the east coast is more exposed:
• The Bay of Bengal is warm and gets about 4 times as many storms as the Arabian Sea.
• Its head is shallow and funnel-shaped, which builds high surges.
• The coast has crowded, low-lying deltas: Mahanadi, Godavari-Krishna and Ganga-Brahmaputra (Sundarbans).

• Result: frequent landfalls, salinised farmland and heavy losses.

• The Arabian Sea is becoming a growing risk:

• Warming seas are making Arabian Sea storms stronger, as seen in Tauktae (2021) and Biparjoy (2023).

• The west coast (Gujarat, Maharashtra, Kerala) has less experience of cyclones and fewer shelters. This is one reason NCRMP Phase II covers these states.

• From relief to preparedness (GS-III):

• 1999: about 10,000 dead. Phailin 2013: about 45 dead. Fani 2019: fewer than 50 dead. [8]
• What made the difference: accurate IMD forecasts, OSDMA's zero-casualty approach, multipurpose shelters and mass evacuation.

• This approach fits the Hyogo and Sendai Frameworks, which focus on reducing risk before a disaster rather than only giving relief afterwards.

• Problems still left:

• salinisation and loss of livelihoods after surges
• weak houses and power networks (Fani wrecked power infrastructure)
• mangrove loss in the Sundarbans
• these call for shelterbelts, mangrove restoration, saline embankments and underground cabling, as funded under NCRMP