Classifying and naming cyclones

Detail

1. Starting point: the low and the depression

• Low-pressure area: a region where air pressure is lower than in the areas around it. Air flows into it from all sides.
• Normal sea-level pressure is about 1,013 mb (millibar, a unit of air pressure).
• Depression: a low that has closed isobars. Isobars are lines on a weather map that join places with equal pressure. "Closed" means the lines form complete rings around the centre.

• A reading below about 1,000 mb usually signals a depression.

• Not every depression becomes a cyclone.

• Many monsoon depressions form over the head of the Bay of Bengal and move west-north-west across central India.
• They bring much of the monsoon rain but never grow to cyclonic-storm strength.
• This is why a "depression warning" from the IMD often means a rain alert, not a storm alert.

2. IMD classification of cyclonic disturbances (north Indian Ocean)

• IMD classification: the India Meteorological Department (IMD) places every low over the Bay of Bengal and the Arabian Sea in a category.
• The category depends on the maximum sustained surface wind. This is the wind speed near the surface, averaged over 3 minutes.
• 1 knot = 1.852 km/h.

Category	Wind (km/h)	Wind (knots)	Named?
Low-pressure area	below 31	below 17	No
Depression	31–49	17–27	No
Deep depression	50–61	28–33	No
Cyclonic storm	62–88	34–47	Yes, from here up
Severe cyclonic storm	89–117	48–63	Yes
Very severe cyclonic storm	118–166	64–89	Yes
Extremely severe cyclonic storm	167–221	90–119	Yes
Super cyclonic storm	222 or more	120 or more	Yes

• How a cyclone grows, step by step:
• Warm sea water heats the air above it. The warm, moist air rises and pressure at the surface falls (low-pressure area).
• Isobars close around the centre, and the Coriolis force (the turning effect of the Earth's spin on moving air) makes the inflowing air spin anticlockwise in the northern hemisphere (depression, then deep depression).

• Condensation releases heat, which drives the system harder. Winds cross 34 knots (62 km/h) and the system becomes a cyclonic storm.

• Recent Indian examples:

• Fani (2019) hit Odisha.
• Amphan (2020) hit West Bengal and Bangladesh. It reached super cyclonic storm strength over the Bay of Bengal.
• Biparjoy (2023) formed in the Arabian Sea and made landfall in Gujarat (Kutch).

3. Saffir-Simpson scale (Atlantic and eastern Pacific)

• Saffir-Simpson Hurricane Wind Scale: a 1 to 5 rating used by the US National Hurricane Center (NHC). It is based only on maximum sustained wind speed [4].
• It does not count other deadly hazards, such as storm surge (sea water pushed onto land by the storm), rain flooding or tornadoes [4].

Category	km/h	knots
1	119–153	64–82
2	154–177	83–95
3 (major)	178–208	96–112
4 (major)	209–251	113–136
5 (major)	252 or more	137 or more

(NHC, scale as updated May 2021 [4])

• Category 3 and above are "major hurricanes". Their winds alone can cause very heavy damage and loss of life [4].
• Do not mix the two scales.
• The IMD averages wind over 3 minutes. The NHC uses 1 minute. A 1-minute average gives a higher number for the same storm, so the figures are not strictly comparable.
• Rough matching:
  • Very severe cyclonic storm (118–166 km/h) ≈ Category 1–2.
  • Extremely severe cyclonic storm (167–221 km/h) ≈ Category 3–4.
  • Super cyclonic storm (222 km/h or more) ≈ Category 4–5.
• The same kind of storm has different names in different oceans: cyclone (Indian Ocean), hurricane (Atlantic and eastern Pacific) and typhoon (western Pacific).

4. Who names north Indian Ocean cyclones

• Worldwide there are 6 RSMCs (Regional Specialised Meteorological Centres) and 5 TCWCs (Tropical Cyclone Warning Centres). Between them they issue cyclone advice and name tropical cyclones [2].
• RSMC New Delhi (run by the IMD) is one of the six. It gives cyclone and storm-surge advice to the 13 WMO/ESCAP Panel member countries [2].
• It names a storm over the Bay of Bengal or the Arabian Sea once the maximum sustained surface wind reaches 34 knots (62 km/h), which is cyclonic-storm strength [2].
• WMO/ESCAP Panel on Tropical Cyclones (PTC): a regional body of the World Meteorological Organization (WMO) and the UN Economic and Social Commission for Asia and the Pacific (ESCAP). It agrees on the list of names.
• Special rule: a cyclone from the South China Sea that crosses Thailand and enters the Bay of Bengal keeps its old name. It is not renamed [2].

5. History of the naming lists

Step	Year	Detail
Decision in principle	2000	27th PTC session, Muscat, Oman [2]
First list starts	September 2004	64 names from 8 countries: Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand [2]
New list decided	September 2018	45th PTC session, Muscat. Five new members added: Iran, Qatar, Saudi Arabia, UAE and Yemen [2]
Rapporteur's report	September 2019	Presented at the 46th session, Nay Pyi Taw, Myanmar. Dr Mrutyunjay Mohapatra (IMD) was rapporteur (the person who brings members together and finalises the list) [2]
New list adopted	April 2020	Adopted by consensus. WMO announced it on 1 May 2020 [2][3]

• Amphan (2020) was the last name on the 2004 list. The new list began after it [2].
• New list (2020): 169 names, 13 from each of the 13 members: Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, UAE and Yemen [2][3].
• How the list is used:
• Countries are listed in alphabetical order [2].
• Names are used in sequence, column by column. The list starts at the first row of column 1 and runs to the last row of column 13 [2].
• Column 1 begins: Nisarga (Bangladesh), Gati (India), Nivar (Iran), Burevi (Maldives), Tauktae (Myanmar), Yaas (Oman), Gulab (Pakistan), Shaheen (Qatar), Jawad (Saudi Arabia), Asani (Sri Lanka), Sitrang (Thailand), Mandous (UAE) [2].
• Biparjoy (2023) was Bangladesh's name in column 2 [2].
• India's first names on the list are Gati, Tej, Murasu, Aag [2]. The public sent in suggestions, and an IMD committee chose India's names [2].

6. Rules for choosing a name

• The name must be neutral to politics and political figures, religious beliefs, cultures and gender [2][3].
• It must not hurt the feelings of any group of people anywhere in the world. It must not be rude or cruel [2].
• It must be short, easy to say and not offensive to any member [2][3].
• Maximum length: 8 letters [2][3].
• It must be sent with its pronunciation and a voice recording [2].
• Never repeated. "Once used, it will cease to be used again." The name must also be new to every other RSMC's list worldwide [2][3].
• The Panel can reject a name that breaks these rules [2].

7. Why cyclones are named

• Each storm can be identified easily [2][3].
• People become aware of a storm while it is still forming [2].
• It removes confusion when two storms exist at the same time in the same region [2][3].
• A name is easier to remember than a number [2].
• Warnings reach a much wider audience, quickly [2].

Prelims Hooks

• The IMD names a north Indian Ocean storm only once it reaches cyclonic-storm strength: 34 knots / 62 km/h. Depressions and deep depressions are not named.
• Super cyclonic storm = 222 km/h (120 knots) or more. Saffir-Simpson Category 5 = 252 km/h (137 knots) or more.
• IMD averages wind over 3 minutes. NHC averages over 1 minute. The scales are not directly comparable.
• On the Saffir-Simpson scale, Category 3 and above = "major hurricane". The scale is based only on wind, not storm surge or rain.
• Naming in the north Indian Ocean was agreed at Muscat (2000) and began in September 2004 with 64 names from 8 countries.
• 2020 list: 169 names, 13 members × 13 names. The five new members were Iran, Qatar, Saudi Arabia, UAE and Yemen.
• Amphan was the last name of the 2004 list. Nisarga (Bangladesh) was the first name of the 2020 list.
• Biparjoy (2023) was a name proposed by Bangladesh. Gati and Tej are names proposed by India.
• Name rules: at most 8 letters, neutral, never repeated. A Pacific storm that crosses Thailand into the Bay of Bengal keeps its name.
• Trap: a "depression" (below 1,000 mb, closed isobars) usually means monsoon rain, not a named cyclone.

Mains Points

• Graded classification helps disaster response (GS-III): each IMD category (depression → super cyclonic storm) triggers its own level of warning. Each step up means more coastal evacuation, a ban on fishing and pre-placed NDRF teams. Odisha's evacuations before Fani (2019) and Gujarat's before Biparjoy (2023) show how timely, category-based warnings save lives.
• Naming as a tool of risk communication: short, neutral, never-repeated names make it easier for the media, disaster managers and the public to follow a storm. This matters most when the Bay of Bengal and the Arabian Sea have storms at the same time.
• Regional cooperation: the WMO/ESCAP Panel grew from 8 to 13 members, and the 2020 list was finalised under an IMD rapporteur. This shows India's role as RSMC New Delhi, a regional provider of weather services to countries around the Indian Ocean, from the Gulf to Southeast Asia.
• Limits of comparison: the scales use different wind-averaging periods (3 minutes vs 1 minute), and the Saffir-Simpson scale ignores surge and rain. A disaster plan should therefore weigh storm surge, rainfall and how exposed the coast is, not just the category number.