IMD classification of cyclonic disturbances

Topic: Air Masses, Fronts, Cyclones and Thunderstorms · NCERT: Beyond NCERT

Meaning

The IMD classification of cyclonic disturbances is the India Meteorological Department's (IMD) way of putting every low-pressure system over the Bay of Bengal and the Arabian Sea into one of eight grades. The grades run from low-pressure area to super cyclonic storm, and the grade depends on the maximum sustained surface wind, which is the wind speed near the surface averaged over 3 minutes.

It matters because each grade sets off a different level of warning. The grade also decides when a storm gets a name: only at cyclonic storm strength (34 knots / 62 km/h) and above.

Explanation

The eight grades (north Indian Ocean)

• 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

• What the grade is based on: only the maximum sustained surface wind. It does not depend on rainfall or on the size of the storm.
• Averaging time: IMD takes the wind averaged over 3 minutes.

The first steps: low and depression

• Low-pressure area: a place where air pressure is lower than in the areas around it, so air flows in from all sides.

• Normal sea-level pressure is about 1,013 mb (millibar, a unit of air pressure).

• Depression: a low with closed isobars.

• Isobars are lines on a weather map that join places with the same pressure.
• "Closed" means the lines make complete rings around the centre.

• A pressure below about 1,000 mb usually means 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 become cyclonic storms.
• So an IMD "depression warning" is often a rain alert, not a storm alert.

How a system moves up the grades

• Stage 1: low-pressure area
• Warm sea water heats the air above it.

• The warm, moist air rises, and the pressure at the surface falls.

• Stage 2: depression, then deep depression

• Isobars close around the centre.

• The Coriolis force (the turning effect of the Earth's spin on moving air) makes the incoming air spin anticlockwise in the northern hemisphere.

• Stage 3: cyclonic storm and above

• Water vapour condenses into cloud and gives out heat. This heat makes the system stronger.
• When winds pass 34 knots (62 km/h), the system becomes a cyclonic storm and gets a name.
• If it keeps strengthening, it moves up to severe, very severe, extremely severe and finally super cyclonic storm.

Naming, which is linked to the grade

• RSMC New Delhi (Regional Specialised Meteorological Centre, run by the IMD) names a storm over the Bay of Bengal or the Arabian Sea once its maximum sustained surface wind reaches 34 knots (62 km/h) [1].
• It is one of 6 RSMCs in the world. There are also 5 TCWCs (Tropical Cyclone Warning Centres) [1].
• It gives cyclone and storm-surge advice to the 13 WMO/ESCAP Panel member countries [1].
• 2020 list: 169 names, 13 from each of 13 members [1][2]. Amphan (2020) was the last name on the old 2004 list [1].

Where it occurs

• Area covered: the north Indian Ocean, meaning the Bay of Bengal and the Arabian Sea.
• Who runs it: the IMD, working as RSMC New Delhi [1].
• Indian examples by grade:
• 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).

• Depressions without cyclones: monsoon depressions over the head of the Bay of Bengal that move west-north-west across central India.

• Elsewhere in the world: the Atlantic and eastern Pacific use the Saffir-Simpson scale. The same kind of storm is called a cyclone in the Indian Ocean, a hurricane in the Atlantic and eastern Pacific, and a typhoon in the western Pacific.

Don't confuse with

• Saffir-Simpson Hurricane Wind Scale: the US National Hurricane Center (NHC) rates storms from 1 to 5 using a 1-minute wind average, not IMD's 3 minutes [3]. Category 5 starts at 252 km/h (137 knots) [3]. IMD's super cyclonic storm starts at 222 km/h (120 knots). The two sets of figures cannot be compared directly.
• Depression vs cyclonic storm: a depression (31–49 km/h, closed isobars) gets no name and usually means monsoon rain. Naming starts only at cyclonic storm level (62 km/h).
• Low-pressure area vs depression: a low-pressure area has winds below 31 km/h and no closed isobars. A depression has closed isobars and winds of 31–49 km/h.
• "Major hurricane" vs super cyclonic storm: "major hurricane" means Saffir-Simpson Category 3 and above [3]. Roughly, an extremely severe cyclonic storm matches Category 3–4, and a super cyclonic storm matches Category 4–5.

Prelims Hooks

• IMD's grade depends only on the maximum sustained surface wind, averaged over 3 minutes. The NHC uses 1 minute.
• Correct order: low-pressure area → depression → deep depression → cyclonic storm → severe → very severe → extremely severe → super cyclonic storm.
• Naming starts at cyclonic storm: 34 knots / 62 km/h [1]. Depressions and deep depressions are not named.
• Super cyclonic storm = 222 km/h (120 knots) or more. Very severe cyclonic storm = 118–166 km/h (64–89 knots).
• Amphan (2020) reached super cyclonic storm strength over the Bay of Bengal. Biparjoy (2023) came from the Arabian Sea and made landfall in Kutch, Gujarat.
• Trap: a "depression" (closed isobars, below about 1,000 mb) usually means monsoon rain, not a named cyclone.

Mains Points

• Graded warnings save lives (GS-III):
• Each IMD grade triggers its own level of warning.
• Each step up brings wider coastal evacuation, a fishing ban and NDRF teams placed in advance.

• The evacuations in Odisha before Fani (2019) and in Gujarat before Biparjoy (2023) show how timely warnings based on the grade reduce deaths.

• The grade does not tell the whole story:

• The scales use different averaging periods (3 minutes vs 1 minute).
• Wind-only scales leave out storm surge (sea water pushed onto land by the storm) and rain flooding [3].

• Disaster plans should therefore look at storm surge, rainfall and how exposed the coast is, not just the category.

• India's regional role:

• As RSMC New Delhi, the IMD gives cyclone and storm-surge advice to 13 WMO/ESCAP Panel countries, from the Gulf to Southeast Asia [1].
• This makes the IMD system a tool of regional disaster cooperation.

Related concepts

• Depression
• Saffir-Simpson scale
• Naming of tropical cyclones

Read more

• Classifying and naming cyclones