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Low, depression or cyclone? How the IMD grades the storms brewing in the Bay of Bengal
The weather office expects a low-pressure area over the Bay of Bengal to become a depression by Sunday. What those words mean, how fast the winds must blow before a system is called a cyclone, and how the warnings are issued.
The short answer
The India Meteorological Department grades weather systems over the north Indian Ocean by their strongest sustained winds. A low has winds below 31 km/h, a depression 31-49 km/h and a deep depression 50-61 km/h; only at 62 km/h does a system become a cyclonic storm, and there are four stronger grades above that, up to a super cyclonic storm at 222 km/h or more. The system now forming in the Bay of Bengal is forecast to reach depression stage by 25 October, and the IMD has not said whether it will become a cyclone.
What is the IMD forecasting this week?
A low-pressure area over the Bay of Bengal that is expected to strengthen into a depression by Sunday morning, 25 October.
In its bulletin on Thursday evening the India Meteorological Department (IMD) said a low was very likely to form over the southeast and east-central Bay of Bengal around Friday, 23 October. It gave a high probability that the system would become a depression by Sunday and keep strengthening until 28 October as it moves west-north-west. Gale winds of 70-80 km/h, gusting to 90 km/h, are forecast over the west-central bay from Monday, and fishermen have been told to stay ashore.
The weather office has not called it a cyclone, and has not said where it might cross the coast. To see why, it helps to know how the IMD names each stage.
What is a low-pressure area?
It is the first, weakest stage: a patch of falling air pressure around which winds begin to circulate, with wind speeds below 31 km/h.
Warm sea water heats the air above it, the air rises and pressure at the surface drops. Winds start to spiral into that area. Over the north Indian Ocean the IMD calls this a low, or a well-marked low when the pattern is clearer on the weather charts. Most lows bring rain and fade away without growing further. One such low off the Tamil Nadu coast has given Chennai and the southern districts heavy rain this week.
When does a low become a depression?
When its sustained winds reach 31 km/h (17 knots).
A depression has winds of 31-49 km/h. If they rise to 50-61 km/h, it becomes a deep depression. These are still not cyclones, but they can bring very heavy rain, rough seas and flooding, which is why fishing bans and port warnings often begin at this stage.
So what makes it a cyclone?
Winds of 62 km/h (34 knots) or more.
At that point the IMD calls the system a cyclonic storm, and it is given a name. Above that, there are four more grades, set by the strongest sustained winds near the centre.
The full scale, in use since 2015, runs: severe cyclonic storm at 89-117 km/h, very severe at 118-166 km/h, extremely severe at 167-221 km/h and super cyclonic storm at 222 km/h and above. Forecasters estimate the strength from satellite images using a system of T-numbers, which rise from 1.0 for a low to 6.5 and above for a super cyclone.
Who names the cyclones?
A group of countries around the north Indian Ocean, through a panel of the World Meteorological Organization and the UN’s Asia-Pacific commission.
The WMO/ESCAP Panel on Tropical Cyclones was set up in 1973. Its members include India, Bangladesh, the Maldives, Myanmar, Oman, Pakistan, Sri Lanka, Thailand and Yemen, with the UAE, Saudi Arabia, Qatar and Iran joining later. Member countries contribute names to a list that is used in order. The IMD’s centre in New Delhi serves as the regional specialised meteorological centre for the region, which is why it tracks and names storms over both the Bay of Bengal and the Arabian Sea.
How do the warnings work?
In stages, getting more specific as a storm comes closer.
When a disturbance looks likely to grow into a cyclone, the IMD issues a pre-cyclone watch to the Cabinet Secretary and senior officials. A cyclone alert follows at least 48 hours before bad weather is expected on the coast. A cyclone warning, at least 24 hours ahead, gives the latest position and intensity, the expected time and place of landfall, the height of the storm surge and the likely damage. About 12 hours before landfall a post-landfall outlook is issued for the districts inland that the storm will cross. The warnings are withdrawn once the system weakens back to a depression.
Why is October a worry for the east coast?
Because the Bay of Bengal often produces strong storms after the monsoon withdraws.
Two years ago, on 22 October 2024, a depression over the bay was forecast to grow into a severe cyclonic storm and cross the coast between Puri in Odisha and Sagar Island in West Bengal on 25 October. The pattern this year looks similar on the calendar, though every system is different. As of Friday the IMD’s outlook for 23 to 29 October shows a high chance of cyclogenesis, the formation of a cyclone, over the bay. Whether this one crosses that line will be clear only once the depression forms and its winds can be measured.
The documents
- Tropical cyclones: classification and warning system (SAARC Disaster Management Centre training material)SAARC Disaster Management Centre, Gujarat, 15 October 2019