The term ‘Cyclone Arnab’ has been widely used on social media and in reports about the latest weather system over the Bay of Bengal. But according to the India Meteorological Department (IMD), there is no officially classified cyclone by that name.
The system that made landfall over Andhra Pradesh late on Wednesday night has been classified by the IMD as a deep depression, bringing heavy rain and strong winds to parts of Andhra Pradesh and Odisha.
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The confusion largely stems from the fact that ‘Arnab’ is an assigned name on the regional list of names for tropical cyclones. The name can be used when a system meets the criteria for a cyclone, but its appearance on the list does not itself mean that every weather system associated with the name has reached cyclone intensity.
Why is ‘Cyclone Arnab’ not officially a cyclone?
According to the IMD, the distinction between a depression, deep depression and cyclone is primarily based on the system’s maximum sustained surface wind speed.
The IMD classifies a depression when maximum sustained surface winds are between 31 and 50 kmph. A deep depression has winds between 52 and 61 kmph, or 28 to 33 knots.
For a system to be classified as a cyclonic storm, sustained surface winds need to cross the 63 kmph threshold.
The weather system over the Bay of Bengal was therefore classified as a deep depression rather than a cyclone.
So, where did the name Arnab come from?
The name Arnab is part of a predetermined list maintained under the World Meteorological Organization (WMO) and United Nations Economic and Social Commission for Asia and the Pacific (UN-ESCAP) panel.
The regional naming system involves 13 countries including Bangladesh, India, the Maldives, Myanmar, Oman, Pakistan, Sri Lanka, Thailand, Iran, Qatar, Saudi Arabia, the United Arab Emirates and Yemen.
Each country contributes 13 names, creating a list of 169 names for tropical storms.
The names are used sequentially when qualifying cyclonic systems develop. Arnab, which means “ocean” in Bengali, was among the names in the queue. Its presence on the list does not mean the current deep depression has officially been named Cyclone Arnab.
What impact has the deep depression had?
Although the system is not classified as a cyclone, it has brought significant rainfall to parts of eastern India.
Odisha has witnessed heavy rain, with authorities preparing for possible flooding and waterlogging. Schools and Anganwadi centres were shut in several districts, including Khurda, Jagatsinghpur, Ganjam, Gajapati and Malkangiri.
The state government placed collectors of 11 districts on high alert and kept cyclone and flood shelters ready.
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Andhra Pradesh has also experienced intense rainfall. According to the Andhra Pradesh State Disaster Management Authority (APSDMA), heavy showers were recorded across multiple districts between 8.30 am on Wednesday and 7 am on Thursday, with more than 80 locations recording over 100 mm of rain.
What is the difference between a depression and a cyclone?
A depression is a low-pressure weather system accompanied by thunderstorms and organised circulation. As wind speeds increase, the system can strengthen into a deep depression and eventually into a cyclonic storm if it crosses the required threshold.
In the current case, the IMD’s classification remains deep depression, despite the widespread use of the term ‘Cyclone Arnab’ online.