What are cyclones, and how are they classified, categorised, and named?

Cyclone Biparjoy is expected to hit the Indian coastline in Gujarat on June 15.

"Biparjoy", a deep depression over the southeast Arabian Sea, intensified into a cyclonic storm and has since intensified into a very severe cyclonic storm.

According to IMD's latest update, the cyclone will cross Saurashtra-Kutch in Gujarat and adjoining Pakistan coasts around the afternoon of June 15. However, the impact is expected to be the harshest in Gujarat, where it will hit as a "very severe cyclonic storm" (VSCS), with wind speed reaching 125-135 km/h, gusting to 150 km/h.

The Indian subcontinent, with a coastline of 7,516 km, is one of the worst-affected regions in the world when it comes to cyclones. The subcontinent is exposed to nearly 10 per cent of the world's tropical cyclones, and a majority of them originate in the Bay of Bengal.

Cyclone origins

Cyclones are caused by atmospheric disturbances around a low-pressure area distinguished by swift and often destructive air circulation. Cyclones are usually accompanied by violent storms and heavy rainfall. The air circulates inward in an anticlockwise direction in the Northern hemisphere and clockwise in the Southern hemisphere.

The word "cyclone" is derived from the Greek word "cyclos", meaning the "coils of a snake", as the tropical storms in the Bay of Bengal and the Arabian Sea appear like coiled serpents of the sea. The term was coined by Henry Piddington.

India and cyclones

Every year on average, two to three cyclones make landfall in India, with one being a severe tropical cyclone or greater. Although most of these cyclones originate in the Bay of Bengal, the Arabian Sea coast isn't free from cyclones either.

Between 1891 and 1990, around 262 cyclones (92 of these were severe) occurred above the east coast, and around 33 cyclones occurred over the west coast, out of which 19 were severe.

Over the last few decades, there has been an increase in the frequency and intensity of cyclones forming over the North Indian Ocean. Historically, the Arabian Sea, which earlier saw one or two cyclones per season, now shows a 52 per cent increase in cyclone frequency. The year 2019 was classified as one of the most active cyclone years that broke several records. As the third-costliest season in terms of damages, the North Indian Ocean basin saw its first Super Cyclonic Storm in over a decade, in addition to breaking the record of three Extremely Severe Cyclonic Storms occurring in one season.

How are cyclones classified?

Cyclones are classified as extratropical cyclones (called temperate cyclones) and tropical cyclones.

The World Meteorological Organisation (WMO) uses the term "tropical cyclone" to describe weather systems in which winds exceed "gale force" (63 km/h). Tropical cyclones are the progeny of ocean and atmosphere, powered by heat from the sea, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain.

Tropical cyclones have four different stages of cyclonic disturbances. The first stage is a cyclonic storm (CS) with a maximum sustained wind speed (MSW) of 63-87 km/h. The second stage is a severe cyclonic storm (SCS) with a maximum sustained speed of 88-117 km/h. The third stage is a very severe cyclonic storm (VSCS) with a maximum speed of 118-220 km/h, and the fourth stage is a super cyclonic storm (SuCS) with an MSW of 222 km/h and above.

Which are the different cyclone categories?

Cyclones are divided into five categories, depending on wind strength.

Category 1 cyclones have a wind speed of 120-150 km/h, and the damage capacity is "minimal".

Cyclones in category 2 have a wind speed of 150-180 km/h, and the damage capacity is "moderate".

Category 3 cyclones have a wind speed of 180-210 km/h, and the damage capacity is "extensive".

Category 4 cyclones have a wind speed of 210-250 km/h, and the damage capacity is "extreme".

Category 5 cyclones have a wind speed of 250 km/h and above, and the damage capacity is "catastrophic".

How are cyclones named?

Cyclones are given different names in different regions of the world. They are known as "typhoons" in the China Sea and Pacific Ocean, "hurricanes" in the West Indian islands in the Caribbean Sea and Atlantic Ocean, "tornados" in the Guinea lands of West Africa and the southern US, "willy-willies" in north-western Australia and 'tropical cyclones' in the Indian Ocean.

According to the WMO, there can be more than one cyclone at a time in a particular geographical location, and thus it becomes important to name them to avoid confusion, facilitating disaster risk awareness, management and mitigation.

Cyclones that form in every ocean basin worldwide are named by the regional specialised meteorological centres (RSMCs) and Tropical Cyclone Warning Centres (TCWCs). There are six RSMCs in the world, including the IMD, and five TCWCs.

As an RSMC, the IMD names the cyclones developing over the north Indian Ocean, including the Bay of Bengal and the Arabian Sea, after following a standard procedure. The IMD is mandated to issue advisories to 12 other countries in the region on the development of cyclones and storms. The 12 countries are Sri Lanka, Bangladesh, Pakistan, Myanmar, Maldives, Iran, Oman, Qatar, Saudi Arabia, the UAE, Yemen, and Thailand.

In 2000, a group of nations called WMO/ESCAP (World Meteorological Organisation/United Nations Economic and Social Commission for Asia and the Pacific), which comprised Bangladesh, India, the Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand decided to start naming cyclones in the region. After each country sent in suggestions, the WMO/ESCAP Panel on Tropical Cyclones (PTC) finalised the list. The panel expanded to include five more countries in 2018 — Iran, Qatar, Saudi Arabia, United Arab Emirates and Yemen.

In April 2020, the IMD released a list of 169 names (13 suggestions each from 13 member countries) of future tropical cyclones that would emerge in the Bay of Bengal and the Arabian Sea.

In recent years, the IMD has started to include names of cultural significance in the list. For example, the name "Amphan", which means "sky" in Thai, was used for a cyclone that hit West Bengal in the year 2020. The IMD's tradition of naming cyclones is a way of creating a sense of community around the shared experience of tropical storms.

In the list released by the IMD in 2020, Bangladesh suggested the name "Biparjoy". It means calamity or disaster in Bangla.