Cyclone Arnab, which formed over the Bay of Bengal, is expected to cross the Andhra Pradesh and Yanam coasts between Bapatla and Visakhapatnam by the forenoon of October 28, according to the India Meteorological Department (IMD).

The weather system intensified from a deep depression into a cyclonic storm on October 26. The IMD has forecast that it could strengthen into a very severe cyclonic storm, with maximum sustained wind speeds of 115–125 kmph and gusts reaching 140 kmph at landfall. Coastal Andhra Pradesh and adjoining southern Odisha are among the areas under alert.

But what exactly is a cyclone, how did Arnab get its name, and what makes the Bay of Bengal favourable for these storms?

What is a cyclone?

A tropical cyclone is an intense low-pressure weather system that develops over warm tropical waters. It brings strong winds, heavy rainfall and, in coastal areas, the risk of storm surges and flooding.

Cyclones form when warm ocean water heats the air above it, causing moist air to rise and creating an area of low pressure. More air moves towards this low-pressure zone, rises and releases heat as water vapour condenses into clouds. Under favourable conditions, this process strengthens the system and its winds begin to circulate around the centre.

In the Northern Hemisphere, cyclonic winds rotate anticlockwise because of the Earth's rotation and the Coriolis effect.

The IMD classifies weather systems according to their sustained wind speeds. A depression has wind speeds of 31–49 kmph, while a deep depression has speeds of 50–61 kmph. Once sustained winds reach 62 kmph, the system qualifies as a cyclonic storm and receives a name. Higher wind speeds place it in the categories of severe, very severe, extremely severe and super cyclonic storms.

How did Cyclone Arnab get its name?

The name Arnab was proposed by Bangladesh and is part of the official list maintained under the World Meteorological Organization (WMO) and United Nations Economic and Social Commission for Asia and the Pacific (ESCAP) Panel on Tropical Cyclones.

The regional list contains 169 names contributed by 13 countries: Bangladesh, India, Iran, the Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, the United Arab Emirates and Yemen. Each country submitted 13 names, which are used sequentially as qualifying cyclones develop over the North Indian Ocean, including the Bay of Bengal and the Arabian Sea.

India's IMD, through its Regional Specialised Meteorological Centre in New Delhi, is responsible for naming cyclones in the region. The name assigned to a storm does not depend on which country it is expected to hit. A cyclone named using a list entry contributed by Bangladesh can make landfall in India.

Naming storms helps authorities and the public distinguish between weather systems, follow forecasts and communicate warnings without confusion. The name itself does not indicate how powerful or dangerous a cyclone is.

Why does the Bay of Bengal see so many cyclones?

The Bay of Bengal has historically recorded substantially more tropical cyclone activity than the Arabian Sea. Research published in the IMD journal MAUSAM has examined differences in the two basins, including sea-surface temperatures, atmospheric moisture and wind patterns.

Warm ocean water: Tropical cyclones draw energy from warm seawater. The Bay of Bengal often has favourable sea-surface temperatures, supplying heat and moisture to developing systems.

High moisture levels: Moist air helps thunderstorms develop and sustain the circulation around a low-pressure centre. The Bay receives substantial rainfall and freshwater inflow from major rivers, which can create layers in the upper ocean that limit mixing with cooler water below.

Wind shear: Vertical wind shear refers to changes in wind speed or direction with altitude. Strong shear can disrupt a developing cyclone by tilting its circulation and separating its thunderstorms from the centre. Lower wind shear is generally more favourable for formation and intensification.

Seasonal conditions: The North Indian Ocean typically has two active cyclone periods: before the southwest monsoon, around April to June, and after it, around October to December. The post-monsoon period is therefore an important window for cyclone development over the Bay.

The Bay's shallow northern waters can push storm surges further inland, increasing the risk of coastal flooding, particularly in low-lying areas. This increases the risk of flooding, particularly in densely populated coastal regions.

The Arabian Sea is not immune to powerful cyclones. Research has documented changes in its cyclone activity, with warmer waters and favourable atmospheric conditions contributing to the development and intensification of storms there as well.

What makes a cyclone intensify rapidly?

A cyclone's strength depends on the interaction between the ocean and atmosphere. Warm water and high moisture levels can supply energy, while favourable wind patterns allow the storm's circulation to organise and strengthen. Low vertical wind shear is particularly helpful because it allows thunderstorms to remain aligned with the cyclone's centre.

However, warm water alone does not guarantee that a cyclone will form or intensify. The initial low-pressure disturbance, atmospheric instability, moisture, wind shear and the storm's movement all influence its development. Forecasts can change as these conditions evolve.

For Arnab, the IMD's forecast points to further intensification before landfall. Authorities have issued warnings for coastal areas, with heavy rainfall and strong winds posing risks to homes, crops, power infrastructure and transport. Residents in affected districts should follow official IMD bulletins and local administration instructions rather than relying on the storm's category or name alone.

How is a cyclone different from other weather phenomena?

A cyclone is a large-scale rotating weather system that develops around a low-pressure centre. Rainfall, thunderstorms and monsoons, by contrast, describe different forms of precipitation or atmospheric activity. While cyclones can produce all three, they are not the same phenomenon. A cyclone circulation can sustain strong winds over a broad area, push seawater inland through a storm surge, and generate heavy rainfall that causes flooding well away from the coast.

Rainfall occurs when water droplets in clouds become heavy enough to fall to the ground. It can result from several weather systems, including monsoons, thunderstorms and cyclones, and does not necessarily involve strong winds or a rotating circulation.

Thunderstorms develop when warm, moist air rises rapidly and forms towering cumulonimbus clouds. They can bring heavy rain, lightning, thunder, hail and strong gusts, but are generally more localised than tropical cyclones.

Monsoons are seasonal wind patterns that bring distinct wet and dry periods to a region. India's southwest monsoon, for instance, carries moisture from the Arabian Sea and Bay of Bengal, producing widespread rainfall. Cyclones, however, are individual weather systems that can form during favourable conditions, including before and after the monsoon.

A tornado is a violently rotating column of air extending from a thunderstorm cloud to the ground. It is much smaller and usually shorter-lived than a tropical cyclone, though it can cause severe damage along its path.

Cyclones can generate several hazards simultaneously, including destructive winds, intense rainfall, inland flooding and storm surges along the coast. Even areas that do not experience the strongest winds may face flooding from heavy rain.