Context: A depression over the Bay of Bengal has intensified into a deep depression, bringing heavy rainfall to Chennai and several districts in Tamil Nadu. The Indian Meteorological Department (IMD) has predicted that the system will further intensify into a cyclone (Cyclone Fengal).

Cyclone

• A cyclone is a mature, organized system of clouds and thunderstorms featuring a clearly defined circulation pattern.

• They are caused by atmospheric disturbances around a low-pressure area distinguished by swift and often destructive air circulation.

• They form over warm ocean waters and are characterized by low pressure at the center, called the "eye," surrounded by strong, rotating winds.

Cyclones are classified as

1. extra tropical cyclones (also called temperate cyclones)

2. tropical cyclones

• Worldwide terminology: Cyclones are given many 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 southern USA.

• Willy-willies in north-western Australia

• Tropical cyclones in the Indian Ocean

Exposure of Indian subcontinent to Cyclones

• The Indian subcontinent (8041 kilometres) is one of the worst affected regions in the world.

• It is exposed to nearly 10 per cent of the world’s tropical cyclones.

• Of these, the majority of them have their initial genesis over the Bay of Bengal and strike the East coast of India.  On an average, five to six tropical cyclones form every year.

• More cyclones occur in the Bay of Bengal than the Arabian Sea and the ratio is approximately 4:1.

• Cyclones occur frequently on both the coasts (the West coast - Arabian Sea; and the East coast - Bay of Bengal).

• Tropical cyclones occur in the months of May-June and October-November.

Key Concepts

• Depression: A depression is a low-pressure area in the atmosphere where the air pressure is lower than the surrounding regions.

• This typically leads to the movement of air from high-pressure areas to low-pressure areas, causing clouds, moisture accumulation, and rainfall.

• Depressions are the weakest form of cyclonic systems and are associated with moderate to heavy rainfall, but they are not yet severe enough to be classified as cyclones.

• Depressions are common in tropical regions and form over warm oceans.

• Deep Depression: When a depression intensifies further, it becomes a deep depression.

• In this stage, the system is characterized by stronger winds (usually around 55-75 km/h), more defined low-pressure areas, and increased rainfall.

• While still not as severe as a full-fledged cyclone, a deep depression can cause significant weather disturbances, including heavy rainfall, strong winds, and flooding in affected areas.