Tropical Cyclones are ferocious storms that originate over oceans in tropical areas and move over to the coastal areas causing violent winds, very heavy rainfall, and storm outpourings.
This is the definition as per NCERT. And the most appropriate definition from the UPSC point of view.
They are known as Cyclones in the Indian Ocean, Hurricanes in the Atlantic Ocean, Willy-willies in the western Australia and Typhoons in western Pacific.
The Tropical Cyclones originate and intensifies over tropical areas (between 8°-20° north and south). The tropical cyclones do not form below 8° latitude and above 20° latitude. These tropical cyclones mostly originate in the Bay of Bengal because it fulfils all the criteria to form the tropical cyclones. Around 3-4 destructive cyclones originate in the Bay of Bengal every year. It causes large scale destructions in the eastern coast of India mainly, Andhra Pradesh, Odisha and West Bengal. It also affects Bangladesh very badly.
Although Arabian sea also falls in the tropical area it experiences less tropical cyclones as it fails to fulfil all of the criteria to form a tropical cyclone. Another reason that the tropical cyclone becomes very destructive in the Bay of Bengal is, the cyclone gets trapped in the Bay of Bengal.
How the Tropical cyclone gets trapped in the Bay of Bengal?
As you can see in the above picture that both the Bay of Bengal and the Arabian sea falls in the tropical area (between 0° to 23.5° north) but we experience more tropical cyclones in the Bay of Bengal than the Arabian sea. There are generally two reasons. First, the Bay of Bengal fulfils all the criteria to form tropical cyclones and the second, the tropical cyclone gets trapped in the Bay of Bengal. How does it get trapped?
Because of the Coriolis force, the winds move clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere. The Coriolis force leads the tropical cyclone to move in clockwise in the Bay of Bengal and get it trapped between the landmasses covering from three sides. All the tropical cyclones do not get trapped in the Bay of Bengal. Only the cyclones that cross 20° latitude gets moved in a clockwise direction and comes again to hit the land.
Favourable conditions for the formation of a Tropical Cyclone:
Large sea surface with a temperature higher than 27°C
A large sea surface is needed to form a tropical cyclone. You would be thinking that all the sea has a large surface. No, there are seas which have continuous islands. So the formation of a tropical cyclone cannot happen there. Large seas get heated up very soon than the small seas. The normal temperature of seas in the tropical area is 20°C. So it takes a few months to get it that much warmer.
Presence of Coriolis force
The Coriolis force helps the winds to rotate clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere.
Small variations in the vertical wind speed
There should be minor variation in the speed of the wind moving along the surface and the wind moving in the sky. If the variation of the speed is high between the two, it will not let the cyclone to get form.
A pre-existing weak low-pressure area
The tropical cyclone is developed between 8° and 20° latitude.
It is understandable that at 8° latitude, there is already low pressure. But at 20° latitude (being close to 30° latitude, a high-pressure area), how can a cyclone develop here?
There may be high pressure and a low pressure area at the same latitudinal belt. So, this is the answer.
Characteristics of a Tropical Cyclone
Do not dig deep into the picture, I will be explaining the formation of a tropical cyclone in another post.
A mature tropical cyclone is characterised by the strong spirally circulating wind around the centre, called the eye. The diameter of the circulating system (the eye) can vary between 150 and 250 km (can you imagine 🙄).
The eye is a region of calm with subsiding air (air coming down from the sky). Around the eye is the eyewall, where there is a strong spiralling ascent of air to a greater height reaching the tropopause. The wind reaches maximum velocity in this region, reaching as high as 250 km/hr.
The diameter of the storm over the Bay of Bengal, the Arabian Sea and Indian ocean is between 600-1200 km. The system moves slowly about 300-500 km/day. The storm peters out/end on the land. It is called the landfall of the cyclone.
Recently in June 2020, a tropical cyclone called ‘Nisagra’ was formed in the Arabian sea that affected the Mumbai. Although it was not very fatal or dangerous as the one developed in the Bay of Bengal.
In May 2020, a cyclone developed in the Bay of Bengal called ‘Amphan‘ causing huge destruction in Odisha and West Bengal. You must read about it. And another cyclone called ‘Fani‘ in 2019 hit the Odisha.