Tropical cyclones are large, rotating, atmospheric phenomena extending horizontally from 150-1000 km and vertically from surface to 12-14 km. These are intense low-pressure areas with a spiral shape. Fierce winds spiraling anti-clockwise in the northern hemisphere blow around the cyclone centre. Cyclones generally move 300-500 km in 24 hours over the ocean.
Cyclones develop from areas of low atmospheric pressure and go through the stages of depression and deep depression before attaining the category of cyclone. They can intensify further to attain categories of severe cyclonic storm, very severe cyclonic storm and ultimately super cyclone when the winds in the storm are of the ferocious speed of 220 kilometer per hour or more, Bach category is recognized on the basis of wind speed as indicated below:
A well-developed cyclone consists of a central region of light winds known as its "Eye". The eye has an average Diameter of about 20 to 30 km, but it can be 40 to 50 km in large cyclones. The eye is an almost cloud-free zone and it is surrounded by a ring of clouds with very strong winds and heavy rain. This area is known as zone of maximum wind. Surrounding this region, winds spiral in the counterclockwise direction in the northern hemisphere, extend outwards to large distances, with speeds gradually decreasing towards the outer boundary of the cyclone.
On an average, about 5-6 cyclones form in the Bay of Bengal and the Arabian Sea every year, out of which 2 or 3 may be severe. More cyclones form in the Bay of Bengal than in the Arabian Sea, The ratio is 4:1. Tropical cyclones in these seas generally form between 50 N and 200 N. There are two distinct seasons of cyclones in our area. One is from April to June (Pre-monsoon) and the other is from October to mid-December (Post-monsoon). May, June, October and November are known for severe cyclonic storms.
Almost the entire east coast is vulnerable to cyclones with varying frequency and intensity. In the west coast, the North West coast (coast north of Mumbai) is more vulnerable as compared to southwest coast (South of Mumbai).