All weather systems need a trigger to originate and continuous heat energy to sustain thereafter. Tropical cyclones are no different and they result from the cyclonic disturbances in the Inter Tropical Convergence Zone (ITCZ). Tropics enjoy the proximity of ITCZ, which keeps oscillating north and south, and carries a potential of having number of cyclonic disturbances (CD’s), at any given time. However, these disturbances need conducive atmospheric conditions, convective instability and weak vertical shear in the horizontal wind flow.
The structure of tropical cyclones is quite complex and the most spectacular part of a matured storm is its ‘EYE’, which has the following features:
- Eye forms at the center of Central Dense Overcast (CDO) region of storm.
- Eye has a diameter of about 10-50 km.
- Eye is cloud free and surrounded by thick wall clouds.
- Eye is calm region with practically no rains.
- Eye is warmer than the surrounding region.
- Lowest surface pressure is observed at the Eye.
- Eye is an indicative of very strong winds spiraling around the center.
- All cyclonic storms may not develop an Eye.
- Sometimes, double Eye is also seen, which is indicative of very high intensity.
- Eye is surrounded by a 10-15 km thick wall of convective clouds, a zone of maximum wind.
- Eye wall is the most dangerous part of the storm.
- Storm surge, torrential rains and high velocity winds are the associated features of Eye wall.
- Eye wall is identifiable in the weather radar as a zone of maximum reflectivity, around the center.
Cyclones often defy studied norms and behave indifferently, in different basins. The single most feature which finds compliance is non formation of cyclones in the South Atlantic Ocean. Possibly, it is on account of sea surface temperature being lower than the critical value of 27˚C. The ITCZ is never seen migrating to the south of equator over Atlantic Ocean.