Why was it so intense and why did it follow the course to Myanmar? According to Dr. M.R. Ramesh Kumar, Scientist & Deputy Director, Physical Oceanography Division, National Institute of Oceanography, Goa, who spoke to this correspondent, said that the seasonal cyclogenesis parameter — low level relative vorticity (at 1.5 Km; 850 hecta Pascals) — on May 1 and 2, was quite high over the Myanmar region.

From surface to 400 hPa (hecta Pascals), convergence exists with all cases of tropical disturbances. This inflow produces cyclonic spin in proportion to the existing environmental vorticity field. The larger this low level vorticity, the larger the chances for cyclone formation.

Mid-tropospheric relative humidity at 500 hPa was conducive for the cyclone to move into Myanmar region on April 29 and 30.

It is generally observed that the cyclones form only in regions where seasonally averaged values of middle level humidity are high.

Other factors being equal, an environment of high middle level humidity is more conducive for deep cumulus convection and greater vertical coupling of the troposphere because entrainment of moist air into updraughts inhibits their growth less than dry air. High middle level humidity is also conducive to high cloud precipitation efficiency.

Further, the vertical wind shear (difference of the 850 hPa wind and 200 hPa wind) also was minimum only over Myanmar as compared to other parts of the Bay of Bengal. Observational studies have clearly shown that the tropical cyclones form under conditions of minimum vertical wind shear of horizontal wind between lower and upper troposphere.

If the cloud cluster ventilation is small, then enthalpy and moisture increase can occur. This will lead to a gradual surface pressure reduction and cyclone formation.

The values of these parameters over the Bay of Bengal were deduced from the NCEP/NCAR (National Centre for Environmental Prediction/National Centre for Atmospheric Research) reanalysis data from April 25 to May 5.

It is not unusual for this rise to be a metre for many cyclones, and 2-3 metres for major cyclones. With mean water levels being elevated, and with strong winds generating high waves, storm surges lead to immense loss of life and property. Nargis was accompanied by a powerful storm surge.