How did ‘Laila' become less ferocious?

Even as late as Wednesday evening, cyclonic storm ‘Laila' was forecast to hit the Andhra Pradesh coast in a ferocious manner. But it lost steam significantly overnight, resulting in a substantially lower loss of life and destruction of property than feared.

How did this miraculous change occur?

A senior meteorologist, who has been associated with the India Meteorological Department, says it could possibly have happened because ‘Laila' moved very close to the coast for a long time on Wednesday and because of a sudden increase in the value of what is called “vertical wind shear” (VWR).

A cyclone gets its energy from the sea through evaporation of water from the sea surface and the flow of strong winds over the sea surface. This evaporated water is soaked by winds and the moisture-laden winds flow into the cyclone's centre through a process called “moisture convergence.”

System weakened

After rapidly moving across the sea towards land, ‘Laila,' on nearing the coast, began moving parallel to it for a long time at a very short distance of about 200 km. Consequently, there might have been a significant cut-off in the supply of moisture from the western side, leading to the reduction in moisture supply and moisture convergence, and consequently, the weakening of the system.

On VWR, the expert says a low value helps a cyclone by ensuring a steady supply of moisture by enabling moisture-laden winds to enter the inner core of the cyclone and then rise up and get out of the system from the top.

All along, ‘Laila' benefitted from a low value of VWR, but the value started rising suddenly on Wednesday. This could have led to a disruption in the ventilation of the air in the vertical column of the cyclone and the consequent weakening of the system, the expert said.

Wind shear relates to a difference in wind speed and direction over a relatively short distance in the atmosphere.

Wind shears are of two types — vertical and horizontal. During the monsoon season, the value of the VWR is generally high, which is why cyclonic storms do not occur during the season.