Laila weakens, monsoon on track

Monsoon watchers appear to have heaved a sigh of relief on the weakening of cyclone 'Laila' as they believe it would now have little impact on the annual rainfall season.

"We are still going ahead with the initial prediction on monsoon. This season, monsoon will be normal and is expected to reach Kerala by May 30," Earth Sciences Minister Prithviraj Chavan told reporters here.

He said all the weather parameters have been factored in while arriving at the monsoon forecast.

A section of the meteorologists said that following the landfall, the cyclone will weaken further as supply of moisture to the weather system will be cut off after it hit the land.

But how the cyclone dubbed as "severe" by the weather office lose its sheen?

In fact, Laila began weakening significantly since last night as it travelled very close to the coast for a long time but did not make a landfall. This could have been because of a sudden increase in the value of 'vertical wind shear' (VWR).

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.

"A cyclone gets energy from sea through evaporation of water from sea surface. This evaporated water is soaked by the winds. The moisture-laden winds flow into the cyclone's centre through a process called moisture convergence," said a top operational forecaster.

"When half of the circulation came over land, this process got cut off from western side leading to reduction of moisture supply as well as moisture convergence. This obviously has affected intensity of the cyclone," he said.

After churning across the Bay of Bengal, 'Laila' virtually came to a halt about 200 km south of the coastal city of Machilipatnam and remained there for quite a long time.

Similarly, shift in the mid and upper air high pressure area could have also played a major role as the intensity of a cyclone depends on the 'vertical wind shear'.

"Low value of vertical wind shear creates ventilation in the vertical column of cyclone so that the moisture laden flow enters into cyclone inner core, rises up and then gets out of the cyclone from top. This way a continuous supply of moisture is ensured," the forecaster said.

Since its formation, 'Laila' had benefitted from a low value of the vertical wind shear but suddenly today, the high pressure area shifted slightly and the value of the wind shear started rising.