Vinson Kurian

Thiruvananthapuram, Nov. 2

THE monsoon-friendly `shear zone' is getting organised from extreme peninsula in tandem with the rain-driving Inter Tropical Convergence Zone (ITCZ), which should trigger another wet session in Tamil Nadu and adjoining coastal Andhra Pradesh from Friday.

A shear zone is where the winds change speed and/or direction rapidly over a given distance. Wind shear along the zone can generate `eddy' currents that are sometimes visible as billow clouds. These small eddies and whirling masses of air move in various directions, generating tremendous turbulence.

According to Dr Akhilesh Gupta of the National Centre for Medium Range Weather Forecasting (NCMRWF), the rains would not peak until a sea-based circulation gets embedded into the shear zone. It is expected that the shear zone will induce the formation of such a circulation sooner than later.

In its update on Wednesday, the NCMRWF said there were two upper air cyclonic circulations over peninsular India. But Dr Gupta clarified that these systems are largely land-based, and not seen aiding intense precipitation. Only the sea-based system, expected to embed itself into the shear zone and based over Bay waters, will get sufficiently accentuated to cause the rains to intensify and spread.

The upper air cyclonic circulation over Rayalaseema and adjoining north interior Karnataka has shifted to north Tamil Nadu and neighbourhood extending up to 0.9 km above sea level. This is expected to bring scattered rainfall over the south peninsula, Orissa, Gangetic West Bengal and north Coastal Andhra Pradesh.

Another cyclonic circulation, located to the north Bay of Bengal and neighbourhood and neighbourhoods, extends up to 1.5 km above sea level.

The third upper air cyclonic circulation over Punjab and neighbourhood, extending up to 3.1 km above sea level, is less marked. Yet another circulation over central Pakistan is likely to move north-northeast into India and cause isolated rains/snow in the hilly regions of northwest India over the next two-three days.

In its forecast for the next four-five days, the NCMRWF said isolated snow/rains are expected over Jammu and Kashmir and Himachal Pradesh over the next two-three days. Mainly dry weather is likely to prevail over rest areas of the region.

In the east, isolated to scattered rainfall activity is expected over Orissa, the Northeastern States and West Bengal. Isolated rains may occur over the rest of the region except over east Uttar Pradesh and Bihar where mainly dry weather may prevail during next 4-5 days. In central India, mainly dry weather is likely to prevail over the region except over Chhattisgarh where isolated rains are expected during the next two-three days.

In the South, isolated to scattered rainfall is likely over the region during the next two-three days. The rainfall activity over Tamil Nadu and Pondicherry and south coastal Andhra Pradesh may increase after 24 hours.

(This article was published in the Business Line print edition dated November 3, 2005)
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