The Hindu Business Line : Air circulation over TN signals rains

An upper air cyclonic circulation has formed over Tamil Nadu and neighbourhood in the latest indication that the moisture-laden and rain-causing band of Inter Tropical Convergence Zone (ITCZ) has become accentuated over extreme southern peninsula.

The ITCZ is a band of low pressure centred on or above the Equator where average solar radiation is greatest, causing the warm air to rise. The part of the ITCZ currently active over the southern peninsula will progressively shift north to cover coastal and south interior Karnataka, Rayalaseema, Telangana and south coastal Andhra Pradesh.

`Sambha' sowing

Dr Akhilesh Gupta of the Department of Science and Technology said the ensuing wet session would sustain over the next 5-6 days. The rains would be just in time for sowing of the winter rice (sambha crop) in Tamil Nadu. It would also help Kerala stock up a sprinkling of dams for purposes of irrigation and power generation.

The most significant aspect of the emerging atmospheric conditions is that the east-west shear zone of monsoon turbulence has established, which itself is a broad-sized convergence zone. It is immaterial if this has an embedded cyclonic circulation or not, Dr Gupta said.

He said this in response to forecasts for the next monsoon `low' off the Tamil Nadu coast, expected to form by Sunday as per latest updates.

N-E MONSOON SIGNAL

In another important observation, he said the accentuation of the ITCZ is part of a `low frequency mode' of 30-40 days for successive rain pulses to arrive.

Based on this, the next pulse due 30 days from now could possibly have the features of northeast monsoon (or winter monsoon) written all over it, as rains begin to soak the Tamil Nadu coast, he said.

Meanwhile, the National Centre for Medium Range Weather Forecast (NCMRWF) said that the well marked `low' over land had drifted away from the overnight perch over west Madhya Pradesh to south Rajasthan and adjoining Gujarat on Thursday. The system is likely to further move west-northwest and weaken.

Under its influence, fairly widespread rainfall with isolated heavy to very heavy falls are likely over north Gujarat region, Kutch and south Rajasthan during the next two days. Thereafter, rainfall over the region is expected to reduce significantly.

Anti-cyclone

The weakening of this system will set the stage for the seasonal anti-cyclone (high pressure zone) to spread from across the border into northwest India and adjoining Gujarat and west Madhya Pradesh.

The sinking air builds pressure (as against a `low' with rising air) to set up trademark cloud-free blue skies, causes humidity levels to drop markedly and leaves early morning dew on the ground in what could be emerging wintry weather conditions.

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