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State poised for a good monsoon

By Our Special Correspondent

THIRUVANANTHAPURAM, MAY 22. With excess summer rain being received, thanks to two low pressure systems, and an early onset of the south-west monsoon, weathermen here say the State is comfortably placed for the coming four months during which it receives the bulk of its rain. The State has received 94 per cent excess summer rain so far - 557.5 mm till May 19 as against a normal of 288 mm. Technically, the precipitation received up to May 31 is treated as summer rain, though the south-west monsoon was declared to have set in on May 18.

Kerala receives a normal rainfall of 1,964 mm during the four months from June to September, the highest precipitation being in July. The Indian Meteorological Department (IMD) has already declared that the country stood to receive a normal south-west monsoon. This time around, the monsoon kept the Met authorities guessing. Tracing the chain of events, the Meteorologist, G. P. Bhaskaran Nair, told The Hindu that a low pressure system had formed about 360 km south-east of Balasore in Orissa on May 13. Due to this, the south-west monsoon was declared to have arrived over the Andamans on May 13 itself. Normally, it takes 15 days for the monsoon to arrive over Kerala after it has broken in the Andamans.

On May 16, the system had become well-marked over the east central Bay of Bengal and by May 17, it had become a cyclonic storm and moved north. Coinciding with this, the westerly winds had picked up over the Lakshadweep islands and Kerala and under the influence of these strong winds, widespread rain was received in Kerala on May 16, 17 and 18.

Low pressure systems are common over the Bay of Bengal and the Arabian sea during May, it is pointed out. Though a well-marked low pressure system had formed in the Bay on May 16 , the Met authorities were not sure that the system would continue. To be on the safe side, they decided to announce a "temporary" onset on May 18 and the same was confirmed with retrospective effect on May 21. According to the Met authorities, the south-west monsoon is declared to have set in if the State gets widespread rainfall for two consecutive days anytime after May 11. The direction and speed of the westerly winds are measured through balloon soundings before the monsoon is formally declared.

During the south-west monsoon, North Kerala receives more rain than the south. The rain is caused by the moisture from the Arabian Sea brought by westerlies blowing at speeds between 90 km and 100 km per hour at a height of about 1.5 km above the sea level.

A good part of the rainfall is generated by the westward passage of depressions or low-pressure systems in the Bay of Bengal. A depression or a "low" is an atmospheric vortex with a central region of low pressure. In the northern hemisphere, in which India lies, the winds blow round the centre in an anti-clockwise direction.

The intensity of the vortex is measured by the strength of these winds. Thus, when the winds round the vortex are strong, the depression is classified as "deep", and with still stronger winds, a deep depression becomes a cyclonic storm. On an average, one to three such systems are observed in the monsoon months, especially in July and August. The horizontal spread of a depression is around 500 km and its average life is around a week. According to figures made available by the Met department here, the earliest recorded onset of the south-west monsoon over Kerala was on May 11 - in 1918 and 1955 - while the most delayed onset was on June 18 - in 1972. There have been many other years in which the monsoon arrived earlier than May 18. The monsoon had set in on May 14 in 1960, on May 15 in 1932 and 1962, on May 16 in 1956 and 1969 and on May 17 in 1925 and 1942. Incidentally, last year, the monsoon had set in quite late - on June 8.

Meanwhile, the Met authorities here have written to the IMD asking for confirmation whether the whopping 117 cm of rain received in 24 hours at Amindivi in the Lakshadweep islands on May 16, is an all-time record.

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