The Hindu Business Line : Wet weather attributed to real-time `troughs'

SKIES in the southern parts of the peninsula are witnessing the interplay of three "troughs of low pressure" arranged in pleats from east to the west, bringing welcome wet weather to some sun-scarred geographies.

The strange configuration has resulted in the dumping of rain progressively across Tamil Nadu and Kerala, apart from parts of Coastal Andhra Pradesh and Karnataka, over the past few days.

Speaking to Business Line, Dr M. Rajeevan, Director-Forecasting, India Meteorological Department (IMD), Pune, said the southern peninsular weather is not abnormal but the timing is earlier than usual. The southern peninsula, especially Kerala, is known to witness a lot of weather activity during this time of the year.

"About 40-50 days before the arrival of monsoon rain, it is common occurrence that a spell becomes active over Kerala, which we call the pre-monsoon rainfall peak. But one is not sure if the current wet weather is related to this rainfall peak.

"But it is safe to assume that the prevailing weather has set in a little earlier than usual and will continue to persist for some more days," Dr Rajeevan said.

According to Mr M.D. Ramachandran, Director, Met Office, Thiruvananthapuram, as of Thursday, none of the systems were "active enough" to carry the ongoing spell in the same vigour beyond the next three days.

But Central Kerala and Lakshadweep are ideally poised to get some good precipitation from the inward moving troughs, piggybacking the easterlies respectively from the Southwest Bay of Bengal and Southeast Arabian Sea.

A third trough was located over Kerala on Thursday straddling the central districts, which could spawn soggy conditions in these areas.

The mechanics of the formation of the "troughs" leading to rain can be explained in the following manner.

A "low pressure" system is caused by relatively warm air ascending from the Earth's surface. As the rising air cools, clouds will begin to form and progressively to rain. "Low pressure" systems are represented in closed isobars surrounding a region of relatively low pressure. Isobars are lines of constant pressure on a weather map connecting points of equal pressure. They are generated from mean sea-level pressure reports and are given in millibars. An elongated extension of isobars away from a low-pressure centre is known as a "trough of low pressure".

The instability of the ascending air will produce quite large vertical development of cumuliform clouds with associated rain showers (such as cumulonimbus cloud). Given ideal conditions, troughs can intensify into depressions, deep depressions and cyclones in stages.

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