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Shower surge hint over weekend

Rainfall in Calcutta is set to intensify on Friday and sustain over the weekend with conditions becoming favourable for heavy flow of moisture from the Bay of Bengal.

“The monsoon trough has moved south and is now passing over Calcutta. In addition to that, a cyclonic circulation has developed over the Bay of Bengal. Both systems are pulling moisture-laden air from the sea, which will lead to extensive clouding,” said Gokul Chandra Debnath, director, India Meteorological Department, Calcutta.

The effects of the two systems were felt on Thursday as the city received rain — sharp spells as well as drizzles — throughout the day.

The rainfall, however, was varied, with the Alipore Met office recording 9.3mm in 24 hours till 8.30pm and the civic pumping station in Birpara, near Belgachhia, recording nearly 30mm.

Calcutta Municipal Corporation sources said the rainfall across the city varied between 2.4mm and 13mm.

The rainfall between the second week of July and the first week of August was the lowest the city has received during the period over the past decade.

If meteorologists’ predictions come true, Calcutta could make up for the deficit over the next three days.

Met department sources said the city received 227mm of rain between July 8 and August 7.

The reason for the poor show, a weather expert said, was the “unchanging orientation” of the low-pressure trough over the past few weeks. A low-pressure trough is an imaginary line connecting low-pressure zones that cause rainfall in the monsoon.

July and August usually are the two rainiest months of the season in this part of the world. The highest rainfall recorded in Calcutta in the July 8-August 7 period was 576mm, in 2006. Last year, the figure stood at 435mm.

Experts said the flow of the south-westerly monsoon winds would sustain for the next few days as the monsoon trough, which has an atmospheric pressure lesser than its surroundings, is pulling air towards itself.

“The rainfall the city has received recently was sporadic and localised. We expect typical monsoon showers, widespread and sustained, over the next few years because of the expected surge in moisture flow. The position of the monsoon trough and the cyclonic circulation are likely to trigger the surge,” said a weather expert.