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Skewed showers in reverse rerun

The north had a lead but the south launched a counter-attack and scored a quick equaliser.

Calcutta on Thursday witnessed a repeat of the day before’s skewed rainfall distribution, with the south battling a downpour when the north was experiencing a drizzle.

On Wednesday, the drizzle-downpour spread was just the opposite.

“The rainfall on both days was not monsoon-like,” said a weather scientist.

The rain on both Wednesday and Thursday was caused by tall thunderclouds that had formed because of a cyclonic circulation over the Bay.

On Wednesday the clouds were more over the northern and eastern parts of the city, triggering around 67.5mm of rain in Birpara (near Belgachhia), 61mm in Dhapa and 60mm in Ultadanga. Kalighat and Chetla, on the other hand, received 14.6mm and 10mm of rain, respectively.

The clouds on Thursday developed over South 24-Parganas and moved towards the city. By the time they reached the northern parts of Calcutta, they were a spent force. “Which is why Jodhpur Park and Chetla received 46mm and 41mm of rain, respectively, but the shower gauge at Ultadanga and Thanthania showed just 3mm,” said a weather department official.

In the extreme south, Kamdahari in Garia was lashed with 89mm of rain, 81mm of which came between noon and 1pm. Patuli got 61mm.

“A typical monsoon shower is usually more evenly distributed. Also, monsoon rain is characterised by daylong drizzle, instead of a burst of downpour the city witnessed on Wednesday and Thursday. This is more like summer rain,” said G.C. Debnath, director, India Meteorological Department, Calcutta.

In summer, tall cumulonimbus clouds form because of heat and humidity. These trigger thunderstorms usually in the evening or late afternoon. Over the past 48 hours, however, the city was getting late-morning showers and the sky was remaining overcast throughout the day.

“There is abundance of moisture in the atmosphere, resulting in the formation of thunderclouds in the morning. The cyclonic circulation is active over the Bay and is likely to remain so for 48 hours. Hence, the flow of moisture will continue and more rain is on the radar,” said a senior Met official.

The rain expected till Saturday will be in the form of thundershowers, which has a tendency of being unevenly distributed. “The clouds that produce thundershowers are small and constantly on the move. They lose and gain in strength depending on certain factors,” he added.

The Alipore Met office recorded only 9.3mm and 19.3mm of rain on the two days, much lower than the average rainfall received by most places in the city around the same time.

The city has recorded 208.1mm of rain so far this month, bringing July’s rainfall deficit down to 11.9 per cent from 17.4 per cent two days ago.