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Wettest month in last-week wallop

- July that began with a whimper ends with a bang to reduce monsoon deficit to ‘normal’ range

Monsoon worked the city’s rain scorecard this July just like Virat Kohli raises a hundred — go slow at the start, stay steady in the middle overs and finish with a flourish.

Calcutta’s wettest month contributed 329mm of rain to the season’s total, short of the target by a few but just enough for the umpires to deliver the meteorological equivalent of a Duckworth-Lewis verdict.

“We have noted a 17 per cent deficit for the month. Up to 19 per cent less or more rainfall than the mean is considered normal,” said Gokul Chandra Debnath, director of the India Meteorological Department, Calcutta.

The average rainfall in July is 399.9mm, a target that had looked far beyond reach after a bone-dry first nine days of the month.

When it rained after that, it was more a trickle than a torrent. The last week was when monsoon almost made up for the dry spell with sustained late hitting.

Debnath said July’s last-lap performance was so good that the combined monsoon score for the past two months showed a surplus of 13 per cent. This stunning piece of statistic, of course, has much to do with June’s Brian Lara-like performance: 400mm of rain for a month-specific surplus of 44 per cent.

The last week of June had brought rain every day, including a “final over” that alone yielded 86mm. The downpour on June 30 had led weather scientists to predict a typically rainy July but the cloud-filled skies decided to shut shop.

“The pattern of rainfall in July was unusual. It was only in the last week that we had the sustained rainfall that July is known to bring,” a senior IMD official said.

If the amount of rainfall was a relief for monsoon watchers, those like Rashmi Sharma were glad that the rain gods didn’t flood the city out in their anxiousness to catch up with the asking rate.

“I really enjoyed the non-stop rain since noon on Sunday. While the rain continued for a day-and-a-half and kept the weather cool, the best part of it was that it wasn’t so heavy that the streets got flooded,” said Rashmi, a resident of Alipore.

For heavy monsoon rain to hit Calcutta and its surrounding areas, weather systems like low-pressure areas, depressions or deep depressions over the Bay of Bengal are a key requirement.

Five such low-pressure areas did form in the bay during July but four of those moved to the Odisha coast and then went inland, the director of IMD-Calcutta said.

“Most of the low-pressure areas moved towards Odisha, though that isn’t uncommon either. A low-pressure trough again formed last Friday and that’s the only one in July that stayed over the Bengal coast and brought rain on Sunday and Monday,” Debnath explained.

A new low-pressure trough formed on Tuesday and even turned into a depression, only for Odisha to again steal Calcutta’s thunder. “The city was a bit unlucky. Had this one stayed put, we would have had more rain,” Debnath said.

Rain is crucial to not only maintaining the balance in terms of weather but also helps keep many diseases at bay. Microbiologists say that steady rainfall diminishes the potency of viruses that cause typical summer diseases.

“Adenoviruses and Rhinoviruses, the main causes of upper respiratory tract infections, were very active in the first part of July because of the hot and humid conditions. But the dip in temperature made them less active,” said a senior virologist who works in a state-run institute.

So would rain in August chase away the viruses responsible for many Calcuttans being under the weather? The flip side of rain, according to virologists, is that Rota and Norwalk viruses responsible for ailments such as gastroenteritis become more active.