Kolkata: Tall, dark cumulonimbus clouds (CB) exploded on Kolkata on Thursday, triggering the heaviest 24-hour rain this June in the city. Alipore recorded 84.6 mm of rain. While the downpour forced the mercury to plunge by around six degrees within an hour, the heavy rain left pockets in the city and surrounding areas in ankle- to knee-deep water. The Met office has not ruled out similar thunderstorm activities in Kolkata and other parts of South Bengal in the next few days to come.Thursday’s thunderstorm was the fourth such heavy rain to lash the city in a week. Accompanied by intense lightning, thunder and gusty wind, the city was taken over by the intense rain for close to two hours on Thursday afternoon. Most of the 84.6 mm of rain occurred within two hours. Even as the rain intensity declined later on, the dark cloud continued to keep the city under its spell for the later part of the day.The day began with bright sun, where there was no shade of rain cloud till noon. Soon, the dense, powerful and towering vertical clouds started gathering in the sky over the city and its surrounding areas. Tall and dark, measuring about 12 km in height, they opened up on the city in no time. Met officials said these were CB clouds, which are associated with extreme weather conditions like thunderstorms, torrential rain, hail and lightning.“These CB clouds developed in and around Kolkata due to the presence of monsoon moisture and heating on the ground. At present, we are getting the warm and moist air on the lower level, whereas the air in the middle level is westerly, which is comparatively less in moisture. In such a case, where there is local heating, it triggers the formation of such powerful convective clouds, which in turn can cause heavy rain along with frequent and intense lightning,” said HR Biswas, head of IMD’s Regional Meteorological Centre (RMC), Kolkata.With no rain on Wednesday, the maximum temperature on Thursday shot up by around four notches from the previous day. The mercury, touching 34.6 degrees, 1.7 notches above normal, had caused enough heating amid the city’s relative humidity of 100%, making the condition ripe for the formation of those cloud columns.Kolkata has been witnessing this ‘cloud burst’ kind of phenomenon on alternative days in the past one week. It began on June 19 when the city was struck by a sharp 50.8 mm of rain. While the next day was a lull on the rain front, the local cloud development triggered 62.1 mm of rain on June 21. Then came the third one on June 23, this time accompanied by a squall sweeping across the city at 75 kmph. It uprooted several trees and snapped overhead wires at different stretches in Kolkata.Met officials said the Thursday storm was in the making due to the rising mercury amid the abundant moisture.“During the monsoon, in the absence of a large system like a low-pressure area, an active trough of the monsoon is at a sluggish phase, our region is prone to this kind of thunderstorm. So till the monsoon flow becomes active, we can expect such weather phenomena in parts of Bengal including Kolkata in a gap of one or two days,” added Biswas.IMD’s observatory at Salt Lake recorded 50 mm of rain, most of which happened during an hour, while Dumdum recorded close to 20 mm of rain. In addition to Kolkata and surrounding areas, districts like West and East Midnapore, Jhargram, Howrah and Bardhaman were also lashed by torrential rain.“The seasonal trough runs from north Rajasthan to Bihar across Haryana and Uttar Pradesh. The upper air cyclonic circulation over North Bay of Bengal & adjoining coastal Bangladesh now lies over Northeast & adjoining Eastcentral Bay of Bengal. In view of favourable wind pattern and strong moisture incursion from Bay of Bengal, thunderstorm and associated activities over some districts of West Bengal. Also, heavy to very heavy rainfall (7-20 cm) at a few places with extremely heavy rainfall (more than 20 cm) at one or two places very likely to occur over some districts of North Bengal during June 27-29,” said a special bulletin from IMD.
