A blanket of darkness enveloped Calcutta on Monday afternoon. Thunder and lightning followed with ferocious intensity and at least five people were killed —three of them schoolgirls who had taken shelter in a tea stall in North 24-Parganas
Why was it so dark?
A column of cloud about one and a half times the height of Everest hung barely 500 metres above Calcutta. Spread across 5,000sqkm, the cloud acted as a gigantic sunscreen. The weatherman called it the
mesoscale phenomenon, a rare collection of an enormous mass of rain cloud. In monsoon, the usual cloud column is about a kilometre tall
Why so much thunder and lightning?
The cloud was cumulonimbus, characterised by thunder and lightning. The larger the cloud mass, the more intense the thunder.
Such clouds are common during summer's Nor‘westers. But they can also form during the monsoon. Monsoon clouds come like waves in a sea, where a string of them is followed by a spell of quiet. During such a quiet period, high temperature and humidity can lead to the formation of an immense mass of cloud. On Monday morning, the mercury in the city rose to 35.8°C, four degrees above normal, and the humidity was around 70 per cent.
Why was it so windy?
The atmospheric pressure was low where it rained and it sucked in air from the sea to replace it.
As the cloud was so widespread, a large amount of wind blew in from the bay on Monday afternoon. Seventy trees were either uprooted or broken from the trunk in Salt Lake. Sector V and parts of blocks like BA, CA, AG and AF were flooded
Heavy to very heavy rain likely on Tuesday, says the weather office at Alipore. The cyclonic circulation has blown away but another low-pressure formation has followed.
The Red Road is shrouded in darkness at 2.59pm on Monday as vehicles are forced to switch lights on. The highrise dotted with lights is the UBI headquarters at Dalhousie. Picture by Pradip Sanyal