Calcutta, Oct. 31: Time: 9 am. Place: Tollygunge Circular Road. Scene: Cars and buses crawling with headlights on.
Darkness descended this morning as thick black clouds developed over the city and neighbouring areas — a phenomenon known as meso-scale in meteorological parlance.
Thirty-five minutes later, there was a cloudburst and hard rain swept by gusty winds came down. The weather office recorded 34 mm rain till 11.30 am.
Heavy rain lashed the city for about 45 minutes before gradually settling down to a drizzle. Even after the rain stopped, the sky remained overcast, beginning to clear in the afternoon and the sun came out around 3.30 pm.
K.K. Chakraborty, the director of the weather section of the regional meteorological centre, Alipore, said that last evening, there was a sudden change in the wind pattern.
“Lot of moisture-laden wind blew in from the Bay of Bengal through the night and helped build huge masses of cumulonimbus clouds about 12-km tall over the city and adjoining areas,” Chakraborty said.
He recalled that a similar darkness had enveloped the city on another Friday, September 20, last year in the middle of the afternoon. The clouds then were much larger.
Four big masses of cloud formed over the city this morning. Chakraborty said the clouds were heavier in south Calcutta than in the north. “So, the rains were also heavier in south Calcutta. Dum Dum recorded only 3 mm of rain,” he said.
The sky would remain cloudy with possibilities of light rain or thundershower in the next 24 hours, the Met office said.
The deep depression that had formed over the Bay of Bengal four days ago weakened and passed through Andhra Pradesh to Konkan and Goa. “It, however, left a trough of low pressure behind over Gangetic West Bengal extending up to Jharkhand and Maharashtra,” Chakraborty added.
This trough caused a change in the wind pattern, leading to induction of moisture from the sea, said another official.
When the moisture-laden wind blew in, it mixed with the dry air on land. “This interaction between the moist and dry air builds up the clouds. These cloud formations, which were nearly a kilometre above the ground, are, however, very sudden and localised and give rise to thunderstorm activity,” the official said.
Civic authorities recorded 50 mm of rain in 45 minutes at Chetla, Kalighat and Tollygunge where roads were flooded. Mayor-in-council in charge of drainage, Mala Roy, said the pumps at Calcutta Municipal Corporation’s station at Chetla had to be pressed into service to clear the water.
The weatherman calmed fears of any immediate possibility of a depression in the sea off the Andhra Pradesh or Orissa coasts. “The satellite picture shows that the bay is clear. This is the time when we keep maximum watch on the sea as, till December-end, it is the cyclone season,” an official said.