Aug. 12: A weather system in Tibet that changed its position overnight led to a drizzle in Calcutta throughout the day and threatened more rain in the next 24 hours.
The high-pressure system was not the only factor at play, a weather scientist said. A cyclonic circulation over east Bihar and sub-Himalayan Bengal, a monsoon trough passing over the same region and another trough over south Assam and north Bengal were the other factors.
Together they brought darkness over the city in the afternoon, he said.
Also, the Celsius dipped as the skies remained overcast. The day's maximum temperature was recorded at 29.6 degrees, three notches below normal.
Some parts of the city received strong spells of rain, though. Baranagar in the north, for example, received more than 39mm and Ultadanga 25.65mm. The rainfall in most other places varied between 3mm and 10mm.
The Met office has forecast light to moderate rainfall, accompanied by thunderclouds, for tomorrow.
The system in Tibet, termed Tibetan High, is the emergence of a high-pressure zone over the Tibetan plateau.
It happens every year and is one of the factors related to the onset of the monsoon in Bengal and neighbouring areas, a Met official said.
"The system, which keeps shifting position, has turned westwards," Sanjib Bandyopadhyay, deputy director general, India Meteorological Department, Calcutta, said.
"It is causing a lot of air divergence - the air in the upper levels of the atmosphere is shifting away. Moisture-laden air from the lower levels is rising to fill the vacuum. This is leading to cloud formation and rain."
The cyclonic circulation over Bihar - 5.8km above the ground level - and the two troughs are constantly drawing moisture from the Bay of Bengal and leading to the showers, he said.
While it rained in south Bengal, it poured in Darjeeling, Kalimpong, Jalpaiguri, Cooch Behar, and Alipurduar in north Bengal. Heavy to extremely heavy (more than 200mm) rain is expected in north Bengal tomorrow, a Met official said.