A cyclonic circulation in Chhattisgarh had led to the formation of the thunderclouds, which have since moved further away. The Alipore Met office recorded wind speeds of 20 to 30kmph for about 30 minutes from 2am before hitting a peak of 60kmph. But since only the outer reaches of the thunderclouds passed by, the rain was light and sparse. He said there had been no forecast of rain in Calcutta because the clouds were expected to float north of the city. The main factor was the absence of the northwesterly wind that keeps mornings cool at least till the end-February.

Source: The Telegraph February 26, 2018 21:22 UTC