The sky was cloudy and sunny in phases on Sunday, with more than one spell of light rain.

Similar conditions are expected to play out in Calcutta over the next few days, said a Met official.

The intensity of the showers has dipped in south Bengal and gone up in north Bengal.

"The upper air cyclonic circulation over northeast Bihar and neighbourhood lay over northwest Bihar and adjoining northeast Uttar Pradesh....The monsoon trough at mean sea level passed through Amritsar, Chandigarh, Lucknow, Chhapra, Bankura, Canning and thence southeastwards to the northeast Bay of Bengal," said a Met bulletin issued on Sunday afternoon.

A Met official said: "The circulation will trigger heavy rain in north Bengal for the next few days. However, the trough is over south Bengal and will lead to some rain here".

“Calcutta is likely to see typical monsoon weather. The sky will take turns in being cloudy and sunny. Rain is likely in the next couple of days. But when it does not rain, the conditions will be hot and humid,” said the official.

On Monday, "light to moderate rain/thundershower" is likely in "most places" in south Bengal. Thunderstorms with lightning and gusty wind (speed 30-40kmph) are likely in some areas.

On Tuesday, "light to moderate rain/thundershower" is likely in "many places" in south Bengal. Thunderstorms are likely in North 24-Parganas, South 24-Parganas, East Burdwan, West Burdwan, Murshidabad, Birbhum and Nadia districts.

Successive weather systems and the position of the monsoon trough ensured that south Bengal got surplus rain in June and July, with some districts receiving more than double the rain they usually get.

North Bengal has received less rain than it usually does because of the same factors

This year, there have been six low-pressure areas over south Bengal.

The monsoon trough is an imaginary line that joins the low-pressure points from the west of India to the east.

The trough keeps oscillating. When it is in the foothills of the Himalayas, north Bengal and Sikkim get more rain. When it descends, south Bengal gets more rain.