A low-pressure area over the Bay of Bengal triggered widespread rain in the city on Thursday. The southwest monsoon is expected to reach Calcutta riding the same system over the next 24 hours, the Met office has said.

“The southwest monsoon has advanced to south Chhattisgarh and south Odisha and eastern parts of Assam and Meghalaya. It is expected to reach Gangetic Bengal, including Calcutta, over the next 24 hours,” said Mrutyunjaya Mohapatra, the IMD director general in New Delhi.

The weather department studies various factors such as amount and distribution of rainfall, wind flow and moisture content in the air to decide whether to declare the onset of the monsoon in a particular area.

The usual date for the arrival of the monsoon winds in Calcutta is June 8. Last year, the monsoon had made a much delayed entry, on June 21.

Apart from widespread rain, the monsoon is marked by wind blowing from southwest, the direction of the Bay of Bengal, instead of from north India.

The change is already taking place, said a Met official in Calcutta.

The minimum relative humidity, a measure of moisture in the air during the daytime, has also been on the higher side. On Wednesday, it was 70 per cent.

The city woke up to a cloudy sky on Thursday. Rain, which came in phases, started in the morning. According to the pumping stations of the civic body, between 6pm on Wednesday and 6pm on Thursday, Cossipore got 25mm of rain, New Market got 22mm, Dum Dum 15mm and Joka 14mm.

The showers were devoid of gusts of wind, typical of squalls that frequented the city in April and May.

Wednesday was hot and sultry. The maximum temperature was 34.9 degrees Celsius but the maximum relative humidity was 92 per cent. The showers brought some relief on Thursday.

“The temperature will come down with rain but will go up again when it stops. It is going to be uncomfortable,” the Met official in Calcutta said.