A bone dry September gave way to a rainy first day of October on Thursday and more wet weather is in store over the weekend.

September traditionally brings with it quite a bit of rainfall in the city. This year, however, there was only a sprinkling of showers, adding up to merely 230.1mm of rain - the lowest September rainfall in Calcutta in a decade (see chart) and well below the average rainfall for the month.

The first seven days of September, the city received only 0.9mm of rain.

"Depressions, low-pressure areas and cyclonic circulations were missing from the vicinity of Calcutta for the most part of September. This explains the lack of rainfall, especially towards the end of the month," said G.K. Das, scientist at the India Meteorological Department, Calcutta. "There were a few favourable wind formations in September but they were separated by long dry spells. Also, there were no strong weather systems close to the city."

Depressions or low-pressure areas have lower atmospheric pressure than their surroundings, helping them pull air towards themselves. When these systems are close to a particular place, they also push up the moisture level, leading to cloud formation and rainfall.

The skies opened up on Thursday as a cyclonic circulation over the Bay of Bengal shifted towards the Bengal coast and the moisture incursion to land increased.

The afternoon showers came after six dry days. "A cyclonic circulation spread over the Andaman Sea moved up and reached the Bay of Bengal, coming closer to Calcutta. This led to the formation of thunderclouds and similar weather is expected over the next two to three days," Das said.

Thursday morning was hot, the temperature crossing 35 degrees Celsius around 1pm when the relative humidity was as high as 56 per cent. This made the weather conditions extremely uncomfortable.

The stifling weather contributed to the formation of tall thunderclouds followed by a quick downpour.

The maximum rainfall was recorded at Palmer Bazaar drainage pumping station in central Calcutta. Maniktala in north Calcutta received 28mm, while Ballygunge in the south got 18mm. The Alipore Met office, however, recorded only 0.4mm of rainfall.

Despite a dry spell, monsoon overall was good in the city this year with June recording 18 per cent surplus rainfall. In Calcutta and the rest of south Bengal, the monsoon arrived late this year but made up for the delay mainly because of a steady supply of low-pressure areas and depressions in July. The month saw 606.1mm of rain in Calcutta.

The Met office has ruled out an early withdrawal of monsoon from the city. "The monsoon first recedes from the west of the country. The withdrawal started in the beginning of September, which is normal. But in Calcutta, monsoon will start receding around October 15," said a senior official of IMD, Delhi.