The August rainfall in the city was the second lowest compared with the corresponding counts in the past 10 years, resulting in a deficit of more than 23 per cent.
The rainfall deficit in June and July was 38 per cent. It came down to 23.33 per cent in August mainly because of increased rainfall activity in the second half of the month.
The aggregate rainfall rose from 96.7mm in the first 15 days to 173.5mm in the next 16 days accounting for the city recording its usual August quota of 17 rain days (days when it rains 2.5mm or more).
Met office records show the city has received 270.2mm of rain last month, against the normal count of 352.4mm (based on the readings between 1971 and 2000).
Only once in the past 10 years did the city receive lesser rainfall in August in 2003, said an official of the Alipore Met office. The August aggregate that year was 185.8mm.
Monsoon rain in south Bengal is mostly triggered by low-pressure areas and the depression over Gangetic Bengal and the Bangladesh coast, which suck moisture from the sea. In their absence, the rain this season is almost entirely dependent on the monsoon trough and cyclonic circulations, said Gokul Chandra Debnath, the director of the Alipore Met office.
There should have been at least 10-12 low-pressure areas and a couple of depressions by now. But there have been only three-four low-pressure areas and no depression at all, Debnath pointed out.
Studies are on to identify the reasons for the sudden change in the wind systems but it would take time to compile the results as the process involves many parameters and data collected over long periods, he added.
Weather scientists said the few low-pressure areas that had formed and the resulting clouds got sucked away towards Odisha.
July is usually the rainiest month of the season, followed by August. But this year July was drier than August, having received 247.3mm of rain, said an official of India Meteorological Department.
Chances of the deficit being made up are slim as the business end of the monsoon is over and rain will be in decline, the official said.