Kolkata gets 1,711.5 mm of rain in an average year, and most of it falls in four months. That is the monsoon, and in this city it is the main event of the calendar.
The rain comes from the Bay of Bengal branch of the south-west monsoon, which lashes the city between June and September. The heaviest falls arrive in July and August, with the two months trading the top spot from year to year.
A tropical savanna climate
Kolkata sits in eastern India and has a tropical savanna climate, Aw on the Köppen scale. The annual mean temperature is 26.7°C. Monthly means run from 18.8°C to 30.6°C. The highest temperature ever recorded is 43.9°C and the lowest is 5°C.
Summers are hot and humid, with temperatures in the low 30s. In dry spells in May and June the maximum often tops 40°C. Winter is short, about two and a half months, and the lows settle at 9 to 11°C in December and January.
Before the monsoon
Early summer, from mid-March to mid-May, has its own weather. Dusty squalls blow in, followed by thunderstorms and heavy rain that break the humid heat. These storms are convective in nature and are known locally as Kal Baisakhi, or Nor’westers.
The city gets 2,234 hours of sunshine a year. April is the sunniest month. The main weather station is at Alipore, an international station, with two more at Dum Dum and Salt Lake.
Smog and haze
Pollution is a major concern. Suspended particulate matter levels are high compared with other major Indian cities, which leads to regular smog and haze. Severe air pollution in the city has been linked to a rise in respiratory ailments.
