Kolkata is subject to a tropical wet-and-dry climate that is designated Aw under the Köppen climate classification. According to a United Nations Development Programme report, its wind and cyclone zone is “very high damage risk”.
The annual mean temperature is 26.8 °C (80.2 °F); monthly mean temperatures are 19–30 °C (66–86 °F). Summers (March–June) are hot and humid, with temperatures in the low 30s Celsius; during dry spells, maximum temperatures sometime exceed 40 °C (104 °F) in May and June. Winter lasts for roughly two-and-a-half months, with seasonal lows dipping to 9–11 °C (48–52 °F) in December and January. May is the hottest month, with daily temperatures ranging from 27–37 °C (81–99 °F); January, the coldest month, has temperatures varying from 12–23 °C (54–73 °F). The highest recorded temperature is 43.9 °C (111.0 °F), and the lowest is 5 °C (41 °F). The winter is mild and very comfortable weather pertains over the city throughout this season.
Often, in April–June, the city is struck by heavy rains or dusty squalls that are followed by thunderstorms or hailstorms, bringing cooling relief from the prevailing humidity. These thunderstorms are convective in nature, and are known locally as kal bôishakhi (কালবৈশাখী), or “Nor’westers” in English.
Rains brought by the Bay of Bengal branch of the south-west summer monsoon lash Kolkata between June and September, supplying it with most of its annual rainfall of about 1,850 mm (73 in). The highest monthly rainfall total occurs in July and August. In these months often incessant rain for days brings life to a stall for the city dwellers. The city receives 2,107 hours of sunshine per year, with maximum sunlight exposure occurring in April. Kolkata has been hit by several cyclones; these include systems occurring in 1737 and 1864 that killed thousands. More recently, Cyclone Aila in 2009 and Cyclone Amphan in 2020 caused widespread damage to Kolkata by bringing catastrophic winds and torrential rainfall.