Kolkata is subject to a tropical wet-and-dry climate that is designated Aw under the Köppen climate classification. 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 often exceed 40 °C (104 °F) in May and June. Winter lasts for only about 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). Often, in April–June, the city is struck by heavy rains or dusty squalls followed by thunderstorms or hailstorms, bringing cooling relief from the prevailing humidity. These thunderstorms are convective in nature, and are known locally as kal baisakhi (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 1,582 mm (62 in). The highest monthly rainfall total, 306 mm (12 in), occurs in August. The city receives 2,528 hours of sunshine per year, with maximum sunlight exposure occurring in March. Kolkata has been hit by several cyclones; these include systems occurring in 1737 and 1864 that killed thousands.
Pollution is a major concern in Kolkata. As of 2008, sulfur dioxide and nitrogen dioxide annual concentration were within the national ambient air quality standards of India, but respirable suspended particulate matter levels were high, and on an increasing trend for five consecutive years, causing smog and haze. Severe air pollution in the city has caused a rise in pollution-related respiratory
