United Nations Development Programme report, its wind and cyclone zone is "very high damage risk". Temperature The annual mean temperature is ; monthly mean temperatures are . 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 months, with seasonal lows dipping to in December and January. May is the hottest month, with daily temperatures ranging from ; January, the coldest month, has temperatures varying from . The highest recorded temperature is , and the lowest is . 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 (), or "Nor'westers" in English. Rainfall 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 . 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. Environmental issues Pollution is a major concern in Kolkata. , sulphur 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 ailments, such as lung cancer. Economy Kolkata is the commercial and financial hub of East and North-East India and home to the Calcutta Stock Exchange. It is a major commercial and military port, and is one of five cities in eastern India (alongside Bhubaneswar, Guwahati, Imphal, and Kushinagar) to have an international airport. Once India's leading city, Kolkata experienced a steady economic decline in