Bangladesh has a tropical monsoon climate characterised by wide seasonal variations in rainfall, high temperatures, and high humidity. Regional climatic differences in this flat country are minor. Three seasons are generally recognised: a hot, muggy summer from March to June; a hot, humid and rainy monsoon season from June to November; and a warm-hot, dry winter from December to February. In general, maximum summer temperatures range between 38 and 41 °C (100.4 and 105.8 °F). April is the hottest month in most parts of the country. January is the coolest month, when the average temperature for most of the country is 16–20 °C (61–68 °F) during the day and around 10 °C (50 °F) at night.

Winds are mostly from the north and northwest in the winter, blowing gently at 1 to 3 kilometres per hour (0.6 to 1.9 mph) in northern and central areas and 3 to 6 kilometres per hour (1.9 to 3.7 mph) near the coast. From March to May, violent thunderstorms, called northwesters by local English speakers, produce winds of up to 60 kilometres per hour (37.3 mph). During the intense storms of the early summer and late monsoon season, southerly winds of more than 160 kilometres per hour (99.4 mph) cause waves to crest as high as 6 metres (19.7 ft) in the Bay of Bengal, which brings disastrous flooding to coastal areas.

Bangladesh has a tropical monsoon climate

Heavy rainfall is characteristic of Bangladesh causing it to flood every year. Except for the relatively dry western region of Rajshahi, where the annual rainfall is about 1,600 mm (63.0 in), most parts of the country receive at least 2,300 mm (90.6 in) of rainfall per year. Because of its location just south of the foothills of the Himalayas, where monsoon winds turn west and northwest, the region of Sylhet in northeastern Bangladesh receives the greatest average precipitation. From 1977 to 1986, annual rainfall in that region ranged between 3,280 and 4,780 mm (129.1 and 188.2 in) per year. Average daily humidity ranged from March lows of between 55 and 81% to July highs of between 94 and 100%, based on readings taken at selected stations nationwide in 1986.

About 80% of Bangladesh’s rain falls during the monsoon season. The monsoons result from the contrasts between low and high air pressure areas that result from differential heating of land and water. During the hot months of April and May hot air rises over the Indian subcontinent, creating low-pressure areas into which rush cooler, moisture-bearing winds from the Indian Ocean. This is the southwest monsoon, commencing in June and usually lasting through September. Dividing against the Indian landmass, the monsoon flows in two branches, one of which strikes western India. The other travels up the Bay of Bengal and over eastern India and Bangladesh, crossing the plain to the north and northeast before being turned to the west and northwest by the foothills of the Himalayas.

Natural calamities, such as floods, tropical cyclones, tornadoes, and tidal bores—destructive waves or floods caused by flood tides rushing up estuaries—ravage the country, particularly the coastal belt, almost every year. Between 1947 and 1988, 13 severe cyclones hit Bangladesh, causing enormous loss of life and property. In May 1985, for example, a severe cyclonic storm packing 154-kilometre-per-hour (95.7 mph) winds and waves 4 metres (13.1 ft) high swept into southeastern and southern Bangladesh, killing more than 11,000 persons, damaging more than 94,000 houses, killing some 135,000 head of livestock, and damaging nearly 400 kilometres (248.5 mi) of critically needed embankments.

Flooding after the 1991 Bangladesh cyclone

Annual monsoon flooding results in the loss of human life, damage to property and communication systems, and a shortage of drinking water, which leads to the spread of disease. For example, in 1988 two-thirds of Bangladesh’s 64 districts experienced extensive flood damage in the wake of unusually heavy rains that flooded the river systems. Millions were left homeless and without potable water. Half of Dhaka, including the runway at the Shahjalal International Airport—an important transit point for disaster relief supplies—was flooded. About 2,000,000 tonnes (2,204,623 short tons; 1,968,413 long tons) of crops were reported destroyed, and relief work was rendered even more challenging than usual because the flood made transportation exceedingly difficult. A tornado in April 1989 killed more than 600 people, possibly many more.

There are no precautions against cyclones and tidal bores except giving advance warning and providing safe public buildings where people may take shelter. Adequate infrastructure and air transport facilities that would ease the sufferings of the affected people had not been established by the late 1980s. Efforts by the government under the Third Five-Year Plan (1985–90) were directed toward accurate and timely forecast capability through agrometeorology, marine meteorology, oceanography, hydrometeorology, and seismology. Necessary expert services, equipment, and training facilities were expected to be developed under the United Nations Development Programme.

Cold weather is unusual in Bangladesh. When temperatures decrease to 8 °C (46 °F) or less, people without warm clothing and living in inadequate homes may die from the cold