Climate

Bangladesh has a typical monsoon climate with the same threefold division of the year that occurs in India, but the cool season (November to February) is generally warmer than India. During the hot season there are rainstorms, some thundery, and during the main rainy season (June to September) the rain is frequent and heavy. Generally, annual average rainfall varies from 1500 to 2500 mm (60 - 100 "), but near the eastern border this rises to 3750 mm (150 "). Rainfall from September to November is less reliable, but is occasionally very heavy and is usually associated with violent tropical cyclones that develop over the Bay of Bengal. Storm waves and sea surges raise the water level along the coast and in the numerous branching water courses of the delta so that widespread flooding of the low-lying areas takes place adding to the devastation caused by the strong wind. Such storms have led to great loss of life and destruction of crops on several occasions.

Although temperatures during the hot season are rather lower than in some parts of India, the heat is made uncomfortable by the high humidity. This damp, muggy season continues throughout the main rainy season, but the heat is rarely dangerous. It is, however, very unpleasant for the unacclimatized visitor. There is no great difference in temperature conditions around the year from one region to another. During the hot season, temperatures are a little higher inland (Dacca - April - average maximum temperature 35 °C / 95 °F) than on the coast (Chittagong - April - average maximum temperature 32 °C / 89 °F). Owing to the greater cloudiness of the rainy season, average daily sunshine hours are least between June and September (about four hours per day), but six - eight hours per day for the rest of the year.

Administration     Slope %   Undefined   0 - 2 %   2 - 5   5 - 8   8 - 16   16 - 30   30 - 45   45   Water

Bangladesh has a subtropical monsoon climate characterized by wide seasonal variations in rainfall, moderately warm temperatures, and high humidity. Regional climatic differences in this flat country are minor. Three seasons are generally recognized: a hot, humid summer from March to June; a cool, rainy monsoon season from June to October; and a cool, dry winter from October to March. In general, maximum summer temperatures range between 32°C and 38°C. April is the warmest month in most parts of the country. January is the coldest month, when the average temperature for most of the country is 10°C.

Winds are mostly from the north and northwest in the winter, blowing gently at one to three kilometers per hour in northern and central areas and three to six kilometers per hour near the coast. From March to May, violent thunderstorms, called northwesters by local English speakers, produce winds of up to sixty kilometers per hour. During the intense storms of the early summer and late monsoon season, southerly winds of more than 160 kilometers per hour cause waves to crest as high as 6 meters in the Bay of Bengal, which brings disastrous flooding to coastal areas.

Heavy rainfall is characteristic of Bangladesh. With the exception of the relatively dry western region of Rajshahi, where the annual rainfall is about 160 centimeters, most parts of the country receive at least 200 centimeters 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 328 and 478 centimeters per year. Average daily humidity ranged from March lows of between 45 and 71 percent to July highs of between 84 and 92 percent, based on readings taken at selected stations nationwide in 1986.

About 80 percent 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, thirteen severe cyclones hit Bangladesh, causing enormous loss of life and property. In May 1985, for example, a severe cyclonic storm packing 154 kilometer-per-hour winds and waves 4 meters 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 kilometers of critically needed embankments. 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 sixty-four 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 runways at the Zia International Airport--an important transit point for disaster relief supplies--was flooded. About 2 million tons of crops were reported destroyed, and relief work was rendered even more challenging than usual because the flood made transportation of any kind exceedingly difficult.

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.