Bangladesh, lying in the subtropical monsoon region to the south of the Himalayas, experiences three major seasons.
Only winter, from December to March, is relatively undisturbed. Cool, stable north-easterlies give warm, dry days and cool nights, with thick fog commonly developing at night over the waterways.
Even in January mean daily temperatures rarely fall below 15°C. At the end of March the situation changes, and from April to June torrential thunderstorms, known locally as Nor'westers, bring very heavy rainfall. These storms occur when high-level troughs in the westerly jet stream, located south of the Himalayas between 9,000 to 12,000 meters up, coincide with depressions moving in from the east-south-east over the Bay of Bengal.
The rains frequently arrive well before the wind reversal commonly associated with the first appearance of the monsoons. These storms and their accompanying rainfall release latent heat on a sufficient scale to be a significant trigger mechanism in the monsoon wind reversal which affects the whole of the Indian subcontinent.
The third major season is that of the monsoon itself. The succession of events which leads to the bursting of the monsoon is highly complex.
In early June, the high-level westerly jet stream shifts to the north of the Himalayas and the Tibetan Plateau. While for most of the year a semi-permanent trough lies over Bengal, this now breaks into two parts, one centered on the Pamirs and the other on Szechwan (Sichuan) further east. Simultaneously, the equatorial westerlies deepen, and it is with this air stream that the cyclones which bring most of Bangladesh's rainfall come.
Climate throughout the country is fairly uniform, with minor modifications being found in the eastern and southeastern uplands.
High humidity (over 75%) is characteristic throughout the year, and both annual and diurnal temperature ranges are generally very small. Summer maxima rarely go above 36°C and minimum temperatures, even in the Hill Tracts, generally remain above 10°C. During the monsoon period the diurnal range of temperature is about 6°C. Rainfall shows a decline from east to west, but even the districts of Rajshahi and Kushtia in the extreme west receive over 1,500 mm per annum. Only in the extreme northeast and southeast do rainfall totals exceed 3,000 mm. Most of the rainfall comes between April and November, and towards the end of the monsoon period severe cyclonic storms commonly develop.
The extensive low-lying areas of the delta are repeatedly endangered by the consequent catastrophic flooding, as occurred in December 1970.
At the apex of the Bay of Bengal, the Bangladesh delta is subject not only to floods from overflowing rivers, but from storm surges which may raise the sea level by as much as 10 feet.
Although the swampy Sundarbans in the southwest are sparsely inhabited and therefore suffer relatively little, the islands in the mouth of the Ganges to the east are settled much more densely, and therefore suffer far more severe damage.
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