Jet Stream a strong current of air that blows through the atmosphere at a high altitude, near the tropopause (10-15 km above the sea level). The main jet streams are in middle and subtropical latitudes. The temperate jet stream flows from west to east. It also travels through a wave-link route, going north and south. In winter, two branches of the polar jet stream are anchored by the himalayas - one in the north and the other in the south. The wind speeds in these jet streams may vary from 150 to 350 km/hr. The southern branch of the polar jet stream causes subsidence in the atmosphere in the northwestern part of India and as a result, a centre of high pressure develops there. A stream of cold air from this high pressure enters Bangladesh during the winter. Occasionally, these winds also bring in atmospheric disturbances that are responsible for the occasional rainfall in Bangladesh during the winter.
As the winter season progresses into the spring and summer, temperature in South Asia rises, thereby pushing the southern branch of the westerly polar jet stream to the north of the Himalayas. It is then replaced by the tropical easterly jet stream. The tropical easterly jet stream is a seasonal and regional flow - operating only in summer over the area that extends from the Indo-China Peninsula to the coast of Saudi Arabia, at an elevation of 15-18 km above the sea level. It is strongest over Peninsular India, where wind speed may reach 150-175 km/hr. [Rafique Ahmed]