More than 30 people are reported to have died in flooding associated with this years monsoon rains in Bangladesh, with over 200 000 people forced to flee their homes for higher ground as a series of rivers have burst their banks. The rains have also triggered a series of landslides, with over 200 in the Cox's Bazar District alone. The country has suffered some of the heaviest rainfall in years, but the bulk of the problems have been caused not by rain falling in Bangladesh, but rain that has fallen upstream in India, causing rivers to swell and cause problems in Bangladesh.
Monsoons are tropical sea breezes triggered by heating of the land during the warmer part of the year (summer). Both the land and sea are warmed by the Sun, but the land has a lower ability to absorb heat, radiating it back so that the air above landmasses becomes significantly warmer than that over the sea, causing the air above the land to rise and drawing in water from over the sea; since this has also been warmed it carries a high evaporated water content, and brings with it heavy rainfall. In the tropical dry season the situation is reversed, as the air over the land cools more rapidly with the seasons, leading to warmer air over the sea, and thus breezes moving from the shore to the sea (where air is rising more rapidly) and a drying of the climate. This situation is particularly intense in South Asia, due to the presence of the Himalayas. High mountain ranges tend to force winds hitting them upwards, which amplifies the South Asian Summer Monsoon, with higher winds leading to more upward air movement, thus drawing in further air from the sea.
Diagrammatic representation of wind and rainfall patterns in a tropical monsoon climate. Geosciences/University of Arizona.
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