Bangladesh occupies 147,570 square kilometers on the southern flank of the Asian subcontinent, bounded by India on three sides and Myanmar along a southeastern strip, with the Bay of Bengal forming the southern maritime border. The country sits almost entirely within the Ganges-Brahmaputra-Meghna delta system, the world's largest river delta, where these three major river systems converge before emptying into the Bay. The delta plain extends across roughly 80 percent of the country's land area, creating one of the flattest inhabited landscapes on earth with elevations rarely exceeding 10 meters above sea level throughout much of the territory. This topography results directly from millennia of sediment deposition carried downstream from the Himalayas, building land at an estimated rate of 1 billion tons of sediment annually. The remaining 20 percent of Bangladesh's terrain comprises the Chittagong Hill Tracts along the southeastern border with Myanmar, where elevations reach 1,064 meters at Keokradong peak, though local claims sometimes cite Tazing Dong at 1,280 meters. The Sylhet Division in the northeast contains the Madhupur Tract and extensive haor wetlands, seasonal water bodies that flood during monsoon months and contract to reveal agricultural land during dry periods.
The Ganges River, known locally as the Padma after it crosses into Bangladesh near Rajshahi Division, forms the principal western waterway, entering from West Bengal and flowing approximately 258 kilometers through Bangladeshi territory. The Brahmaputra, called the Jamuna after its confluence with the Tista River inside Bangladesh, runs southward through the central corridor for roughly 240 kilometers. These two rivers meet near Goalundo in Dhaka Division to form the Padma proper, which then converges with the Meghna River south of Chandpur to create the combined flow that empties into the Bay of Bengal through multiple distributary channels. The Meghna itself represents the union of the Surma and Kushiyara rivers descending from Sylhet Division. Together, these river systems drain a combined catchment area exceeding 1.7 million square kilometers, with approximately 93 percent of that watershed lying outside Bangladesh in India, Nepal, Bhutan, and Tibet. This upstream dependency means Bangladesh receives water flow and sediment loads determined almost entirely by precipitation and land use decisions beyond its borders. Annual sediment discharge through the combined delta system ranges from 500 million to over 1 billion tons depending on monsoon intensity, continuously reshaping riverbanks, creating new char islands, and extending the delta front into the Bay of Bengal.
The Sundarbans mangrove forest occupies approximately 10,000 square kilometers across the southwestern delta where the Ganges-Brahmaputra system meets the Bay of Bengal, split between Bangladesh (6,017 square kilometers) and India's West Bengal. This forest represents the world's largest contiguous mangrove ecosystem, characterized by intertidal mudflats, tidal waterways, and salt-tolerant tree species adapted to brackish water conditions. The dominant tree species include sundari (Heritiera fomes), from which the forest derives its name, gewa (Excoecaria agallocha), and keora (Sonneratia apetala). Tidal influence extends far inland through the Sundarbans, with tidal ranges exceeding 3 meters during spring tides, creating a dynamic environment where salinity gradients shift with seasonal freshwater flows from upstream rivers. The forest provides critical storm surge protection for inland areas, with mangrove stands reducing wave energy and absorbing tidal surges during cyclones. UNESCO designated the Bangladesh portion as a World Heritage Site in 1997, recognizing both ecological significance and biodiversity including populations of Bengal tigers, spotted deer, estuarine crocodiles, and multiple dolphin species.
Bangladesh experiences a tropical monsoon climate classified as Aw under the Köppen system across most of the delta plain, transitioning to Cwa in elevated portions of the Chittagong Hill Tracts. Three distinct seasons structure the annual cycle: a hot humid summer from March through May, a monsoon period from June through October, and a cooler dry winter from November through February. Pre-monsoon temperatures peak in April and May when daily maximums regularly exceed 35°C in Dhaka, with recorded extremes reaching 40.5°C in Rajshahi during April 2014. Winter temperatures remain mild with January minimums averaging 12-15°C in Dhaka and occasionally dropping to 5-7°C in northern districts like Dinajpur and Rangpur. Coastal areas experience smaller temperature ranges due to maritime influence, with Chittagong maintaining warmer winter minimums near 15°C. Relative humidity remains persistently high, exceeding 80 percent during monsoon months and rarely dropping below 50 percent even during winter dry periods in most locations.
Rainfall distribution follows pronounced seasonal and geographic patterns. The southwest monsoon, driven by the seasonal reversal of atmospheric circulation over South Asia, delivers the overwhelming majority of annual precipitation between June and September. Dhaka receives approximately 2,000 millimeters annually, with July typically contributing 350-400 millimeters alone. The northeastern Sylhet Division intercepts moisture-laden winds ascending the Meghalaya plateau, creating Bangladesh's wettest zone where locations like Sylhet city record 4,000 millimeters annually and surrounding areas sometimes exceed 5,000 millimeters. Cherrapunji and Mausynram, immediately north across the Indian border in Meghalaya, rank among earth's wettest locations with annual totals exceeding 11,000 millimeters, demonstrating the extreme orographic enhancement in this region. Western districts including Rajshahi Division receive notably less precipitation, averaging 1,400-1,600 millimeters annually due to rain shadow effects and greater distance from maritime moisture sources. The monsoon withdrawal occurs during October, often bringing a secondary precipitation peak associated with post-monsoon tropical depressions and cyclones tracking northward through the Bay of Bengal. Winter months from December through February contribute minimal precipitation, typically less than 50 millimeters total across these three months in most locations.
Tropical cyclones pose the most severe meteorological hazard. The Bay of Bengal generates roughly five to six tropical storms annually, with peak frequency during April-May and October-November when sea surface temperatures exceed 26°C and atmospheric conditions favor development. The funnel shape of the Bay concentrates storm surge effects along the Bangladesh coast, with the shallow bathymetry and wide continental shelf amplifying surge heights. The 1970 Bhola cyclone produced storm surge estimates of 6-10 meters, killing an estimated 300,000 to 500,000 people in coastal districts, though exact figures remain uncertain. Cyclone Sidr in November 2007 generated sustained winds of 215 kilometers per hour at landfall near the Sundarbans, with surge heights reaching 5 meters and causing approximately 3,500 deaths despite improved warning systems and expanded cyclone shelter networks. Cyclone Nargis struck Myanmar's Irrawaddy Delta in May 2008, demonstrating similar surge vulnerability in adjacent deltas. More recently, Cyclone Amphan in May 2020 brought winds of 155 kilometers per hour to coastal Bangladesh, with surge heights of 3-4 meters in Khulna Division. The government has constructed over 4,000 cyclone shelters since 1970, predominantly in coastal districts, and implemented community-based early warning systems that have substantially reduced mortality during recent storms despite increasing population density in hazard-prone areas.