Bangladesh is a low-lying, riverine country located in South Asia with a largely marshy jungle coastline of 580 km (360 mi) on the northern littoral of the Bay of Bengal. Formed by a delta plain at the confluence of the Ganges (Padma), Brahmaputra (Jamuna), and Meghna Rivers and their tributaries, Bangladesh's alluvial soil is highly fertile, but vulnerable to flood and drought. Hills rise above the plain only in the Chittagong Hill Tracts in the far southeast and the Sylhet division in the northeast. Straddling the Tropic of Cancer, Bangladesh has a tropical monsoon climate characterized by heavy seasonal rainfall, high temperatures, and high humidity. Natural disasters, such as floods, tornadoes, and tidal bores affect the country yearly. Bangladesh also is affected by major cyclones, on average 16 times a decade. A cyclone struck the southeastern coast in May 1991, killing 136,000 people. Cyclone Sidr struck the southwestern coast on November 15, 2007, affecting not only the coastal districts of the administrative division Khulna but also about half of the tropical forest Sundarbans.