An imposing mountainous mass lies in the south central part of Sri Lanka, with the highest point being the peak of Pidurutalagala at 2,524 metres (8,281 feet). Two plateaux-Nuwara Eliya and the Horton Plains-extend across the upland area. These are major centres of commercial tea plantations. North of the mountains, and extending south, is the dry zone-an arid and gently rolling plain.

Rivers and streams broken by rapids are numerous in the mountainous south central region of Sri Lanka. The Mahaweli Ganga is the longest river, emptying into the Indian Ocean south of Trincomalee. Other rivers are the Kelani, the mouth of which is near Colombo; the Kalu, which reaches the sea near Kalutara on the southwest coast; and the Aruvi Aru, which flows northwest across the dry zone to a point near Mannar.

Its location near the equator means that Sri Lanka has an extremely hot and humid climate. At the higher elevations, however, the climate is cool, and in the dry zone the humidity is relatively lower. The average annual temperature is 32°C (90°F) in the lowlands and 21°C (70°F) in the higher mountainous regions.

Precipitation is determined by seasonal and regional variations. The monsoon season in the southwest lasts from May to November, during which time the rainfall is exceptionally heavy. In the northern dry zone, most of the precipitation—roughly 1,016 millimetres (40 inches)—annually occurs during the monsoon season, which begins in the first week of November. Most crops in the dry zone require irrigation nonetheless. The hills and lowlands of the southwest section, known as the wet zone, normally have some rainfall throughout the year, but heavier precipitation occurs in May and June and again in October and November.

Richly endowed with biodiversity, Sri Lanka has a long tradition of natural resource conservation. Nearly 12.1 per cent (1996) of the land is protected, although much wildlife habitat has been lost and a number of species continue to be threatened by poachers. There is coastal degradation from mining activities and increased pollution from the tourist trade, and freshwater resources are being polluted by industrial waste and sewage. Some wet-zone forests have been removed for timber export and replaced by commercial crop plantations. Sri Lanka is a participant in several international agreements on biodiversity, endangered species, hazardous wastes, the banning of nuclear testing, ozone-layer protection, and wetlands, among others.