Kerala’s 38,863 km² landmass (1.18% of India) is wedged between the Arabian Sea to the west and the Western Ghats—identified as one of the world's twenty-five biodiversity hotspots—to the east. Lying between north latitudes 8°18' and 12°48' and east longitudes 74°52' and 72°22', Kerala is well within the humid equatorial tropics. Kerala’s coast runs for some 580 km (360 miles), while the state itself varies between 35 and 120 km (22–75 miles) in width. Geographically, Kerala can be divided into three climatically distinct regions: the eastern highlands (rugged and cool mountainous terrain), the central midlands (rolling hills), and the western lowlands (coastal plains). Located at the extreme southern tip of the Indian subcontinent, Kerala lies near the centre of the Indian tectonic plate; as such, most of the state is subject to comparatively little seismic and volcanic activity. Geologically, pre-Cambrian and Pleistocene formations compose the bulk of Kerala’s terrain.
Eastern Kerala lies immediately west of the Western Ghats's rain shadow; it consists of high mountains, gorges and deep-cut valleys. 41 of Kerala’s west-flowing rivers, and 3 of its east-flowing ones originate in this region. Here, the Western Ghats form a wall of mountains interrupted only near Palakkad, where the Palakkad Gap breaks through to provide access to the rest of India. The Western Ghats rises on average to 1,500 m (4920 ft) above sea level, while the highest peaks may reach to 2,500 m (8200 ft). Just west of the mountains lie the midland plains composing central Kerala; rolling hills and valleys dominate. Generally ranging between elevations of 250–1,000 m (820–3300 ft), the eastern portions of the Nilgiri and Palni Hills include such formations as Agastyamalai and Anamalai.
Kerala’s western coastal belt is relatively flat, and is criss-crossed by a network of interconnected brackish canals, lakes, estuaries, and rivers known as the Kerala Backwaters. Lake Vembanad—Kerala’s largest body of water—dominates the Backwaters; it lies between Alappuzha and Kochi and is more than 200 km² in area. Around 8% of India's waterways (measured by length) are found in Kerala. The most important of Kerala’s forty four rivers include the Periyar (244 km), the Bharathapuzha (209 km), the Pamba (176 km), the Chaliyar (169 km), the Kadalundipuzha (130 km) and the Achankovil (128 km). The average length of the rivers of Kerala is 64 km. Most of the remainder are small and entirely fed by monsoon rains. These conditions result in the nearly year-round water logging of such western regions as Kuttanad, 500 km² of which lies below sea level. As Kerala's rivers are small and lack deltas, they are more prone to environmental factors. Kerala's rivers face many problems, including summer droughts, the building of large dams, sand mining, and pollution.
With 120–140 rainy days per year, Kerala has a wet and maritime tropical climate influenced by the seasonal heavy rains of the southwest summer monsoon. In eastern Kerala, a drier tropical wet and dry climate prevails. Kerala's rainfall averages 3,107 mm annually. Some of Kerala's drier lowland regions average only 1,250 mm; the mountains of eastern Idukki district receive more than 5,000 mm of orographic precipitation, the highest in the state. In summers, most of Kerala is prone to gale force winds, storm surges, cyclone-related torrential downpours, occasional droughts, and rises in sea level and storm activity resulting from global warming. Kerala’s maximum daily temperature averages 36.7 °C; the minimum is 19.8 °C. Mean annual temperatures range from 25.0–27.5 °C in the coastal lowlands to 20.0–22.5 °C in the highlands.