Kerala is wedged between the Lakshadweep sea and the Western Ghats. Lying between north latitudes 8°18' and 12°48' and east longitudes 74°52' and 77°22', Kerala experiences the humid equatorial tropic climate. The state has a coast of length 590 km (367 mi) and the width of the state varies between 35 and 120 km (22–75 miles). 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; hence, most of the state is subject to comparatively little seismic and volcanic activity. Pre-Cambrian and Pleistocene geological formations compose the bulk of Kerala’s terrain.
The eastern Kerala region consists of high mountains, gorges and deep-cut valleys immediately west of the Western Ghats' rain shadow. Forty-one of Kerala’s west-flowing rivers, and three of its east-flowing ones originate in this region. 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). Anamudi is the highest peak at an elevation of 2,695 metres (8,130 ft). Just west of the mountains lie the midland plains comprising central Kerala, dominated by rolling hills and valleys. 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 Agastya Mala and Anamala.
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 River(130 km), the Valapattanam (129 km) and the Achankovil (128 km). The average length of the rivers of Kerala is 64 km. Many of the rivers 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. The rivers also face problems such as sand mining and pollution. The state experiences several natural hazards such as landslides, floods, lightning and droughts. The state was also affected by the 2004 Indian Ocean Tsunami.
A catastrophic flood occurred in Kerala in 1341 AD that drastically modified the terrain and consequently affected the history. The flood resulted in changing the course of the river Periyar, recession of Arabian Sea by several miles downwards making the Kuttanad region cultivable, closure of the Muziris (Kodungalloor) harbour and creation of a new harbour at Kochi.
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.
During summer, Kerala is prone to gale force winds, storm surges, cyclone-related torrential downpours, occasional droughts, and rises in sea level. The mean daily temperatures range from 19.8 °C to 36.7 °C. Mean annual temperatures range from 25.0–27.5 °C in the coastal lowlands to 20.0–22.5 °C in the eastern highlands.