The thumb-shaped state is characterized by wholly mountainous terrain. Almost the entire state is hilly, with an elevation ranging from 280 metres (920 ft) to 8,585 metres (28,000 ft). The summit of the Kangchenjunga is the highest point which falls on the border between Sikkim and Nepal. For the most part, the land is unfit for agriculture because of the precipitous and rocky slopes. However, certain hill slopes have been converted into farm lands using terrace farming techniques. Numerous snow-fed streams in Sikkim have carved out river valleys in the west and south of the state. These streams combine into the Teesta and its tributary, the Rangeet. The Teesta, described as the "lifeline of Sikkim", flows through the state from north to south. About a third of the land is heavily forested.

The Himalayan ranges surround the northern, eastern and western borders of Sikkim in a crescent. The Lower Himalayas in the southern reaches of the state are the most densely populated. The state has 28 mountain peaks, more than 80 glaciers, 227 high-altitude lakes including the Tsongmo Lake, Gurudongmar and Khecheopalri Lake, 5 hot springs, and more than 100 rivers and streams. Eight mountain passes connect the state to Tibet, Bhutan and Nepal.

Sikkim's hot springs are known for medicinal and therapeutic values. The most important hot springs are at Phurchachu (Reshi), Yumthang, Borang, Ralang, Taram-chu and Yumey Samdong. They have high sulphur content and are located near river banks. Some also emit hydrogen. The average temperature of the water in these hot springs is 50 °C (122 °F).

Climate

The climate ranges from sub-tropical in the south to tundra in the northern parts. The tundra-type region in the north is clad by snow for four months a year though the temperature drops below 0 °C (32 °F) almost every night. The peaks of north-western Sikkim are perpetually frozen. Most of the inhabited regions of Sikkim, however, witness a temperate climate, with the temperatures seldom exceeding 28 °C (82 °F) in summer or dropping below 0 °C (32 °F) in winter. The mean monthly temperature in summer is 15 °C. The state has five seasons: winter, summer, spring, and autumn, and a monsoon season between June and September. The average annual temperature for most of Sikkim is around 18 °C (64 °F). Sikkim is one of the few states in India to receive regular snowfall. The snow line ranges from 20,000 feet in the north to 16,000 feet in the south. During the monsoon, heavy rains increase the possibility of landslides. The record for the longest period of continuous rain is 11 days. In the northern region, because of high altitude, temperatures drop below −40 °C (−40 °F) in winter. Fog also affects many parts of the state during winter and the monsoons, making transportation perilous.