A boat on the Tonle Sap

The most distinctive geographical feature is the lacustrine plain, formed by the inundations of the Tonle Sap (Great Lake), measuring about 2,590 square kilometers (1,000 sq mi) during the dry season and expanding to about 24,605 square kilometers (9,500 sq mi) during the rainy season. This densely populated plain, which is devoted to wet rice cultivation, is the heartland of Cambodia. Most (about 75%) of the country lies at elevations of less than 100 metres (330 ft) above sea level, the exceptions being the Cardamom Mountains (highest elevation 1,813 m / 5,948 ft) and their southeast extension the Dâmrei Mountains ("Elephant Mountains") (elevation range 500–1,000 m or 1,640–3,280 ft), as well the steep escarpment of the Dângrêk Mountains (average elevation 500 m / 1,640 ft) along the border with Thailand's Isan region. The highest elevation of Cambodia is Phnom Aoral, near Pursat in the centre of the country, at 1,813 metres (5,948 ft).

Cambodia's temperatures range from 21° to 35°C (69° to 95°F) and experiences tropical monsoons. Southwest monsoons blow inland bringing moisture-laden winds from the Gulf of Thailand and Indian Ocean from May to October. The northeast monsoon ushers in the dry season, which lasts from November to March. The country experiences the heaviest precipitation from September to October with the driest period occurring from January to February.

It has two distinct seasons. The rainy season, which runs from May to October, can see temperatures drop to 22 °C and is generally accompanied with high humidity. The dry season lasts from November to April when temperatures can raise up to 40 °C around April. The best months to visit Cambodia are November to January when temperatures and humidity are lower.