Climate of Cambodia

Houses on stilts above the flooded waters of Kampong Phluk, Tonle Sap Lake

The climate of Cambodia is tropical and shaped by the monsoon, which divides the year into a dry season and a rainy season. The rains swell the Mekong and push water up the Tonle Sap River into the Tonle Sap lake, a yearly flood that fisheries and farming depend on.

Seasons

The dry season runs from November to April, when the northeast monsoon brings drier, cooler air. The rainy season runs from May to October, when the southwest monsoon draws moist air in from the Indian Ocean. Sources divide the country into three climate zones: the coast and mountains of the southwest, the central plains around the Mekong and the Tonle Sap, and the north and northeast.

From late July there can be dry spells of ten days or more within the rainy season. This break is known as the "short dry season", and farmers often hold off planting so that rice seedlings are not lost.

Rainfall

Most of the country receives between 1,000 and 2,500 millimeters of rain a year. The lowlands around the Tonle Sap get about 1,200 to 1,900 millimeters, and the land east of the Mekong about 1,500 to 3,000. The western coastal lowlands are the wettest, with over 3,000 millimeters. The driest areas lie in the rain shadow of the Cardamom Mountains and the Elephant Mountains, in Kampong Speu, Koh Kong and Pursat provinces.

Temperature and humidity

A country report by an official of Cambodia's National Committee for Disaster Management (NCDM), published by the Asian Disaster Reduction Center, gives an average temperature range of 21 to 36 °C, with the hottest weather in April and May. On the central plains, April is the warmest month, just before the rains begin, and temperatures seldom pass 40 °C. December and January are the coolest months, when nights can drop below 15 °C. July is the most humid month and February the driest; humidity runs at about 65 to 70 percent in January and 85 to 90 percent in August.

The Tonle Sap flood pulse

In the dry season the Tonle Sap lake drains into the Mekong. In the flood season the Mekong rises so high that the Tonle Sap River reverses, and water flows back into the lake. A policy report on the lake places the peak of this reverse flow in May and June. The lake can barely drain until the Mekong falls again, in September and October. The lake grows from about 2,500 square kilometers in the dry season to about 15,000 in the wet season. The water that comes in carries sediment, nutrients and fish. It floods the forests, wetlands and fields around the lake, where fish and other wildlife breed. Across the wider Mekong basin, the flood season lasts from June to November and carries about 70 to 80 percent of the river's yearly flow. In this season fish move onto the floodplains to spawn and feed.

A 2022 study of sixty years of records found that the flood pulse has weakened. The average yearly reverse flow into the lake fell by 56.5 percent, from 48.7 cubic kilometers in 1962 to 1972 to 31.7 in 2010 to 2018. Wet-season lake levels in 2010 to 2019 were 1.05 meters lower than in 1996 to 2009, shrinking the lake area by about a fifth. The authors found that upstream dams alone could not explain the decline and pointed to local causes such as irrigation and riverbed erosion.

Floods and droughts

Cambodia faces floods, droughts and storms, among other weather hazards. The flood of 2000 led the Mekong River Commission to draw up a strategy for flood management. In 2011 floods affected 350,000 households, more than 1.5 million people, in 18 of the country's 24 provinces. The NCDM report records 250 deaths and about 630 million US dollars in losses. Floods in 2013 reached 20 provinces and 377,354 households and took 168 lives.

Droughts come from late or erratic rains and fall hardest on rice farmers who rely on rain or river water; the 2013 report put irrigation coverage at around 20 percent. The droughts of 2004 to 2005 and 2009 to 2010 brought low flows on the Mekong and heavy crop losses in Cambodia and northeastern Thailand.