1.3. Weather

a. Climate

Cambodia has a tropical monsoon climate and is influenced by various factors including: (i) its location in the Inter-Tropical Convergence Zone; and (ii) the monsoon which forms two distinct seasons, described as follows.

• The dry season from November to April is associated with the northeast monsoon, which sends drier and cooler air.

• The rainy season from May to October, in which rainfall is largely derived from the southwest monsoon drawn inland from the Indian Ocean.

Three climatic zones have been defined for the country: the coastal and mountainous area of the southwest, the central plains which include the Mekong River and the Tonle Sap Lake, and the North and Northeastern region (See the Climatic Zoning Map of Cambodia attached.)

b. Rainfall

Annual rainfall varies by region. In the lowland area around Tonle Sap Great Lake, annual rainfall ranges between 1,200 to 1,900 mm. East of the Mekong River, rainfall generally averages between 1,500 to 3,000 mm. The heaviest rainfall, over 3,000 mm per year, occurs along the coastal lowlands in the west. The lowest rainfall occurs in the rain-shadow region of the Elephant Mountain and the Cardamom Mountains in the southwest and western regions (Kampong Spueu, Koh Kong and Pousat Provinces). Generally, the average annual rainfall over most of the country is between 1,000 and 2,500 mm.

Precipitation also varies widely from year to year. From the latter part of July, there may be periods without significant rainfall for ten or more days at a time; this is referred to as the "short dry season" (Farmers tend to delay planting during this time to minimise the risk of damage to rice or crop seedlings.) (FAO-1994)

c. Temperature

In Cambodia, temperatures vary according to the three climatic zones:

• Coastal and mountainous areas: Temperatures are generally high with little variation between seasons. Temperatures can fall below 20 degrees C in the coldest months.

• Central plains: Temperatures are generally also high with little variation between seasons. Monthly temperatures range from 25 degrees C in January and can rise to 40 degrees C; it is rarely higher than 40 degrees C in April, the warmest month before the start of the rainy season. The coldest temperature is below 15 degrees C in December and January. The mean temperature is approximately 25 degrees C.

• Northern and North-eastern areas: Temperatures are high during the hottest and coldest months. The mean temperature is normally 25 degrees C in the lowlands and 20 degrees C in the highlands.

d. Humidity and evaporation

Humidity and evaporation rates change with the seasons. The most humid month is July and the driest month is February. Generally, humidity ranges between 65 to 70 percent in January and 85 to 90 percent in August. Annual evaporation rates range from 2,000 to 2,200 mm. The highest evaporation rate occurs in March and April at 200 to 240 mm and the lowest evaporation rate in September at 120 to 150 mm.

e. Winds

There are few records on wind speed data for Cambodia, but referring to data from neighbouring countries, we can estimate that mean wind speeds are low, at an average of 2 m/s average for most parts of the country. In the southeast, strong winds occur from February to April and result in relatively strong winds in the southern part of the country. From August to October, general turbulence in the upper layer leads to unstable conditions across the continents of Southeast Asia. The wind moves to the northeast and sometimes results in strong, steady winds from November to January.

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Table of contents > 1. Overview

1.0. Background Information

1.1. Physical geography

1.2. Topography and administrative boundaries

1.3. Weather

1.4. Hydrology