Cambodia occupies 181,035 square kilometers in mainland Southeast Asia. The country shares a 1,158-kilometer border with Thailand to the west and northwest, an 800-kilometer border with Laos to the northeast, and a 1,158-kilometer border with Vietnam to the east and southeast. The Gulf of Thailand forms the southwestern coastline across 443 kilometers. The Kingdom of Cambodia extends from approximately 10 to 15 degrees north latitude and 102 to 108 degrees east longitude.
The Mekong River defines Cambodia's hydrological structure. The river enters Cambodia from Laos at Stung Treng Province and flows south through the country for 486 kilometers before splitting into the Mekong and Bassac River channels near Phnom Penh. These channels continue into Vietnam and the Mekong Delta. The Mekong's annual flood pulse carries an estimated 475 cubic kilometers of water through Cambodia. During the dry season from November through May, the river at Phnom Penh measures approximately one kilometer wide with depths of eight to ten meters. During the wet season from June through October, the river expands to several kilometers in width with depths exceeding fifteen meters in the main channel.
Tonle Sap Lake functions as the Mekong's natural reservoir through a hydraulic phenomenon that reverses river flow seasonally. The Tonle Sap River connects the lake to the Mekong at Phnom Penh. During the dry season, water flows from the lake southeast into the Mekong. When monsoon rains swell the Mekong from June onward, the increased volume forces the Tonle Sap River to reverse direction, flowing northwest and filling the lake. The lake expands from approximately 2,700 square kilometers in the dry season to more than 16,000 square kilometers during peak flood in September or October. Depth increases from an average of one meter in April to nine meters in October. This seasonal expansion deposits nutrient-rich sediment across the floodplain and creates one of the world's most productive freshwater fisheries, yielding an estimated 300,000 to 400,000 metric tons of fish annually.
The Central Plains surround Tonle Sap Lake and extend across the lower Mekong basin. This alluvial floodplain sits at elevations between five and thirty meters above sea level. The plains encompass approximately 70 percent of Cambodia's land area. Rice cultivation dominates this landscape, with paddies covering an estimated 2.8 million hectares. The soil consists primarily of recent alluvial deposits mixed with older sedimentary formations. The flatness allows seasonal flooding to extend across vast areas, creating wetland ecosystems that support fish migration between the lake, rivers, and rice fields.
The Cardamom Mountains rise along Cambodia's southwest boundary with Thailand. This range extends approximately 160 kilometers from northwest to southeast, with peaks reaching 1,500 to 1,800 meters. Phnom Aural, Cambodia's highest point, stands at 1,813 meters in the eastern Cardamoms. The mountains consist of Mesozoic and Paleozoic sedimentary and metamorphic rock overlaid with crystalline formations. Dense evergreen rainforest covers the slopes, receiving 3,800 to 5,000 millimeters of annual rainfall on western exposures facing monsoon winds from the Gulf of Thailand. The Cardamom Mountains Protected Forest encompasses approximately 1.4 million hectares, making it mainland Southeast Asia's largest remaining contiguous rainforest.
The Elephant Mountains form a southern extension of the Cardamom range, running parallel to the coast. These mountains reach elevations between 500 and 1,100 meters, with Phnom Bokor at 1,079 meters as the most prominent peak. The Bokor Plateau sits at approximately 1,000 meters elevation, creating a distinctive highland microclimate. French colonial authorities established a hill station on the plateau in the 1920s to escape coastal heat. The Elephant Mountains drain westward into Kampong Som Bay and the Gulf of Thailand through short, steep rivers.
The Dângrêk Mountains mark Cambodia's northern boundary with Thailand across approximately 350 kilometers. This escarpment rises 180 to 550 meters above the Cambodian plain, forming the southern edge of the Khorat Plateau. The range consists of horizontally bedded sandstone from the Mesozoic era. Unlike the wet Cardamom Mountains, the Dângrêk range receives 1,200 to 1,600 millimeters of annual rainfall, supporting deciduous dipterocarp forest rather than evergreen rainforest. Preah Vihear Temple sits atop the escarpment at approximately 525 meters elevation, constructed in the 11th and 12th centuries during the Khmer Empire.
Cambodia's 443-kilometer coastline along the Gulf of Thailand extends from the Thai border at Koh Kong Province southeast to the Vietnamese border at Kep Province. The coast features a mix of sandy beaches, mangrove forests, and rocky headlands. Kampong Som Bay forms the primary indentation, providing the deep-water anchorage that supports Sihanoukville port, which handles approximately 500,000 twenty-foot equivalent units of container traffic annually. Ream National Park protects 21,000 hectares of coastal habitat including 15,000 hectares of marine area. Koh Rong Island and Koh Rong Samloem lie 25 kilometers offshore from Sihanoukville, representing the largest of approximately 60 islands in Cambodian waters.
Cambodia experiences a tropical monsoon climate driven by the seasonal reversal of prevailing winds. The southwest monsoon dominates from May through October, drawing moisture-laden air from the Indian Ocean and Gulf of Thailand. This period accounts for 80 to 90 percent of annual precipitation. The northeast monsoon prevails from November through March, bringing dry continental air from the Asian landmass. April represents a transitional period with variable winds and the year's highest temperatures.
Phnom Penh records a mean annual temperature of 28.3 degrees Celsius. Monthly mean temperatures range from 26.1 degrees Celsius in December to 30.2 degrees Celsius in April. Daily temperature variation exceeds annual variation, with typical dry season days reaching 32 to 35 degrees Celsius and cooling to 22 to 25 degrees Celsius at night. The capital receives an average of 1,400 millimeters of rainfall annually, with October typically recording 250 to 290 millimeters as the wettest month. January and February each average less than 10 millimeters.
Siem Reap in northwest Cambodia records similar temperature patterns to Phnom Penh but receives slightly less rainfall at approximately 1,350 millimeters annually. The city sits at 15 meters elevation on the northern shore of Tonle Sap Lake. September and October deliver the heaviest rainfall, coinciding with the lake's maximum expansion. The Angkor Archaeological Park surrounding Siem Reap shows significant microclimatic variation, with tree-covered temple complexes maintaining temperatures two to three degrees Celsius cooler than open areas during midday.
Battambang Province in western Cambodia receives 1,250 to 1,500 millimeters of annual rainfall, with the city of Battambang recording a mean annual temperature of 28.0 degrees Celsius. The province occupies the Central Plains adjacent to Tonle Sap Lake, with elevations ranging from ten to thirty meters. Rice cultivation relies entirely on monsoon rainfall and seasonal flooding, as the dry season from December through April typically produces less than 50 millimeters combined.
Kampot Province on the southern coast experiences heavier rainfall than interior provinces due to orographic lifting as monsoon winds rise over the Elephant Mountains. Kampot city receives approximately 1,900 millimeters annually, with October averaging 400 millimeters. The coastal location moderates temperature extremes slightly, with mean annual temperature of 27.4 degrees Celsius and smaller diurnal variation than inland areas. The province's famous Kampot pepper grows in red basaltic soils at elevations between 100 and 600 meters on the mountain slopes.
The Cardamom Mountains receive Cambodia's highest rainfall, with western slopes recording 3,800 to 5,000 millimeters annually. Weather stations in Koh Kong Province near the Thai border have measured over 5,000 millimeters in particularly wet years. This extreme precipitation supports evergreen rainforest distinct from the deciduous forests covering drier regions. Cloud cover persists through much of the wet season, reducing solar radiation and maintaining high humidity. The mountains experience over 200 rain days per year, compared to 120 to 140 rain days in the Central Plains.