Cambodia divides into two principal seasons determined by the southwest and northeast monsoons. The dry season runs November through April, defined by the northeast monsoon bringing continental air from China. The wet season spans May through October under the southwest monsoon drawing moisture from the Indian Ocean and Gulf of Thailand. This binary pattern governs water levels in the Mekong River and Tonle Sap Lake, which in turn determine accessibility to archaeological sites, fishing patterns, and agricultural cycles across the Central Plains.
November through February represents the cool dry season. Daytime temperatures in Phnom Penh average 26-28 degrees Celsius, dropping to 20-22 degrees at night. Siem Reap records similar ranges. Humidity falls to 50-60 percent. The Cardamom Mountains and Dângrêk Mountains experience cooler conditions, with Bokor Plateau reaching minimums of 15 degrees. Rainfall averages below 20 millimeters monthly in Phnom Penh during this period. December and January see the lowest precipitation. Tonle Sap Lake begins receding from its wet season maximum of approximately 12,000 square kilometers toward its dry season minimum of 2,500 square kilometers. This contraction occurs gradually through April. Angkor Wat and surrounding temples at Angkor Thom receive the highest visitor numbers November through February, with January peak attendance often exceeding 200,000 international arrivals to Siem Reap province monthly according to Cambodia Ministry of Tourism data from recent years.
March and April constitute the hot dry season. Temperatures in Phnom Penh and Siem Reap regularly exceed 35 degrees Celsius, with April peaks reaching 38-40 degrees. Kampot and coastal Sihanoukville measure slightly lower due to marine influence, typically 32-35 degrees. Humidity rises to 65-75 percent even without rain. Dust accumulates on roads and temple stones. Water levels in Tonle Sap Lake reach annual minimums, exposing broad mudflats and concentrating fish populations. Rural communities dependent on the lake ecosystem face food security pressure during these months. Electricity demand peaks due to air conditioning load, occasionally causing outages in provincial areas. Tourist numbers decline through April as heat intensifies. Khmer New Year falls in mid-April, typically April 14-16, based on the traditional solar calendar. Domestic travel surges during this three-day festival. Hotels in Siem Reap, Sihanoukville, and Kampot fill with Cambodian families. International visitor numbers drop as locals occupy tourism infrastructure.
May marks the wet season transition. Rainfall arrives irregularly. Phnom Penh may record 120-160 millimeters in May, but precipitation falls in intense afternoon thunderstorms rather than sustained periods. Temperatures remain high, 33-35 degrees, but humidity jumps to 75-85 percent. June through August form the established wet season. Monthly rainfall in Phnom Penh averages 150-170 millimeters. Siem Reap receives similar amounts. The Cardamom Mountains and Elephant Mountains record far higher totals, with some stations exceeding 400 millimeters monthly. The southwestern slopes facing the Gulf of Thailand receive the heaviest monsoon impact. Rain typically falls in late afternoon or evening bursts lasting one to three hours. Mornings often clear. Temperatures moderate slightly to 30-32 degrees due to cloud cover, but humidity remains 80-85 percent. Angkor Wat's moats and Bayon Temple's water channels fill. Vegetation greens across temple complexes. Moss grows rapidly on stone surfaces, requiring constant clearing by conservation teams from APSARA Authority.
The Tonle Sap system defines Cambodia's wet season hydrology. The Mekong River swells from Himalayan and Tibetan Plateau snowmelt combined with monsoon rains across its catchment. Flow volume increases force water backward up the Tonle Sap River, which reverses direction annually. This reversed flow refills Tonle Sap Lake, expanding it from approximately 2,500 square kilometers in April to 12,000-16,000 square kilometers by October. The lake depth increases from one to two meters in dry season to nine to fourteen meters at wet season peak. Flooded forests around the lake fringe become nursery habitat for fish species. This annual flood pulse supports Cambodia's inland fishery, one of the largest freshwater fisheries globally by tonnage. Fishing communities around Kampong Thom and Kampong Chhlang provinces relocate with water level changes, moving floating houses and fish traps as the lake expands and contracts.
September and October bring peak rainfall. Phnom Penh averages 230-260 millimeters in October, the wettest month. Flooding occurs in low-lying districts of the capital, particularly areas near the Bassac River confluence. Siem Reap town experiences similar flooding in certain neighborhoods. Rural roads become impassable without four-wheel drive vehicles. Some Angkor temple complexes see reduced accessibility. Beng Mealea, located 65 kilometers east of Siem Reap, becomes difficult to reach when laterite roads soften. Preah Vihear Temple on the Dângrêk Mountains escarpment remains accessible but approach roads deteriorate. Coastal areas including Sihanoukville and Kampot receive intense rainfall. Koh Rong Island and Koh Rong Samloem experience rough seas. Ferry services reduce frequency or suspend operations during storm periods. The Bokor Plateau receives heavy fog and rain, limiting visibility.
October also hosts Bon Om Touk, the Water Festival, celebrating the Tonle Sap River's flow reversal back toward the Mekong. The festival occurs on the full moon of the Buddhist month of Kadeuk, typically late October or early November. Phnom Penh hosts dragon boat races on the Tonle Sap River where it meets the Mekong. Teams from provinces across Cambodia compete. The festival spans three days. Over one million people converge on Phnom Penh according to estimates from recent festivals. Hotels fill completely. River viewing areas become densely crowded. In 2010, a stampede on a bridge connecting Diamond Island to the mainland killed 347 people, leading to enhanced crowd control measures in subsequent years.
Tourism patterns show clear seasonality. Cambodia Ministry of Tourism statistics indicate November through March accounts for approximately 60 percent of annual international arrivals. Siem Reap receives disproportionate visitor concentration, with Angkor Archaeological Park selling over two million tickets annually in recent pre-pandemic years. Daily visitor caps do not currently exist for Angkor Wat itself, though authorities have discussed implementation. December and January see the highest daily visitor counts, sometimes exceeding 10,000 people entering the park. Sunrise viewing at Angkor Wat draws crowds of 1,000-2,000 people on peak December and January mornings, concentrated at the reflecting pools west of the temple. February and March remain busy but slightly reduced. April sees declining numbers due to heat. May through October represents low season, with visitor numbers dropping 40-50 percent compared to peak months. Hotels and guesthouses in Siem Reap offer reduced rates during wet season, often 30-50 percent below high season prices.
Weather impacts vary by destination type. Angkor temples can be visited year-round. Rain during wet season falls predictably in afternoon, leaving mornings clear for temple exploration. Stone surfaces become slippery when wet, requiring caution on stairs at Angkor Wat's central towers, Phnom Bakheng, and Pre Rup. Ta Prohm's tree-root-entwined structures present particular slip hazards. Coastal areas see more significant wet season impacts. Sihanoukville beaches experience rough surf and limited sunshine May through October. Island resorts on Koh Rong reduce operations or close entirely during wettest months. Diving and snorkeling conditions deteriorate due to reduced visibility from sediment runoff. Ream National Park's mangrove boat tours continue but encounter choppy water.