In Cambodia, it is warm all year round, even though there are several distinct periods affecting the best time to visit Cambodia. The climate of Cambodia is characterized by the presence of two very influential monsoons which alternately mark the seasons. From May until October, the monsoon from the southwest becomes dominant.
The effects of the monsoon are felt by the presence of heavy rains and strong winds. The humidity is very active during this period. In contrast, there is the northeast monsoon is blowing from November to March and brings very little rainfall.
Taking into account the weather, the months of December and January are the best times to travel to Cambodia because during this period, rains are rare and humidity is somewhat cumbersome. Starting from February, the heat and humidity become unbearable, but the best attractions are less crowded and you'll have time to admire them.
Located in the south of the country, the city is built at the confluence of four rivers, and experiences a tropical climate with a rainy season from May to October and a slightly cooler dry season from November to April. The rainy season brings short, sharp afternoon showers to Phnom Penh, which often result in flash floods, and the wind is stronger, the rainy season is a good time to visit Phnom Penh because there will be far fewer tourists around and the rain showers are mostly confined to the afternoons, so you can plan around them. The dry season is when Phnom Penh experiences the least rainfall, and can be split into two sections – the cool season, which runs from November to January, and the hot season, which runs from February to May. Temperatures in the cool season range from around 21 to 32°C (70 to 90°F), while the hot season sees temperatures soaring to overbearing levels, reaching as high as 40°C (104°F) – which feels worse because humidity is also high at this time of year. The most popular (and so the most crowded) time to visit Phnom Penh is in December and January, when temperatures are at their most pleasant and rainfall is at its lowest.
Cambodia can be visited at any time of year, because even during the rainy season the weather in Siem Reap and other parts of Cambodia is never so bad that it makes it impossible to visit. Whatever the weather in Siem Reap is like, you can always take a tour around the Angkor temples. The best time to visit Siem Reap is January for most people as this is one of the coolest months of the year and it is also dry season. During January the skies are clear most days so it perfect for sunrises and sunsets.
At the end of January, Sihanoukville begins to warm and continues to get hotter through July (maximum 35?). After February, cool 'mango showers' occasionally blow in from the north. December through June is said by some local outfitters to be the best scuba weather with clearer (though cooler) waters than the rest of the year.