Cambodia: When to Go
Cambodia is a tropical country, meaning it is warm all year, but there are still 3 distinct seasons:
Dry season: November-February
The best time to visit, with temperatures in the mid to high 20's celsius (but be prepared for the occasional chillier night). It is also the most popular season, and the most expensive - Christmas and New Year, particularly.
Hot season: March-May
Humidity and temperatures rise steadily, peaking in the mid 30's in Phnom Penh, which makes the middle of the day an uncomfortable time for doing anything active. Be prepared for lots of dust, too, which all turns to mud in...
Rainy season: June-October
The southwest monsoon from the Gulf of Thailand drenches the whole of Cambodia. Dirt roads can become impassable and some areas flood, especially in September-October. But in Angkor the mornings are usually dry and you can have the temples to yourself, if you don't mind holing up in the afternoon, and being flexible on overland travel (or just using flights).