The country is warm all year round, though there are several distinct seasons affecting the best time to visit Cambodia. There is little rain between November and May, the so-called dry season, which itself divides into two distinct phases.
The cool season (Nov–Feb) is the peak time for tourism – mild enough to explore the temples in comfort but warm enough to sunbathe by the coast. Humidity and temperatures rise slightly during the hot season (March–May), with Phnom Penh and Battambang seeing peak daytime temperatures of 33–35°C. This is an excellent time to visit the coast, although Angkor is usually bakingly hot.
Visiting Cambodia during the rainy season (roughly June–Oct) can present certain practical challenges, but it is also a fascinating time to see the country as it transforms into a waterlogged expanse of tropical green under the daily monsoon deluges (fortunately, the rains falls mainly in the afternoon; mornings are generally dry). Getting around (particularly in September and October) isn’t always easy: dirt roads turn to mud and flooding is commonplace. Not surprisingly it’s also the quietest time for tourism (even Angkor is relatively quiet) and the countryside is at its lushest.