The wet season comes courtesy of the southwest monsoon which blows from May to October, bringing with it some 75% of Cambodia's annual rainfall. Not surprisingly, the wet season is characterised by rain, and during the peak of wet season from July to September it can rain as much as two out of every three days. However, the rainy days are usually just a few hours of heavy downpour and not all-day rain, although the latter do occur.
From a more cheerful perspective, monsoonal Cambodia is also a beautiful country to travel around in. The roads are not dusty and the lush greenery of the country returns. Angkor Wat in particular can be stunning during the wet season -- the murals have a more unique appearance and feel. Observing Angkor Wat with a lightning storm as a backdrop is an electrifying experience. There are also fewer tourists going about in the country, so if you prefer to dodge the crowds, wet season can be a good time to visit.
Regionally, the Cardamom Mountains get the heaviest rain in the country, while the entire coastline gets rough seas and a lot of rain.
The dry period runs from October to April, when the dusty northeast monsoon arrives. Blowing like a hair-dryer set to high, the northeast monsoon dries out the country very quickly. While November and January are quite cool (high C20s) by April, the weather can be scorching and very dry. Characterised by heat and dust, this season coincides with Cambodia's peak tourist season when travellers arrive in their droves between November and January to take advantage of the lack of rain, enjoy the sun and the relatively cooler months.
Cambodia's beach strips at Kep, Sihanoukville and Ko Kong bask in brilliant sunshine with clear calm waters and if you're a beach person, dry season is the best time for you.