Wet season in Cambodia
Cambodia's wet season comes courtesy of the southwest monsoon, which blows from May to October, bringing with it some three-quarters of Cambodia's annual rainfall. Not surprisingly, wet season is characterised by rain, and during the peak of wet season from July to September it can rain every day. Rainy days however tend to have a few hours of heavy rain rather than being all-day downpours, though the latter do sometimes occur -- you will get wet travelling in a Cambodian wet season.
Aside from getting drenched, the main disadvantage of travelling in wet season in Cambodia relates to flooding and degraded road conditions. The bulk of roads in Cambodia are dirt and in wet season they turn to heavily rutted and pot-holed mud pits. Travelling in rural areas, particularly the north and northeast of the country, can be slowed considerably. You will still be able to go just about anywhere, it will just take longer.
A secondary problem are bridges being out, but this is becoming less of an issue as the quality of bridgework is improving. Cambodia's arterial routes -- namely Phnom Penh to Siem Reap, Phnom Penh to Battambang and Phnom Penh to Sihanoukville -- are all-weather, sealed affairs and far less of a problem than the unsealed roads.
Tips for visiting Cambodia during wet season
On the upside, monsoonal Cambodia can be a beautiful country to travel in. The dust is all gone -- turned to mud unfortunately -- and the lush greenery of the country returns. Angkor Wat in particular can be stunning in wet season -- the birdlife is far more obvious and the ruins have a special appearance and feel. Observing Angkor Wat with a lightning storm as a backdrop is an electrifying experience (couldn't resist!). Far fewer travellers are in-country, too, so if you prefer to dodge the crowds, the monsoon 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.
Dry season in Cambodia
Cambodia's dry season 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 fast. While November and January are quite cool (high C20s) by April, the weather is scorching and oh-so-dry it will take your breath away. Characterised by heat and dust, this season coincides with Cambodia's peak tourist season -- travellers arrive in their droves between November and January to take advantage of the lack of rain and relative cool. By March, travelling can be uncomfortable and hot, while April can be excruciating.
As the country dries out, badly rutted roads get graded and trip times improve dramatically, though they get incredibly dusty. Cambodia's beach strips at Kep, Sihanoukville and Ko Kong bask in brilliant sunshine with clear calm waters -- if you're a beach bum, dry season is the season for you.