Cambodia is blessed with one of Asia's simpler weather systems and despite having two distinct weather seasons you can travel in Cambodia all-year-round. In general, the entire country is subject to the same weather patterns, mainly due to the relatively uniform altitude and latitude throughout Cambodia.
There are two distinct seasons – dry (October to late April) and wet (May to late September). Within each season there are variations in temperature, with the final few dry months leading up to the wet season (March and April) and the early months of the wet season (May and June) usually being the hottest of the year with temperatures in excess of 35°C at times.
Humidity is at its height during March and April whilst the coolest months of the year tend to between October and December, however this is cool for Cambodia but far from chilly (avg temperatures 24°C - 26°C).
The Dry Season
October - April/Early May
Cambodia's dry season lasts from October to April, when the dry north-east monsoon arrives, characterised by hot wind blowing across the entire country. Whilst November to January are quite cool (high 20°C's) by April the weather is scorching making early morning and late afternoon Angkor Temple tours, with a few hours by the hotel pool at lunchtime, the preference for many.
Thanks to the hot weather this is unsurprisingly the season when Cambodia's tourist numbers peak.
In more remote parts, the roads are at their best and journey times are shorter because of this. Kep and Sihanoukville on the south coast are popular during this season as they bask in the brilliant sunshine and sea conditions are very favourable.
The Wet Season
Early/Mid May - October
Cambodia's wet season comes courtesy of the southwest monsoon and lasts from May to October, bringing with it almost 75% of Cambodia's annual rainfall. Across Cambodia, throughout much of the rainy season, daytime temperatures average between 25°C and 27°C. The early months of the wet season (May – July) remain very hot with infrequent rainfall usually in the form of short downpours. In the latter months (late July – September) the rains tend becomes more constant and is heavy at times, especially in coastal and rural regions.
Travel in the more remote corners of the country is almost impossible due to the state of the roads and journeys into the north east are inadvisable during the peak wet season because of this. There is also very limited access to Bamboo Island (near Kep) due to high seas.
On the upside, traveling in monsoonal Cambodia does have its advantages. The dust is gone and the lush greenery of the countryside returns. Angkor Wat in particular is stunning in the wet season, with moats brimming with water and a severe drop in visitor numbers.