The weather is another item to consider while planning your trip. As one of my Cambodian friend said, there are three seasons in Cambodia - hot, hotter, hottest ;-) But it is not quite so.
There are two distinctive seasons in Cambodia – dry (around October to late May) and wet (starting from June to mid October). Within each season there are variations in temperature, with the final few dry months leading up to the wet season (March and May) and the early months of the wet season (June and July) usually being the hottest of the year with temperatures in excess of 35+°C at times.
Humidity in Cambodia is at its height during March and May whilst the coolest months of the year tend to be between October and December. However, this is cool for Cambodia but far from chilly (average temperatures 24°C - 26°C).The dry season lasts from October to May, when the dry northeast monsoon arrives, and is characterized by hot wind blowing across the entire country. The period from November to January is quite cool (high 20°C's) at night and early morning and it is the best time for the temples visit in the morning and heading back to the hotel for lunch and to relax by the pool and starting exploring the sites again in the late afternoon. Many rivers have low water level making them
From January to May, 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 for sea side activities, scuba diving, snorkelling etc.
During the wet season the southwest monsoon blows to the northeast across the country and lasts from June to October, bringing with it 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 tends to become more constant and is heavy at times, especially in coastal and rural regions. Many roads become impassable.
The seasons have an important impact on the activities you expect to engage in and need to be taken into account while planning your trip. You may wish to check out this link: Weather to Travel