Like all tropical countries, Cambodia has two seasons: Dry and rainy.
The dry season lasts from November to April, and the rainy season from May to October. The first is very dry, with a temperature of around 25 to 30° Celsius. The rainy season is hot, with some days approaching 40° Celsius, and also wet, with short but heavy down pours almost every day. Some localized flooding can occur.
The months of April and May are oppressive, so it is not a recommended time to visit for those that cannot deal with excessively hot days. The tourist high season coincides with the most friendly weather from November to March, because it is not too hot. The landscapes are splendid just after the rainy season, because everything is green.
However, for those wishing to stretch their travel dollars further, then a visit to Cambodia in the low seasons will allow you to obtain some great discounts on accommodation. If you plan your day to get your sight-seeing and other activities done early or late in the day, and relax in the middle of the day at your hotel and guesthouse, then you can avoid the heat of the middle of the day.
Hanging out by the swimming pool through the middle of the day is a great way to relax. If your hotel or guesthouse does not have a pool, remember that many hotels will allow you to use their pool for the day for a small fee, which often includes a complimentary drink.
It should be remembered that even if there are two seasons, it is hot all year round in Cambodia, as compared to many western countries! The temperature rarely falls below 20°C, except in the north of Cambodia, where the temperatures are cool during the night. Bring a hat and sunscreen and remember to drink lots of water.