Phnom Penh has a tropical climate with two clear seasons. The dry season runs from November to April, with cool nights and pleasant days in November and December (lows around 20°C, highs in the high 20s). March, April and early May are punishingly hot — daytime temperatures regularly exceed 35°C and the humidity is brutal.
The rainy season runs from May to October. Mornings are usually clear; afternoon thunderstorms arrive most days, dump rain for an hour or two, and clear off. The countryside is at its lushest, the Mekong runs high, and there are far fewer tourists.
The best window for a first visit is late November through February, when the weather is genuinely pleasant. The Water Festival (Bon Om Touk) in early-mid November is the most spectacular time of year to be in the city — three days of dragon-boat races, fireworks and crowds along the river — but accommodation gets very busy and prices spike. The Khmer New Year in mid-April is the other major holiday; the city empties as everyone heads home to the provinces, and many restaurants and shops close.