Cambodia has two seasons, affected by the monsoon winds. The northeastern monsoon blowing toward the coast ushers in the cool, dry season in November, which lasts through February, with temperatures between 65°F (18°C) and 80°F (27°C). December and January are the coolest months. It heats up to around 95°F (35°C) and higher in March and April, when the southwestern monsoon blows inland from the Gulf of Thailand, bringing downpours that last an hour or more most days. This rainy, humid season runs through October, with temperatures ranging from 80°F (27°C) to 95°F (35°C). The climate in Phnom Penh is always very humid. Thanks to climate change, Cambodia now experiences rainstorms in the dry season, cool temps in the hot season, and a lot of unpredictability. Bring your umbrella, although higher-end resorts usually offer one along with your bathrobe and slippers.
It's important to book in advance if you plan on visiting during mid-April's New Year celebrations or for the Water Festival in Phnom Penh in November. Strangely, the New Year is one of the best times to see the capital—at least in terms of lower rates and crowds—because the majority of Phnom Penh residents come from somewhere else and they all go home for the holidays.