The cool season (November through February) is the best time to visit Mondulkiri. Temperatures in Sen Monorom stay in the mid-twenties during the day and drop to 12–15°C at night — genuinely cool by Cambodian standards and a relief from the heat of the lowlands. The roads are dry, the forest trekking is comfortable, and the landscape, while less lush than in the rains, has a clear, sharp quality.
The wet season (May–October) brings heavy rains, forest paths that turn to mud, and roads that can become impassable in the more remote areas. On the positive side, the waterfalls are dramatically fuller and the landscape intensely green. Bou Sra in particular is best in the wet season when the volume of water is highest. October and November, as the rains ease, combines reasonable roads with lush landscape and thinner crowds.
Avoid the hottest months (March–April) if possible; even at altitude, temperatures reach 35°C and the landscape is parched.