Koh Kong in October

Dry season runs November through April, and that's your window. December through February brings the coolest temperatures - still hot by most standards, but bearable for hiking. March and April turn brutal, with temperatures hitting 38°C (100°F) regularly. But here's the trade-off: waterfalls run strongest during and just after rainy season. Ta Tai Waterfall becomes a thundering cascade in October, then shrinks to a trickle by March. Rainy season (May through October) makes many roads impassable. The Cardamom Mountains disappear behind walls of rain. Some eco-lodges close entirely. November offers the sweet spot - waterfalls still flowing, roads mostly dry, and temperatures dropping from unbearable to merely sweltering.