October through April is the dry season and the best time for trekking: trails are dry, river crossings manageable, and the mountain views clear. November through February brings cooler temperatures (evenings can drop below 10°C) that make walking comfortable. March and April are hot but remain relatively dry.
The rainy season (May–September) turns the trekking trails to mud and makes some river crossings impassable or dangerous. Kayaking on the Nam Tha is best during the wet season when water levels are adequate; in the dry season the river can be too low for comfortable paddling. The forest is lush and the landscape most dramatic in the rains.
The Laos-China Railway has changed the seasonal dynamics somewhat: Luang Namtha is now year-round accessible from the rail network, so pre-travel day can be removed from the itinerary calculation.