Sapa has a mountain climate that is substantially cooler and wetter than the Vietnamese lowlands. The key seasons to plan around:
September to November is the best time. The rice harvest turns the terraces gold in September and October — this is when the landscape is at its most photogenic. November brings cooler, drier days and clear visibility. This is the peak season for trekking and photography.
March to May brings spring flowers — cherry blossom, peach blossom, and wildflowers across the higher slopes — and warming temperatures. Cloud is common but the light can be beautiful. This is the second-best period.
June to August is the rainy season. The paddies are lush and brilliantly green, but rain can make trekking muddy and unpleasant, and leeches are active. Views are often obscured. Still viable but bring waterproofs.
December to February is cold and often foggy. Temperatures can drop below freezing, and snow occasionally falls on Fansipan. The cloud can sit in the valley for days at a stretch. The mist has a ghostly beauty but visibility is poor and trekking is limited. Bring warm layers — far more than you'd think necessary for Vietnam.