The best months to visit Aba Prefecture are May to June and September to October. Spring brings wildflowers to the plateau meadows and the rivers run full from snowmelt; the weather is mild and the tourist crowds at Jiuzhaigou have not yet peaked. Autumn — especially October — produces the most spectacular leaf colours at Jiuzhaigou and Huanglong, though it is also the busiest and most expensive period.
July and August are peak summer months when Han Chinese tourists arrive in large numbers. Expect long queues at Jiuzhaigou, higher prices, and congested roads. The weather is warm and generally clear, which makes the lakes at their bluest.
Winter (November to March) is cold and access roads can close after snowfall. Jiuzhaigou is partially frozen and very quiet; Huanglong closes entirely in winter. Temperatures in Aba town can drop well below zero. However, the monastery towns are at their most atmospheric in the cold season when tourist buses are absent.
Altitude is a factor year-round. Aba town sits at around 3,300 metres; Huanglong reaches 3,600 metres. Spend a night at lower altitude before heading onto the plateau if you are susceptible to altitude sickness.