The best seasons are spring (April–May) and autumn (September–October). Summers in Lanzhou are hot but drier than much of China — the city sits at 1,500 metres and the altitude moderates the temperature somewhat. July and August can reach 35°C. Winter is cold (below freezing at night) and occasionally dusty from the loess plateau's famous sandstorms, though it is not bitterly cold by northern Chinese standards.
If you are continuing west into Xinjiang, the overland route through Zhangye and Dunhuang is most comfortable from May to October. For the Bingling Si day trip, avoid the winter months — water levels on the reservoir can affect boat access, and it is cold enough to make the visit less enjoyable.