Puno's altitude — 3,830 metres — overrides Peru's general climate story. The seasons here are dry and wet rather than warm and cool. The dry season (May to October) is the peak time to visit: cold, brilliantly clear, with deep blue skies and crisp views across the lake. Days are mild, around 14 to 18°C, but nights can drop well below freezing.
The wet season (November to April) brings heavy afternoon downpours, slippery streets, and far fewer tourists, but mornings are often clear. The single best fixed date in the calendar is the Fiesta de la Virgen de la Candelaria in the first two weeks of February — among the largest religious festivals in the Americas, with hundreds of dance troupes and tens of thousands of pilgrims. Book hotels months in advance for the festival.
At any time of year, the altitude is a real consideration. Allow at least 24 hours to acclimatise before any strenuous activity, drink coca tea liberally, and ideally come from another high-altitude city (Cuzco, Arequipa) rather than directly from the coast.