Peru has three main climatic zones: the tropical Amazon jungle to the east; the arid coastal desert to the west; and the Andean mountains and highlands in the middle of the country. In the Andes, which have altitudes over 3500m, average daily temperatures fall below 10°C (50°F) and overnight temperatures can dip well below freezing. Travelers flying straight into Cuzco (3326m) should allow time to acclimatize.
From June to August is the dry season in the mountains and altiplano (Andean plateau); the wettest months are from December to March. It rains all the time in the hot and humid rainforest, but the driest months there are from June to September. However, even during the wettest months from December to May, it rarely rains for more than a few hours at a time. Along the arid coastal strip, the hot months are from December through March. Some parts of the coastal strip see rain rarely, if at all. From April to November, Lima and other areas by the Pacific Ocean are enclosed in garúa (coastal fog, mist or drizzle) as warmer air masses off the desert drift over the ocean where the cold Humboldt Current hits.
The El Niño effect, which occurs on average every seven years, is when large-scale changes in ocean currents and rising sea-surface water temperatures bring heavy rains and floods to coastal areas, plunging tropical areas into drought and disrupting weather patterns worldwide. The name El Niño (literally ‘the Child’) refers to the fact that this phenomenon usually appears around Christmas. The El Niño in the winter of 1997–98 was particularly traumatic for Peru. El Niño is usually followed the next year by La Niña, when ocean currents that cool abnormally create even more havoc and destruction.
Peru’s climate has two main seasons – wet and dry – though the weather varies greatly depending on the geographical region. Temperature is mostly influenced by elevation: the higher you climb, the cooler it becomes.
The peak tourist season is from June to August, which coincides with the cooler dry season in the Andean highlands and summer vacation in North America and Europe. This is the best (and busiest) time to go trekking on the Inca Trail to Machu Picchu, or climbing, hiking and mountain biking elsewhere.
People can and do visit the highlands year-round, though the wettest months of December to March make it a wet and muddy proposition. Many of the major fiestas, such as La Virgen de la Candelaria, Carnaval and Semana Santa, occur in the wettest months and continue undiminished even during heavy rainstorms.
On the arid coast, Peruvians visit the beaches during the most hot and humid time of the year, from late December through March. In central and southern Peru, the coast is cloaked in garúa (coastal fog) for the rest of the year. Although the southern beaches are deserted then, the coastal cities can be visited at any time. In the north, the coast usually sees more sun, so beach lovers can hang out there year-round.
In the eastern rainforest, of course, it rains. The wettest months are December through May, but even then it rarely rains for more than a few hours at a time, so there’s still plenty of sunshine to enjoy. Follow the locals’ example: briefly take cover during the heaviest downpours. It’s not a big deal.