Picking the best time to visit Peru is complicated by the country’s physical characteristics; temperatures can vary hugely across the country. Summer along the desert coast more or less fits the expected image of the southern hemisphere – extremely hot and sunny between December and March (especially in the north), cooler and with a frequent hazy mist between April and November – although only in the polluted environs of Lima does the coastal winter ever get cold enough to necessitate a sweater. Swimming is possible all year round, though the water itself (thanks to the Humboldt Current) is cool-to-cold at the best of times. To swim or surf for any length of time you’d need to follow local custom and wear a wetsuit. Apart from the occasional shower over Lima it hardly ever rains in the desert. The freak exception, every ten years or so, is when the shift in ocean currents of El Niño causes torrential downpours, devastating crops, roads and communities all down the coast. The last really heavy one was in 1983, though there have been several El Niños since then.
In the Andes, the seasons are more clearly marked, with heavy rains from December to March and a warm, relatively dry period from June to September. Inevitably, though, there are always some sunny weeks in the rainy season and wet ones in the dry. A similar pattern dominates the jungle, though rainfall here is heavier and more frequent, and it’s hot and humid all year round.
Taking all of this into account, the best time to visit the coast is around January while it’s hot, and the mountains and jungle are at their best after the rains, from May until September. Since this is unlikely to be possible on a single trip there’s little point in worrying about it – the country’s attractions are invariably enough to override the need for guarantees of good weather.