The peak season for travel in Peru is between April and October. Although these months are winter south of the equator, it is locally considered summer in the Andes of Peru because it is the dry, sunny season, and the best time of year for travel and trekking. From mid-November through mid-March you can expect cloudy days and some rain, but it is nonetheless suitable to travel in Peru any time of year for non-trekking trips.
Climate is also determined by three distinct geographical regions. The Peruvian coast is warm and arid with temperatures ranging from 65° to 85° F. In the Andes temperatures range from 30°-40° F at night to 60- 75° F during the day. Trekkers approaching higher altitudes need to be prepared for temperatures near or below freezing at night. In the Andes, April and May have spring-like weather, with fewer tourists, wildflowers and vibrant green vegetation, and local crop harvests.
The lowland Amazon rain forest regions of Tambopata and Manu in Peru are far enough south of the equator that you can enjoy a somewhat cooler and drier climate compared to very hot regions of the Amazon like Iquitos, Peru. Do expect temperatures to be in the 80's to 90's F., and fairly high humidity. Occasionally, cold fronts from the south sweep in to southwestern Amazonia, pushing daytime highs down to 50° F and nighttime lows into the 40's. Of course, always be prepared for rain any time of year in the Amazon. From November through April rain can continue for hours or occasionally even days. Around 80% of the annual rainfall occurs during this season.