Peru is divided up into three distinct geographical regions: the mountain highlands, the jungle and the coast.

The Mountain Highlands (Cusco, Lake Titicaca, etc).

Mid April - October.
This period is the dry season, with hot, dry days and cold, dry nights, often hovering just above freezing, particularly in June and July. May is perhaps the best month with the countryside exceptionally lush, yet with superb views and fine weather. You'll find the flowers in full bloom, the grass green and the streams full. Peru's summer is from June to August which coincides with the dry season and summer holidays in North America and Europe. You'll find Cusco a pretty cosmopolitan city with tourists from all over the globe converging on Machu Picchu.

November - Mid April
This is the wet season with most rain in January and February. It's usually clear and dry most mornings with outbursts of heavy rain in the afternoons. The daily temperatures are typically mild with only a small drop at night. 

The Coast (Lima, Nasca, Arequipa).

December - April
This is summertime on the coast where the weather is hot and dry and ideal for swimming and getting a tan. Temperatures on average range from 25 - 35°C. There is little or no rain during these months. The beaches around Lima and the North can become packed during the months of January and February which coincide with school holidays.

May - November
From May to November the temperature drops a bit and you'll find blankets of sea mist engulfing the coast from the south right up to about 200 km north of Lima. At this time of year only the northern beaches such as Mancora and Punta Sal are warm enough to provide pleasant swimming.

The Jungle

April - October
This is the 'dry' season with daily temperatures averaging 30-35°C. However cold fronts from the South Atlantic are common when the temperatures can drop to 15°C during the day and 13°C at night.
The dry season is the best time to visit the jungle regions . there are fewer mosquitoes and the rivers are low, exposing the beaches. It's also a good time to see nesting and to view the animals at close range, as they stay close to the rivers and are more easily seen.

November - March
This is the wet season, hot and humid, when you can expect heavy rain at anytime. It only rains for a few hours at a time, so it's not enough to spoil your trip. Wellington boots are a must though, as some of the jungle trails can become small rivers.

Average Temperatures and Rainfall