The only thing predictable about Peru's weather is its unpredictability. In fact, it's not uncommon to experience all four seasons in a single day. Generally speaking, Peru has two seasons, wet and dry, but in a country as geographically diverse as Peru, local weather patterns vary greatly.
In the highlands, the dry season is between June and October, but even the mountains' wet season isn't always really wet and the dry season sees its share of downpours. Temperatures during the day in the dry season can get hot making shorts rather inviting. At night, temperatures can drop quickly and a warm sweater makes walking around the plazas comfortable.
The Amazon experiences...you guessed it, rain heat and more rain and heat! The average daytime temperature is 30-32° C (85-90° F), while nights can cool down to 20° C (68° F). They do have a wet and wetter season with the wet (dry) season corresponding with the dry season in the highlands. The desert coast is arid. From January to March the sky is clear and the air tends to be hot and sticky. Peruvians go to the beach at this time. The rest of the year the gray coastal mist known as guara moves in and the sun is rarely seen.