The climate in the Andes can basically be divided into two seasons, the wet season and the dry season.
The dry season is normally between May and September, with generally sunny days, warm evenings and often very cold nights.
The rainy season is from October to May, though rains usually come only between January and March. The Inca Trail is usually closed for these months due to the possibility of landslides or mudslides ("huaycos").
For information about temperatures, please, visit Cusco destination, clicking on the left bar.