Cusco has a definite wet and dry season: however the months in which these seasons start and end seem to vary considerably from year to year.

November through May is the rainy season in Peru. Typically, Cusco and Machu Picchu receive heavy afternoon thunder showers throughout this period, although extended periods of drier weather can occur. During this season, the valleys will be green and lush--and sometimes muddy. At high elevations, temperatures may fall below freezing, especially at night, and there may be occasional snow or hail. February and March are often times of extremely heavy rains, flooding, and corresponding transportation delays. However, the rainy season is also the time when the number of tourists is at its lowest. 

Peru Cusco

For the remainder of the year, the weather is much drier and cooler; the number of tourists increases significantly, the landscape tends to be drier and browner, and the river levels are lower. Frost can be expected in the evenings.