The famous Inca Trail treks are part of an extended net-work sistem. Located in the Southern Peruvian Andes in Cusco region. So the usual year-round weather on the Inca Trail is clearly marked into two big seasons: dry and wet. From May to October is usually the dry season with a lower chances of getting rainy days, mostly sunny and hot. But cold at night especially in winter time. From November to April is the wet season, you can expect mainly heave rainy days, thunder storms, cloudy and usually there is less visibility on the mountains.
For the classic Inca Trail hikers should bring a good four season sleeping bag or you can hire it here in Cusco in our office. Also a change of clothes for sunny days and a good rain gear. The area for the 2 day inca trail is in the sub-tropil cloud forest so the weather is warmer and humid. Only at Machu Picchu you can expect very changeable weather, usually unpredictable. But still nice and sunny days at winter season (June to August).
If you are hiking the Short Inca Trail to Machu Picchu sleeping bags are not necessary to bring. Because you will stay at the hotel in Aguas Calientes. But rain gear is very essential and especially in the wet season (November to April). Hiking the short Inca trail is the way to see Machu Picchu from the Sun gate.