Many people are surprised to learn that Machu Picchu falls into a subtropical climate category. It is filled with orchids, bromeliads and some pesky insects. Also known as the Cloud Forest, Machu Picchu is surrounded by higher mountain peaks, frequently shrouded in mist. This is perhaps one of the reasons that the Spanish never discovered it. It is considerably lower altitude (hotter and more humid) than Cusco and many of the other treks.
The weather changes often in Machu Picchu. Typically, during the dry season (April-November) the early mornings are foggy, and can be chilly. In the late mornings, the clouds clear and the sun comes out, so it can get quite hot. Layers that you can peel off or add on, are best. Sun protection, insect repellent and a good hat are essentials. However, please keep in mind that sometimes there is still rain in June and sunshine in December. A rain poncho is highly recommended, no matter what season you visit. When it rains, the ancient stones can become slippery, so having shoes with a good tread is helpful, and perhaps, a rubber-tipped walking stick.
The rainy season is from mid-November to mid-April, with February being the wettest month. The Inca Trail is closed during February, although the Machu Picchu ruins are still open.
Of course, weather is unpredictable. No matter what month you are in Machu Picchu, please make sure that you have rain gear, including a waterproof jacket, pants, poncho and waterproof gloves.