The final layer in the Three-layer System of hiking apparel is the Outer Shell. These are typically windbreakers and other such pullover jackets. Their main purpose is threefold. The outer shell serves to protect you from the elements such as high winds, rain, and snow. In addition, they vent moisture that can build up as you exert energy. The outer layer also takes the brunt of any scratching or scraping made by trees and brush you may encounter on your hike.