The primary purpose of the outer shell is to provide protection from the wind and rain. Outdoor clothing outer shells exist in many different forms from basic waterproofs and Soft Shells, to highly breathable, fully Waterproof Jackets.
For active use on the hill, an outer shell with a high level of breathability is likely to be required. High levels of waterproofness and breathability are available with products incorporating fabrics such as GORE-TEX and eVent.
GORE-TEX specifically is a porous fabric that contains billions of tiny holes. Each of these holes is too small for water molecules to pass through, but large enough for air molecules, creating a fully waterproof, highly breathable garment.
In addition to leading branded technologies like GORE-TEX and eVent, most brands also produce outdoor clothing outer shells using their own alternative brand fabrics. Brands such as Berghaus and The North Face produce outer shells using their own high performance, branded fabrics such as AQ, HyVent and HyVent Alpha.