The three-layer-rule provides optimal running conditions for every runner in unpredictable weather, especially in the cold and rain. The combination of the 3 layers protects from any weather and improves your performance in the process. Layer 1:
The tight fitting base layer is the functional layer that directs the sweat away from the body, so you stay dry and don't cool down. Layer 2:
The mid-layer insulates to protect against the cold. This could be a hoodie or a fleece that leaves enough room between layers 1 and 2 for insulation. Layer 3:
The water and windproof shell layer protects against anything mother nature wants to throw your way. Make sure the material is nice and breathable so the extra warmth has somewhere to go.