What is layering?
Dressing in layers is the most effective way to stay warm and dry in the outdoors. This simple concept allows you to make quick adjustments to regulate your warmth based on your activity level and changes in weather.
Each layer has a very specific function. Your base layer (the layer closest to your skin) regulates moisture, your mid layer (insulating layer) protects you from the cold, and your outer layer (shell layer) protects you from wind, snow, and rain.
Layer #1: Base Layer
The main purpose of a base layer is to help get sweat off your skin (regulate your temperature by transporting sweat from your skin to the outer surface of the fabric). While cotton may be incredibly comfortable when it's dry, it won't keep you warm in cold weather when it's wet, as it will completely saturate. More technical fabrics from companies such as Arc'teryx, Helly Hansen, Salomon, and SmartWool are now available that are far more effective at moving sweat away from your skin to a place where it can evaporate.
Layer #2: Mid Layer
The mid layer, or insulating layer, keeps you warm by trapping air close to your body. Natural materials such as Merino wool and goose down are fantastic insulators with excellent warmth-to-weight ratios (they pack down well). Merino wool offers soft, reliable warmth, even when wet, but Goose down is nearly useless as an insulator when it gets wet.
Synthetic fabrics such as Thinsulate and Polartec are light, breathable, and insulate even when wet. Their warmth-to-weight ratio isn't quite as high as with down, but it's better than classic fleece (quite bulky).
Layer #3: Outer layer
The outer layer, or shell layer, is for stopping wind, rain, and snow. With shells, you need to decide on how waterproof/windproof the item needs to be versus how breathable. While a large plastic bag would keep you very dry from the outside, you would quickly become soaking wet from the inside due to sweat not being able to go anywhere. Laminated membranes such as Gore-tex however, allows sweat to escape by letting it evaporate while at the same time stopping 100% of precipitation from entering from the outside. For more strenuous activities, you can choose outer layers that are more breathable (but water-resistance will be reduced).