The key to ensuring you are properly prepared to layer effectively is to select an arsenal of garments that you can put on or take off as weather conditions dictate so that the body's core temperature remains essentially constant. Layering begins with the base layer -- the clothing directly against the skin. The next layers, as many as needed (within reason), are the insulation layers. And finally, the outermost layer is the protective layer, the barrier against moisture and wind.
Understand that in putting together the "ideal layering system," what you will be doing is creating a system that effectively combines elements of breathability, wicking (transporting moisture), rapid drying, insulation, durability, wind-resistance and water-repellence, without adding much weight or impeding freedom of movement, and all with just a few garments. The articles of clothing you choose on a particular day will depend on your intended aerobic level and the anticipated weather conditions.
A good salesperson at your local outdoor specialty store can be invaluable in helping to educate you regarding the choices you can make with the layering options your favorite store is able to provide you.
Understanding the layers
The layer against the skin is, essentially, a second skin that keeps the wearer dry, warm and comfortable. This layer is the workhorse in a moisture-management system, moving moisture through it all day long. It is the first layer you will put on and the last layer you will take off, and unless you are going to run around naked, this is pretty much a non-optional layer.
Thick or thin, special weaves or not, merino wool or synthetic, the primary intent of a base layer is to manage moisture (wick or transport perspiration away from the skin) while adding a degree of insulation. How much insulation you require in this layer will depend on how cold the environment will be coupled with your anticipated level of activity.
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