Layering during active outdoor pursuits like snowboarding allows you to customize your own "personal climate" to your exercise level and the conditions around you. You won't need as much insulation when you're working hard, but you'll need to be able to warm up if you stop moving, and outdoor temperatures may change by tens of degrees thanks to warming from the sun, temperature inversions and altitude changes as you go up or down the hill. You may also find yourself alternately subjected to rain, snow, sleet or bright sun. Dressing in layers allows you to remove or add layers as necessary, so that you are always comfortable no matter what the conditions.
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Select a base layer that fits snug to your skin and wicks moisture away to help keep you dry and comfortable, no matter what your exertion level. Most base layers come as separates -- shirts and pants -- and you should make sure to wear a base layer on both the top and bottom of your body. Some base layers have special features, like middle zips or a no-button fly, to facilitate bathroom breaks.
Put on one thick layer, or two thin layers, of insulating material over the base layer. Again, this should cover both tops and bottoms. Two of the best materials for this insulating layer are fleece and wool. You might also combine a thick fleece with a windproof vest as part of your middle layer.
Add a shell layer over the top of your other layers. Most outerwear will be made of tightly woven nylon or polyester and, to be suitable for snowboarding, should be both waterproof (to keep melting snow out) and breathable (to avoid keeping moisture and condensation from perspiration inside with you).
Repeat the layering procedure for your gloves if necessary. This is based on personal preference--some snowboarders will prefer to wear only their durable snowboarding gloves, while others may prefer wearing a thin, light liner glove inside the heavier gloves. Those venturing far into the backcountry may wish to carry overmitts that fit over the snowboarding gloves for extra insulation.
Avoid the temptation to layer your socks too thickly. A thin liner sock and an insulating sock is fine, and if there's extra room in your boot liners you can add an extra sock to take up the space. But if you add too many sock layers, you may squeeze your feet enough to reduce the circulation, which will actually leave you colder than if you hadn't added the socks.
Add a hat to your layering system. Again, the choice of how much to layer is based on personal preference; you might use a thin hat or helmet liner that fits underneath your snowboarding helmet, then add a thicker insulating hat of wool or fleece to keep your head warm when the helmet isn't on. Or, simply use the thick insulating hat on its own.