Why Wear Layers? Three Layered Clothing Systems

Why Layer?

Everyone knows to wear layers, but no one really says why. It is not "warmer" as is oft quoted by nervous parents who are sending their kids out in the snow. Here are the real reasons:

• Avoiding sweat is the primary reason for layered clothing systems. The purpose of a layered clothing system is so you can REMOVE ITEMS as you get warmer and ENSURE YOU DON'T GET WET from sweat. Once you are wet, you lose a lot of control about how warm or cold you are going to be.
• Layers allow you to adapting your clothing to the ambient temperatures of the day or location. If you are in and out of shade, or wind, or find your self at low or high altitudes—or the weather changes during the day—then layered clothing allows you to adapt.
• Layers can be more easily specialized to the nature of your work or environment. Mechanics wear coveralls for good reason: this sacrificial layer can get dirty without worry—and Knights wear plate armor to protect from pointy objects—but both mechanics and knights wear underwear (we hope).  You get the point.

If you're going to be in the outdoors: NO COTTON.  It's no joke: Cotton gets wet and doesn't dry out--and that's a dumb way to die.  Synthetic materials (polypro, nylon, etc.), Merino wool, and down are your only choices in the winter.

Three Layers of Clothing

I find it helpful to always think of clothing as a three layer system:

1. Baselayer. This layer of clothing is close—sometimes tight—to your skin, and it is generally just a single layer, though some folks will double up in the cold. The purpose of this layer is to move moisture (sweat) away from your body so that it can evaporate. The purpose of keeping your skin dry is so that you retain control over the forces of evaporation and conduction: evaporation cools you (think of wind on wet skin) and conduction does too. Obviously this layer should also be comfortable.  Wool or synthetics are best—and don't forget your underwear.
2. Insulating Layers.  Zero or more layers of clothing are added to insulate your body. These layers should be BREATHABLE if you are moving around and generating heat: Wool or fleece is ideal.  Down is a very good choice for insulating layers if you are not moving around and are in no danger of sweating (since damp or wet down doesn't insulate). Outer garments in this system should be easily and quickly removable so you can adjust your insulation levels to whatever activity you are doing; if they are not easy to remove, you won't bother, and then you'll get soaked with sweat.
3. Protective Layers. This outer layer protects you from the weather: primarily precipitation (wet again!) andheat loss from wind (convection), but you should also worry about abrasion (from tree branches, ski lifts, tools you're using, backpacks, etc.). A windproof and (truly) waterproof shell is often a good choice here--but make sure it's tough enough to handle whatever environment you're in.

Be careful of clothing that tries to combine these layers into a single garment since that will reduce the effectiveness of the layers above and below and limits the amount of control you can exercise. For example, if you wear a silnylon down jacket as an insulating layer, that will reduce the breathability of the layers below (versus a fleece or sweater).Remember that this layering system applies to your head, feet, and hands too: use a base layer (wicking beanie, liner socks, and liner gloves), an insulator (fleece beanie, fleece gloves, and wool socks) and a protective layer (tough wool hat, waterproof boots, and over-mittens).Don't forget glasses, sunglasses or goggles to protect from debris, UV, snow-blindness, frostbite, paparazzi, etc..