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Momma always told you to layer your clothing to stay warm just like her momma told her. The mystery is: what the heck is that layering thing?  Is it as simple as a tee-shirt and a jacket? Does it matter what kind of clothing it is? Yes and yes. Layering is all about flexibility.  Remove clothes when you are hot, adding when you are cold. But, beyond the basics, it is critical when trying to stay warm in very cold temperatures. Here’s why.
Your body loses heat 4 ways.  Hard to believe isn’t it? Some heat is lost through Convection or the transfer of heat through flowing liquid or air. One molecule picks up the heat from you and then goes away. The next molecule picks up some more heat and goes away, etc. Nature hates cold, so heat always tries to go to the cold place to warm it up. Air and fluid are very effective at allowing heat to travel. This is one reason why it is so dangerous to be wet in cold weather or in water that is colder than your body.  The heat flows from your body trying to heat up the water. However, an air space that can’t get out of your clothing is VERY effective at insulating your body.
Radiant heat is transfer by light, primarily by invisible infrared light. The sun can warm you even without any air present.
Conductive heat loss happens through solids or liquids, like leaning against a cold object. This is when the heat passes from one molecule to the next and without moving; it passes that heat on to another molecule.
 
Evaporative heat loss (which is a form of convection) happens when water steals the heat in order to become a vapor. Since liquid flows fast and easy, the heat loss can be extreme. The temperature of the part in contact with water can drop below the air temperature, which makes you feel cold.
How to stop Convection.  Use a tight layer against your skin to prevent movement of air.  Multiple layers of any type of clothing that traps air will perform nicely. Have an outer shell that is an efficient windbreaker. Many garments use “CFM” to quantify the “windproofness” of the material. This number is the cubic feet of air movement. A “0” number is totally windproof.  No air can penetrate.  FLEECE is 200cfm while ‘Windstopper’ fabric is 0cfm.
Air layers stop convection. So, all types of insulation are convection-stoppers. Some are more efficient than others. Down is an excellent choice for trapping air.  The “loftier” the down, the more air gets trapped and the more insulating power it has.  Down is one of the best layering pieces, especially INSIDE, under your waterproof jacket. Down, however, becomes useless if it gets soaking wet, so many people who may be exposed to soaking rain use synthetic insulation.
Stop Conductive heat loss. Don’t lean or sit on cold objects.  Put foam covers over your metal-framed glasses so the sides do not touch your face. Leave all metal jewelry at home. Don’t sit on the tailgate of your truck or the metal rack of your ATV.  When you sit or lie on your insulation layers, you compress them, which severely affects their ability to prevent convective heat loss. This is why you must use a sleeping pad under your sleeping bag because you have compressed all the insulation by lying on it.  The foam sleeping pad then provides insulation from the cold ground. Foam seat pads (or heated ones) do the trick to keep the cold metal of a tree stand or stool from robbing your heat.  Gloves are key here since your hands are the part that touches most cold surfaces when you are outside.
To kill Evaporative heat loss, you must control where your perspiration goes.  This is very difficult. You do not want moisture on the surface of your skin because the entire outer surface will then become cold.  You must allow your sweat to wick through the layers to the surface in a slow controlled fashion. The trick is having a waterproof layer on the outside to keep water out while letting the water that is in evaporate to the outside. Newer high-tech fabrics with special coatings actually help this process. They contain small enough “holes” in the fabric that allow sweat VAPOR out but keep water drops from getting in.
Encouraging radiant heat exchange.  Sit in the sun!
DRESSING IN LAYERS
Use four main layers: SKIN layer, Middle layer, Outer layer, and the Shell layer..
The skin layer needs to wick moisture away from your skin and must feel soft and comfortable. This top and bottom layer is the most critical component of a layering system. It is also the layer most people don’t own and must purchase before a trip. Long underwear serves two purposes: first, to trap a layer of warm air next to your skin; second, to wick away the perspiration your body produces during activities. Polypropylene or name brand polyester-based underwear is required for any trip. Do not buy cotton - it must be synthetic. These modern synthetics are nonabsorbent, so they stay dry next to your skin. And they do a superior job of wicking away moisture before it can soak and chill you. Keep to light or "mid" weight for active use and "expedition" weight for more passive activities or severe conditions. Silk is a natural alternative to synthetics. Silk will not absorb moisture and allows other layers to easily slide over it.
The middle layer provides insulation. This layer affords you the most versatility as your activity intensifies or temps fluctuate. Acceptable fabrics for this layer include wool or synthetics. Fleece or "Polartec" is by far the best performing mid layer you can buy. Fleece comes in many different styles and is rated in accordance to how densely knit the fibers are. Microfleece is tightly knit and offers good wind protection and warmth. If you are looking for more insulation, fleece with a rating of 300+ offers greater density thus more air space to capture heat. Higher rated fleece is not as windproof as microfleece. Other fabrics include wool or acrylic sweaters, heavy weight wool jackets or button up shirts. Wool sweaters are a good middle layer.
 
The outer layer is focused less on warmth and more on staying dry. It is the outer layer that protects insulating layers from wind & water - the 2 prime culprits in convective and conductive heat loss. This is where Gore-Tex and its variations come into play. GORE-TEX is a water and windproof fabric that allows for venting of internal heat. Several companies have their own version of the technology. Some coats come with a variety of features, such as full hoods and pit zips to vent heat. A Gore-Tex shell is the premium waterproof upper layer but nylon or Teflon coats offer good protection with slightly less venting ability. A PVC or plastic coated rain jacket will not breathe, nor will a nylon jacket repel rain for long periods of time. A wind breaker is designed to keep the wind off when it's not raining. A raincoat is too heavy to serve as a windbreaker. If you own a more technical piece of clothing that you know will repel water and breathe well enough to serve as a windbreaker, you may bring only one jacket.
 
If you layer up your chest well, but only wear cotton jeans, you will be very cold. Cotton doesn't insulate well & doesn't block the wind. Your lower body has more surface area than your upper body.   The warm blood that leaves your core will loose its heat as it travels down your legs to your feet and back, where your core has to warm it up. You'll get cold. Once you reach this point, your body will constrict the blood vessels going to your legs and feet to maintain your core temperature, making your legs even colder. Gore-Tex, Teflon, or nylon shell pants pants are great bottom layers but with the lower body, since venting isn’t as much of a priority as it is around the core of the body.
 
The head layer.  It is important to keep your head and neck warm. While most people know that humans lose a great of heat through their heads, people often neglect their necks.  A hat, such as a wool cap, fleece hats or a balaclava is required to reduce the significant amount of heat loss from the head. Baseball caps do not provide adequate insulation. A balaclava looks like a sock, sized for your head, with a oval hole in the side for your eyes.A neck gaiter is also another way to team up your headwear to protect the whole head and neck area.
The feet layer. How do we keep our feet warm? Layer them just like the rest of your body. First, put on a very light long sock made of either polyester or silk. Second, goes on a heavier sock that will cover your calves -middle layer-remember. Now, you need a shoe that is large enough to let you wiggle your toes and trap heat. If your shoes are too small for these layers, you'll compress the insulation making it less effective and your toes will be jammed together, so they'll get cold. High insulated boots work well because they cover your ankles and calves. Whatever boots you decide on, don't over tighten them. Avoid cotton socks for your feet. Wool or wool blends for the middle layer will keep your feet warm and dry. If you will be sitting for a long time, use overboot insulators.
The Hands layer. Layering works here also. Start with a lightweight poly or silk glove liner, then an intermediate glove (wool, windstopper) if you need it and finally an outer glove. Don't make the mistake of trying to cram your hand with these layers on into a glove that's too small. You need airspace around your fingers to keep them warm. That’s why mittens are more effective than gloves. Since we have to manipulate ammo, remove scope covers, etc, many will want to use gloves or “system gloves” (mittens with takedown tops).
DRESS WARMLY!