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Just thought I'd point it out in case someone wasn't aware, if you are on a trip, and you find your cold weather gear lacking, wrap your poncho around your bag. It'll prevent quite a bit of heat loss via convection by providing a windproof barrier and provide a greater layer of insulation. Not very breathable, but it's a compromise if you're underprepared in a survival situation.
Putting a "sportsman's" reflector blanket on the ground, will reflect the heat back to you and the freshness back to the earth. Twice as much under you as over will serve you best, its isolation from the ground that is important in a cooler setting.
First, lets review. Heat leaves your body in the following ways:
Conductive Heat Loss occurs when contact is made between your body and a colder surface. It can be minimized by not sitting on the cold ground, especially on snow. Conduction occurs 25 times faster with wet clothing than with dry. Prevention: Thickness of insulation.
Convective Heat Loss occurs when your body heat warms the air adjecent to your body; that air then rises and moves away from your body and fresh colder air replaces it. Wind increases the speed of heat loss through convection. The mipact of heat loss from convection is measured by the windchill factor. This same process happens when you are submerged in cold water, but it happens much faster than in air because of the greater density of water. An important element in dressing for the backcountry is trapping the air around the body. Prevention: Windproof garments.
Radiant Heat Loss is caused by the escape of infrared radiation from the body. It is minimized by wearing insulative fabrics or those with reflective fabric that reflects the heat back to the body. Prevention: Thick layers of unsulation or reflective material.
Evaporative Heat Loss occurs when perspiration (water) on the skin evaporates, drawing heat from the body. Changing water from a liquid to a gas takes a lot of energy. This is why sweating helps cool you off when you are hot. In hot weather, evaporation is essential in colling your body down to prevent heat illnesses. However, when it's cold you want to minimize the amount of sweating to reduce evaporative heat loss. Prevention: Fabrics that move water away from the skin and vapor barriers.
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I see where the reflective blanket will help with radiant heat loss, and I agree that the key is insulation from the ground to prevent conductive heat loss, but I was refering to a way to assist in preventing convective heat loss if you are without a shelter or it's very drafty, but you do have a bedroll, and hopefully your wet weather gear. It's just one more thing to keep in the back of your mind in case you are in a situation where you haven't had the opportunity to build a shelter, etc. etc.
Sapper
Oooh. Ooooh. You missed one!!!!! Respiration. I just learned that today so I just HAD to throw it in.:D
I've never heard that before. I knew respiration was a major source of fluid loss in cold enivorments, as your body will use the moisture to warm the air coming into your lungs, but I never thought about it as a souce of heat loss as well. It makes total sense though. Where did you learn that, if you dont mind me asking?
Sapper
Tony posted a thread on videos yesterday (I think) and one of the links was to Ron Hood from Hood's Woods. He listed the 5 mechanisms for heat loss as:
Conduction
Convection
Radiation
Respiration
Perspiration (wet skin)
As I said in Tony's thread, I didn't have a formal knowledge of those mechanisms. I guess I knew them intuitively but had never heard them stated that way.
Here is a link to Tony's thread:
http://www.wilderness-survival.net/f...ead.php?t=1561
I was actually just pulling your chain a bit.
a little conceptual understanding of the sciences goes a long way. thermodynamics is no different. the exacting mathematics may have no relevance to your life at most times, but the concepts are fundamental.
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