Loc: Bay Area, California, USA
Yeah, this gets right back to the original question. Theoretically a down jacket/bag warms up and radiates out heat too, on an icy day, but we all know the jacket keeps warmth in. So what exactly are the effects of clothing and insulation??
Sometimes I astonish myself with how little I contribute <img src="/forums/images/graemlins/grin.gif" alt="" />
I have been waiting for a chance at work to answer this question, mostly because my heat transfer textbook is at work. Although some people have given questionable answers, they arenít so bad that I need to say they are wrong. They are just a different way of seeing the same phenomenon. Mostly, I agree with Paddy Crow. She probably took the same type of heat transfer class that I took.
So, I am going to say that there are three types of heat transfer; conduction, convection, and radiation. Evaporation is a combination of more than one. The one that is the strongest depends on how prevalent the other ones are at the time. If you want to say that evaporation is a fourth heat transfer mechanism, that is fine, wonít hurt my feelings.
Letís pretend that you are placed in a vacuum, and have no contact with any object. Convection and conduction are zero, and radiation is 100%. Or, you could be placed naked on top of a mountain with 100 mph winds. Radiation and conduction are so small because convection is so strong. Conduction is highest when there is no convection, and you are touching something with a high thermal conductivity (like copper), and it is much colder than you, and much larger than you.
Now, letís look at real world. You are standing outside with a decent wind, and it is between 30įF and 40įF. You also have a windproof jacket with 2 inch loft of insulation. What your jacket does is create 2 inches of stagnant air. So, next to your skin, convection is greatly reduced. That leaves radiation and conduction. Because air has a very low thermal conductivity (almost the lowest), the conduction of heat from your skin to the outside is also greatly reduced. The thicker the insulation, the more conduction is reduced. So, your body loses most of its heat through radiation. This, of course, is only if the thickness of the insulation is great enough to drop the heat lost through conduction to be lower than the heat lost through radiation. If you are staying warm, then radiation, conduction, and convection are all small. I would say that convection is where the most heat is lost when you are out in the woods, especially since convection plays off the other two.
For your clothing choice, I would recommend trying to combat all forms of heat loss. They all play a role, and one shouldnít be ignored over another. <img src="/forums/images/graemlins/grin.gif" alt="" />
I've taken a vow of poverty. To annoy me, send money.
Loc: Portland, OR
would it make any difference if the first layer (or any layer for that matter) is white or black?
This is an interesting theoretical question, but I have a hunch that any actual difference this makes would be so mind-bogglingly tiny that it would be dwarfed by the heat loss from simply breathing or shifting your body.
Heat transfer to and from the body occurs via the following 4 mechanisms:
Hate to get ticky-tacky, but did anyone else notice that 2 + 10 + 65 + 30 = 107%?
I don't recall if it was this forum or another, but I went through this whole deal about a year ago, even getting down to black-body radiation and total heat loss from respiration.
Breathing turns out to be a large heatsink for your body. Basically, the air you breath in has to go from its current temperature and humidity right up to body core temp. and 100% humidity. That's a significant amount of heat loss. There are masks, like PSolar and PolarWrap, that use heat exchangers to recover heat and moisture from exhaled air, then use it to warm and humidify the inhaled air.
Of course, not many of us lay out under the stars buck nekkid and those that do are probably beyond help. <img src="/forums/images/graemlins/grin.gif" alt="" />
The things we do to insulate ourselves also shield us from losing heat through radiation. The simple calculation being referenced is the potential heat loss in an "ideal" situation. Real world heat loss is going to depend on many factors- clothing, a tent, and a sleeping bag completely changes the equation. If it didn't, we'd be in trouble.
First off I say more folks should lay out under the stars buck nekkid <img src="/forums/images/graemlins/cool.gif" alt="" /> it's a real eye opening experience <img src="/forums/images/graemlins/grin.gif" alt="" />
You just gave me an idea.....
PEPPER SPRAY AIN'T BRAINS IN A CAN!