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@freemo So ideally you'd want something "black" to visible light and "white" at infrared wavelengths to both absorb external radiation and reflect internal black body radiation inwards. Assuming you can only have one or the other, a quick back of the envelope calculation suggests that the sun's radiation, which is on the order of a kilowatt per square metre and would be irradiating roughly your frontal area (let's call it one square metre, total 1kW) at polar midsummer, is the dominant effect over your body heat, which I estimate as 2000 kcal/day, roughly 100W. So I'll go with black.
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@khird No, actually in the arctic the ideal would be something white to all wavelengths, but especially white to IR. The reason being is you want to retain your own body heat and as such white will cause less of your IR heat to be radiated.
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@freemo why does that dominate over the benefits of absorbing maximal energy from solar radiation? It looks like it's about an order of magnitude more energy in this mechanism
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@khird Because the IR temperature int he arctic is far below your body temperature. The sun is low int he sky so the IR temperature is already low. But it also makes up a very small percentage of the number of degrees/sphere around your body with everything else being extremely cold in terms of IR. So the overall incident IR temperature is far below body temp thus causing net loss.
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@freemo the sun may be a small portion of the sky, but the temperature difference between the sun and your body is thousands of kelvin, whereas the difference between your body and your other surroundings is tens of kelvin. Since radiative transfer scales with the fourth power of temperature, I don't think you can neglect that mechanism (and the ballpark numbers I ran above suggest that it actually dominates over the IR transfer). On top of that, the snowy surroundings are going to be reflecting sunlight at you from a much greater area.
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@khird The percentage of the sphere around you taken up by the sun is 0.00001205% everything else around you is several degrees below 0C. Overall the suns influence is not significant enough.
If you want to do a practical test in the middle of winter take an IR thermometer and point it at the sun when the sun is low in the sky. The sun will only take up a small portion of the cone and most of it will encapsulate the sky around it. The IR temp you measure will be surprisingly low (10 - 30C). Considering the IR thermometer is seeing on a small portion of the sphere of influence itself the actual total IR temp experienced across the whole sphere would be quite a bit below body temp.
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@khird I managed to find a chart to sorta prove my point. It compares white with gray but you can extrapolate to understand white would be even better than gray.
Notice that at cold temperatures lighter colors present less radiative loss than darker. This is outside temp not inside.
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@freemo thanks! What's the context here? In particular, was this experiment conducted under sunny conditions?
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@khird that i dont know, it is from a second hand source that got it from a paper that i couldnt find. I only recognized it as consistent with a paper I saw some years back where I learned that white in the arctic improved heat retention. I am having trouble finding the actual paper.
