Kiwi,
Wait a minute here, This beer can in the trench trick is something I need to know more about. This sounds like a very useful piece of information. Do I need to carry my own sand bag around with me or will dirt do? I need to know or I will end up wasting good beer experimenting.
Wind chill. . . There are a number of ways that heat is transferred into the atmosphere, convection, conduction, evaporation and several others which can work separately or in combination. It has been a little over 20 years sense I had to really think about this and I do not have any reference material here.
Evaporation as I recall is a very different mechanism, and works differently in hot Vs. cold. evaporation requires energy to provide the power for the transfer, heat is the energy source that provides the power. (I think). It would be somewhat like pouring water on a hot engine block. The water turns to steam, the heat from the engine block provides the power, and gas used up, leaving the engine cool. This may be semi-related to the old gas powered refrigerators (and perhaps your beer in the trench trick!)
Any way, when the water is gone the canvas does not remain cool. Wind chill results from an object which is warmer than air temp. heating the air surrounding it. the warmer air will dissipate, requiring the object to provide more heat to reheat the surrounding air. When the air is moving the object must transfer heat faster, therefore cooling itself faster. When the object reaches the same temp. as the air, the transfer stops. There is no longer any power source to provide the needed energy.
A person exposed to wind chill is a slightly different proposition, as the person has a built in heat source, and will attempt to produce more heat to replace the heat that is lost. The wind requires that the person produce heat at a faster rate to replace the heat that is being lost. The question that the "wind chill factor" attempts to answer, is what temperature would it have to be (with no wind) to cause body to lose heat at the current rate.
We are pretty badly off topic here. If this thread gets lost, or we get asked to go elsewhere, please feel free to email me (esp. with the beer in the trench trick) although I think there may be enough interest in the beer to gain us a little forbearance here.