Applying physics to understanding overwintering is useful, but may be  
limited in understanding the whole picture.  Obviously, a bee cluster  
cannot escape the realities of heat loss through radiation, conduction  
and convection.  At the same time, the cluster is a living, breathing  
entity with variable responses through the day and through a witner  
season depending on external conditions and dynamics within the bee  
cluster.  During winter, clusters have sharp gradients of  
temperatures, where the interior bees can be at "body temperature" and  
exterior bees fairly close to chill comma.  Some of the insulative  
properties of thick-walled cavities may be beneficial at the moderate  
low temperatures of mild winters or in areas that get cold at night  
during the growing season.  At extremely low temperatures (and with  
wind) insulation per se (to deal with conductive heat loss) may not be  
as important as preventing convective heat loss.

The winters of most of Europe and of overwintering areas for most of  
the beekeeping in the US require some attention to the realities of  
thermodynamics, but pale in comparison to those of some areas of the  
upper Midwest and Canada. 

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