I notice that the discussison of clustering tends to oversimplify what is a very complex phenomenon.   I propose that the clustering of summer or fall bees is very different from the clustering of an overwintering or springtime colony. 

There is a tendency to ignore the season, the nutrional state and history of the cluster, and the possible accumulation of feces in  the bees.   

Is there brood?  Even if not, humidity and water economy are important, too,  since if the ambient air is very dry, they have to get water somewhere or make it.

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