A very simplistic explanation is that air temperature and humidity inside the hive, outside of the cluster, tends to track that of the outside air.  All that a natural or an artificial cavity does is buffer the extremes.  The amount of buffering depends on how tight the skin of the cavity is, thermally and to humidity.  My impression is that colonies in the more humid (and cold) winters benefit from an upper entrance/opening, while that same situation may be unnecessary or problematic in very dry and cold climates.

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