I'm no meteorologist nor physicist, but have been looking into relative
humidity, dew points, vapor pressure, and the evaporation
and sublimation of water (ice directly to vapor).  I've also reviewed the
recorded temperature isotherms in wintering hives (Owens, Szabo).

The bees appear to have mixed feelings about upper entrances, and may move
away from them, or perhaps block them with their bodies.
Owens felt that the main advantage of an upper entrance was to allow
cleansing flights.

As far as condensation being a problem, I suspect that one could predict it
from maps of dew points and average winter temperatures.  Please refer to
the attached map.

The combs around and below the cluster rarely drop as low as the outside
ambient temperature, and may get cold enough for the water vapor escaping
from the cluster to freeze.  Since the temperature of those combs is above
that of the outside air, the water (whether frozen or not) will have a
greater
vapor pressure than the local atmosphere, and the water will tend to
evaporate.  In areas with very low dew points, the condensed ice may
evaporate out
completely, and the colony may need to conserve water to prevent
desiccation.

But as the ambient temperature rises above freezing, any moisture
condensing on the combs would be in the liquid state, and its evaporation
rate would be relative
to how much the cluster was warming the combs outside the cluster.  It
would also depend upon the dew point--the temp at which water vapor would
no longer evaporate.

Once the temperature of the outer combs exceeds that of the insulating
shell of bees--around 50F--any water would be expected to evaporate, at a
rate relative to the relative humidity (which can be calculated from
temperature and dew point).

I'm guessing that those who report moisture and mold problems lie in areas
in which the dew point is close to the temperatures of the outside combs,
which are often in the 20-40F range.  Take a look at the map above (keeping
in mind that actual daily dew points are very much dependent upon the
day-to-day weather conditions, not the average).

So my question to the List is whether those who are reporting moisture
problems tend to fall into one of the colored dew point gradients in the
map above.  Please let me know whether or not you observe moisture/mold
issues in uninsulated hives, and the dew point gradient your hives are in.

Please email [log in to unmask]  In the subject line please write Y or
N for moisture problems, and the dew point isotherm from the map above
(e.g., 20-30).  Example: Y 20-30.

Thanks in advance.

-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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