I'm very curious about the observations of moisture condensing in the hive
outside the cluster.
van Nerum and Buelens found that a healthy cluster is damn near airtight --
so much so that the oxygen content drops substantially, and the CO2 content
rises.

That being the case, it's clear that moisture is not freely escaping by
convection.
Other researchers have observed that moisture does not noticeably condense
on the bees forming the cluster mantle, which the bees keep at around 55F.

A number of researchers have measured cluster relative humidity.  From what
I've seen, RH rarely exceeds 75% in the cluster.  Those of you with
humidistats in your hives please confirm or let us know otherwise!

So take a look at the dewpoint calculator at http://www.dpcalc.org/
Click to solve for dewpoint.
Slide the RH arbitrarily to 70%.
Then slide the temperature around in the range of winter cluster core
temperatures -- 70F to 95F.
The dewpoint at those core temps ranges from 60F to 84F.
What that means is that moisture will condense on the combs within the
cluster before it ever reaches the bees in the mantle, much less ever
escaping the cluster without forced ventilation.

As I explained in my recent article "The Prevention of Dysentery," based
upon the findings of Möbus, Sachs and Tautz, and Toomema, it appears that
instead of venting moisture out the top of the cluster, a healthy cluster
vents out the bottom.

So my question then is how their hypotheses are supported by observations.
I've seen photos by beekeepers who have placed plexiglas over the top of
the hive to look for condensation.  With some insulation above, there
wasn't much.  Ditto with Toomema's condensers -- most condensation occurred
at the bottom.

So why are some beekeepers observing condensation above and on the outer
combs?
I see this occasionally, but in retrospect, didn't make a point to check to
see whether the colony was healthy or behaving "normally."

Folks, bees have been dealing with the laws of physics for a long time, and
appear to have winter ventilation of moisture pretty well worked out when
they are living in a tree cavity.  So what we need to understand are the
conditions that prevent them from preventing condensation on the combs of
hives.  Is the cavity too large for the cluster?  Are they unable to direct
exhaust air flow out the entrance?  Are colonies that don't manage moisture
doing so because they are sick?

I don't know the answers, but I sure do know the questions!

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

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