> on the other hand, we clearly observe that plenty of moisture does indeed get convected out of the cluster (the condensation on the cover).  So the bees apparently "allow" some moisture to escape.

Here are some incomplete fragments of thoughts to seed discussion:

One must consider that the 'air' in the cluster must be recycled through a number of bees as it makes its way out of the tight cluster, at least when the cluster is down from the hive top.  Either that, or some mechanism must allow the air to escape through the tightly-packed bees as you suggest.

If the bees were to re-breathe all the the air as it passes outwards, then each bee would in turn -- I imagine -- modify the humidity of that air and necessarily exhale it at less than 100% RH for that point in the cluster.  Since temperature drops progressively (but not linearly) outward from the centre, one would have to wonder at this point, since it would seem that each cooler bee would have to _remove_ moisture in such a scenario, whereas reason would suggest that the metabolism of each bee would be adding moisture, not removing it. ?????

As a point of interest, water vapour is quite a bit lighter than air and, as a result air containing water vapour rises, independant of temperature.  That is why we see a layer of clouds on a fair day.  Moisture-laden air rises from the surface and expands (causing cooling) until it encounters a temperature gradient which results in 100% RH and condensation occurs spontaneously, forming the ranks of familiar fair-weather clouds.

Consideration of these various fragments of ideas leads me to the hypothesis that air is taken in all around the cluster and "exhaled" from the top of the cluster.  While low in the hive, the temperatures above the cluster are not much different from ambient as I recall.  Therefore moisture-laden air rising from the cluster is likley to condense and freeze.  If weather moderates, then this water descends on the cluster.  When the cluster, however, is up against a lid, especially an insulated one, the water only condenses around the periphery of the cluster and cannot drip on the bees.

This discussion makes one realise why insulating lids of wintering hives in frigid environments, even if the hive is vented, is so important to successful wintering.

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