Ok - I've just read a post in which somebody is blaming the lack of an upper entrance for his colony losses ... so I thought this might be a good opportunity to revisit the old chestnut of which is best during winter - 'upper or lower ventilation' ?
Warm, moist air rises - obvious isn't it ? Or is it ... ?
Now although perfectly true as a statement, there are two things inherently unsatisfactory about how this warm moist air comes into being: the first is that air becomes 'moist' by the process of evaporation of liquid water into water vapour - and as anyone who's sat in a boat out at sea wearing wet clothes will tell you - evaporation causes chilling, to such an extent that it often results in hypothermia.
So, from evaporation 'moist air' is generated - only that air isn't warm. Quite the contrary - it's actually quite cold - and a parcel of cold air, being denser than the air around it, will initially descend, and NOT ascend.
Should it descend and encounter a substantial opening to the atmosphere, it will quite literally 'fall out of the hive' - but if such an opening isn't present, then condensation will occur upon the cold floor surface (evidence for which is often seen as black mould).
The second issue is that eventually, these molecules of cold air will begin to be warmed to whatever the ambient temperature happens to be in that part of the hive. At this point - the water vapour component of the air, having a lighter molecular weight than the air which holds it, will indeed begin to rise - but only at the cost of reducing the local internal temperature of the beehive.
The above may go some way to explain how BOTH upper and lower ventilation can remove water vapour from a beehive - that is, the moist air initially descends, and only later does it begin to ascend. However, from a temperature conservation point-of-view, adequate bottom ventilation is clearly to be preferred.
Interestingly - with those using conventional hives - Europeans tend to favour bottom ventilation in winter, whereas Americans appear to prefer upper ventilation.
To flesh this issue out a little more:
Water has a molecular weight of around 18, whereas the gas nitrogen has a molecular weight of 28, and oxygen 32 - therefore molecules of water vapour are significantly lighter than those of dry air, and so will rise - which of course is why we see clouds high up there in the sky. And, as a one-time chemist, I used to hold the simplistic view that moist air rises ... and that was the end of the matter.
However, I was to discover a paper by Harry D. Tiemann, 'Kiln Drying of Wood for Airplanes', 1919. Now Tiemann was no slouch when it came to chemistry - a subject he even taught for a while - and so would have been well aware of the molecular weight argument. However, on page 15 of his paper (Bulletin 509 : The Theory of Kiln Drying), he writes:
Tiemann's principles went on to form the basis of the Oregon kiln-drying industry which today has an annual turnover of many millions of dollars, and yet - ironically - American beekeepers have yet to embrace these same principles in the removal of humidity from within their beehives." ... evaporation is of itself a cooling operation, and calculation shows that the effect of evaporation is to increase the density of the humid air in spite of the fact that more vapor has therefore been added. [my emphasis] This means that there is a natural tendency of the air to descend as it passes through the lumber, particularly when rapid drying is taking place. The arrangement of the pile of lumber and the kiln, therefore, should be such as not to oppose but to assist the natural gravity tendency. [again, my emphasis] Success or failure sometimes hinge on this point. Extensive observations under all kind of conditions and in all kinds of kilns have shown that this theory of the downward tendency of the air through the pile is the correct principle."
Perhaps this is the point at which I say ... "Tin Hat ON".