The above video discusses the following:

The shape of a bee cluster varies depending on the depth of the frame.

In conventional Langstroth vertical hives, due to a relatively shallow (just 9″ deep individual frame) the bee cluster tends to take a form of an ellipsoid – it looks like an American football. In horizontal hives with deep frames, such as in a horizontal Layens hive, the shape of a cluster tends to be purely spherical like a soccer ball or a basketball. 

When a frame is a combined frame where just the top part originally comes from a conventional 9” deep shallow frame, you can see the old darker brown trace of the previous oval-shaped brown comb. But once combined within the deep 16” frame, with the greater depth the new capped brood cells already exhibit a circular shape. Given the fact that the bee cluster spans multiple frames, an oval shape or a circular shape of brood areas on an individual frames is the evidence of 3-dimentional clusters shaped like either ellipsoid or sphere respectively. 

An ellipsoid shape looks like American football, while a sphere is like a soccer ball, or if you prefer, a basketball. Compared to an ellipsoid, a sphere is a shape with the higher volume to surface area ratio. And that means that given the same number of bees within these 2 cluster shapes, the spherical shape exposes fewer bees to cold than the ellipsoid shape. 

So the bees wintering in an ellipsoid cluster in conventional Langstroth hives with shallower individual frames, have to work harder and use up more energy and consume more honey to maintain the same temperature in the center of the cluster: around 93°F (34°C) In contrast to single (long) box horizontal hives, a typical conventional Langstroth vertical hive consists of multiple smaller boxes stacked up vertically where each box contains a row of much shallower frames and there’s always a gap between lower and upper frames in-between the individual boxes. 

2 examples are shown when during cold winters, the winter bee cluster died trying to bridge the above gap during a cold snap: the bees from a lower brood box starved while moving upwards after consuming the last bits of a honey band on a lower frame but due to a cold snap were unable to bridge the gap between lower and upper frames where there were more honey stores. Multiple bees have died with their heads down in the cells.

 For horizontal single-box hives, for every locality with cold winters, we can actually calculate the safe width for a band of honey above the bees on a contiguous single frame so that the bees will winter successfully. 

In Maine zone 5 the maximum time that the bees will remain in the winter cluster is 5 months: from Nov 1 to Mar 31. That’s the maximum time when it can be continuously so cold here that the bees will only be able to go upward 1mm per day consuming the honey from the honey band on top. 1mm per day for 5 months is 150 mm or 5.9” – let’s round it up to 6” – that’s the safe width of the honey band above the bees.

So in Maine, any brood frames with top honey bands less than at least 5” (like this one) should be taken out in late Fall – they will be unsafe for wintering. The biggest winter cluster of bees can be up to a basketball size & the diameter of the regulation size basketball is 9.5”, we’ll round it up further to an even 10” in diameter (that’s 25 cm). So, the winter bee cluster of a maximum of 10” in diameter will start wintering under the honey band of a maximum of 6” in width – that means that the frame that will be safe for wintering in Maine zone 5 should have the maximum depth of 16”. And it so happens that 16” is exactly the depth of a frame in a Layens horizontal hive!