This is why I enjoy these conversations on Bee-L so much.

I apologize for my Engineerese :), I know I get carried away. It is how my mind works.

Randy both your last comments were great and made me think during my cold dog walk.

I agree with you (Heinrich) about the cluster respiratory process. Scott Debnam does a great job of explaining this on the colony level (starts around 2:30) - https://www.youtube.com/watch?v=bOZNhvIBDOM&t=1s (Presented at our recent WAS mini-conference).

My theory (that I still need to prove if not already done by others) is that natural convection (changes in internal air/gas temperatures) and the buoyancy effect drive air flow (in thermally stable homes with bottom entrance only). It can be "increased" with well placed ventilating bees during warmer temperatures in winter or in summer. This breathing of the cluster to release CO2, warm gases (waste heat off the cluster) is the driver of natural convection in the hive (especially those without a top entrance or bee tree with the typical bottom hole entrance). Changes in temperature in the internal air therefore the density changes will drive natural convection. Limited active fanning required based on internal external T differential. The second very interesting comment by Randy about the gas volume change from O2 to CO2 (during the metabolic process) at seems to be a pseudo pumping system that would keep pushing the cluster gases out just by pure expansion. 

Another driver that I see in both my monitored colonies is the presence of cold "channels" that would naturally allow cooling internal gases to drop towards the lower entrance. It is logically always against one of the hive walls which also creates a natural condenser. 

Why would this be important?
When the cluster is at its most vulnerable (dealing with extreme cold) it cannot afford to have bees "force" air to move via active fanning (except the cluster natural respiration). Many beekeepers use top vent holes and different approaches to "help" warm moist air out of the hive (thermosyphon process) which works in most climates, including mine, but a colony housed in a thermally stable enclosure with a bottom entrance only has access to some natural means of moving air out.

Observations don't prove anything, but they are useful at highlighting potential mechanisms at work or asking questions.

Observation 1:
During winter, colonies are noisier during warm periods (-5C to 5C), bees are more active, more fanning can be heard. Delta T (in Vs Out) is small. Wet condensation can be seen on the bottom board draining through the bottom screen. Internal hive Ts >10C

Observation 2:
During winter, colonies are the quietest during very cold weather, bees are in tighter clusters, internal hive temperatures are from 0C to 10C below the cluster, entrance has large amounts of hoar frost buildup. Hoar frost is a sign of warm moist air coming into contact with cold air causing the air moisture to quickly change from vapour to ice. The picture I shared showed hoar frost across a 5 inches of surface which highlights the air coming out of the hive has considerable velocity/energy. 

Observation 3:
During a summer nectar flow, ventilation bees can be heard inside, landing board is soaking wet at 1st light from condensation occurring at lower entrance from flow of moist air out of the hive. Delta T is ~30C (Tin35C vs Tout 5C)

It would be interesting to record some internal acoustics, CO2 levels at the entrance, and more granular (1 minute) interval temperature measurements.

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