- how can it be higher when the brood temperature is 36C
The brood temp is 36 but the hive box interior temperature may be colder than the outside temperature. Most of the thermal mass in a hive is honey. It is not generating heat. If your bees are in cluster, the cluster is warm but outside the cluster may not be. I don't know what your hive box temperature is because you are using boxes made of insulation material rather than wood. But if you have an uninsulated wooden box with SBB, the hive box interior temperature will be lower than the outside temperature once the sun comes up in the Spring Honestly, I don't know why your bees use so much food in the winter. If they stay in cluster and your insulation is so good, why do they use 40kg of sugar? My bees use about 10kg. And they are not insulated. And the wind comes in the bottom. :-D
- and why it should keep bees in cluster?
If the hive has SBB, it will be colder inside the hive box at night and the bees will go into cluster. If the box is sealed and made of insulation, maybe the bees are not in cluster and use more food?
Bees keep the hive warm, not sun. Can't you understand that.
I understand that the sun does not warm your
hives. :-D But if you have an SBB and you live in the sunny South, the sun will warm up the outside temperature warmer than the hive box interior and the flow of air through the SBB then warms up the interior air. Our temperature could be 25 degrees at night and rise to 75 or 80 the next day. And there is a heating effect of the sun shining directly on the hive also. So the hive interior temp (not the brood or the cluster) may fall below freezing The next morning as the outdoor temperature rises and the sun shines on the hive box, the interior temperature of the hive will rise and the bees will come out of cluster.
What I think in Spring is that is it enough that bees can fly to willows and get pollen.
Why should they warm up early?
Well Finski, I guess if they get up earlier, they will have more time to gather pollen? And since most hive that starve do so in the Spring, I like to have my bees out getting as much nectar as they can. So your lazy bees like to stay in their warm bed and sleep late? :-D
Many beekeers use here mesh floor. Many are mad with them. Some keep them open all the time, some close them for winter. A mesh floor is a religion. So called "modern beekeeper's bottom. - heh!
We close our about 90% in Winter. That blocks the wind but still gives plenty of ventilation. But I'm not religious about anything. If I see that closed boxes worked better, I would use closed boxes.