Physics for beekeepers: temperature in the hive
For this fifth installment of the Physics for beekeepers series, I’ve decided to use this recent question, which is representative of several I’ve received this month.
“I’ve heard that some beekeepers leave a medium super of honey on the hive ‘just in case.’ But others say do not leave the hives with an over-abundance of honey because one of two things will happen: They’ll either kill/weaken themselves trying to maintain it, or the honey will simply get too cold and won’t be usable which may or may not result in them dying of starvation with honey inches away from them.”
The cluster stays warm, the hive does not
Natural systems do not waste energy and honey bees are no exception. To survive the winter, a cluster of bees must keep itself warm. While it does this efficiently, it makes no attempt to heat the entire space within the hive.
The warmest place within a hive is in the center of the cluster. The temperature of the cluster decreases as you move toward the outside. The bees on the outside get so cold that they must rotate to the inside. If the inside of the hive were uniformly warm, this rotation would be unnecessary.
Of course, there is some heat lost from the cluster into the surrounding air, and because heat is lost, the bees must continually generate more. If you put your hand close to a heated iron, for example, you can feel the heat. The heat loss from the iron is similar to the heat loss from the cluster. You don’t have to move far away before you no longer feel it. The same is true inside the hive: the temperature drops rapidly as you move away from the cluster.
Nevertheless, the air inside the hive is slightly warmer than the ambient outside air. This is because the hive box itself provides a small amount of insulation. But the R-value of a pine board is not much, which means the difference in temperature between the inside air and the outside air is not great.
Heat rises from the cluster
There is one place in the hive that is warmer than the others, and that is the space immediately above the cluster. That is because warm air rises. One beekeeper in France measured the temperatures in his hive when the outside air temperature was 44°F. He measured 95° in the center of the cluster, 71° immediately above the cluster and 52° in other empty portions of the hive. Other beekeepers have found similar temperature gradients.
For this reason, an insulating layer placed above the bees reduces the rate of heat loss from the hive. Styrofoam, wood chips, or a layer of another material will slow the loss through the roof. But even this has its limits. For one thing, as the internal temperature gets warmer in comparison to the outside air, more heat is lost through the walls, so overhead insulation alone does not conserve as much heat as insulating the top and sides. It is a complex system with no easy answers.
A super of honey will not kill your bees
I often hear people say they don’t leave extra honey supers on their hives because it is too much for the bees to heat. Aside from the fact that the bees will not attempt to heat it, a super of honey is much different than an empty super. An empty super provides more space for the rising heat to go without additional benefit, so it is not a good idea.
But a super of honey has many advantages. Besides being a supply of food, a super of honey is a good overhead insulator. Also, because it is very dense, it has a high heat capacity. That means small or rapid fluctuations in external temperature do not readily change the temperature of the honey. In other words, a large supply of honey stabilizes the internal hive temperature.
Also, a super of honey slows the air flow from bottom to top through the hive. That is because the air passing through the narrow spaces between the frames rubs against the irregular surfaces of the comb, so the air flow is considerably disturbed. This is a good thing: you want some air flow through the hive to remove moisture, but you don’t want a wind tunnel. A super of honey, then, provides food, insulation, temperature stability, and reduced air speed through the hive.
Honey doesn’t need to be kept warm
“If bees can’t eat cold syrup, why can they eat cold honey?” In fact, the honey isn’t cold when the bees eat it.
When you bring groceries home from the store, do you store them in a warm oven? Of course not. And the bees don’t need to store honey in a warm spot either. Remember, heat rises from the cluster, so the honey above the cluster is plenty warm. Even honey close to the sides of the cluster will be warm enough.
As the warm honey is consumed, the cluster slowly moves toward more honey, and as they get close, that honey begins to warm. The bees warm their food on an as-need basis—just like we do. As I mentioned in the beginning, natural systems do not waste energy. It would be a total waste to keep all the honey warm all the time.
In addition, honey in cold storage is far less likely to be ravaged by other insects because they don’t like cold food either. If the bees kept their honey stores too warm, predation would increase.
Moderation in all things
Regardless of the above, I think some degree of moderation is prudent. You don’t need to leave three supers of honey stacked on your brood boxes. In fact, you may not need any. But if you think your bees might run short of food, there is nothing wrong with leaving a super of honey. Would I leave three? Probably not. You can keep one in reserve and add it later, if necessary, or perhaps one super plus supplemental feeding might be enough. Since every situation is different, the beekeeper must use judgement.