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.

Rusty
HoneyBeeSuite

Comments

Anubis Bard
Reply

Thanks for these kinds of posts. Quick question. In a place with moderately cold winters, (Rhode Island) should I leave the screened bottom boards open or drop the wooden inserts in over the screen to close it up? My guess is that it doesn’t matter, so I was going to leave well enough alone. But no one around here seems to mention it, when talking about setting their bees up for the winter, and I keep forgetting to ask.

Rusty
Reply

Andy,

In my opinion, there is no need to close off the screened-bottom board in most parts of the country. I would say it depends mostly on how much wind you have, but generally no. The air flow through the hive will be determined by the size of your upper opening. If the opening is small and restricts the air going out, not much air can come in the bottom because those two amounts have to be equal. Also, hot air rises, so warm air is not dropping out of the bottom of your hive, nor is cold air flowing up into the hive, except for the amount that replaces what left through the top.

I haven’t closed up the bottoms in years, and I know beekeepers in MA who don’t either. People on the open plains with a great deal of wind tend to close up, and that makes sense because the wind blows away the envelope of warm air just outside the hive.

Anubis Bard
Reply

Thanks for the response – it’s pretty much what I thought (which probably means that people have told me that before and I just forgot). But now I’m concerned about the upper opening, because on two of my three hives the telescoping cover sits right on top of an inner cover that has no side opening in it. On the third the inner cover has a slot in it, but when there’s no super on the outer cover just sits over that as well. Do I have to craft some sort of opening up there?

Rusty
Reply

Andy, you can cut a small front opening in your inner covers, you can add an Imirie shim, or you can drill a hole in the upper corner of the top brood box. Any of these are enough to allow the moist air to escape.

cgrey8
Reply

I have followup questions about ventilation. I don’t recall it being answered in the other physics posts, so I’ll risk being proved wrong and ask.

I know in the winter, it’s a good idea for most areas to block off a screened-bottom board to reduce the airflow so the colony can preserve heat. I assume the same is true of a screened top board? Or do you simply replace a screened top board for a standard board along with insulation on the top?

Is it also recommended/suggested to insert the entrance reducing block to further restrict air flow? I realize you don’t want them choked off completely. They do need air or it gets a bit humid inside. But it seems like that might help. Does it really? If so, which restriction should be used (narrow, med, wide)?

Rusty
Reply

Chris,

Yes, you should remove the screened inner cover in the winter and replace it with your standard inner cover. I restrict the entrance in the winter, no so much to reduce airflow (after all, air can come in through the screened bottom board) but to keep out vermin . . . mice and their kin. Remember that the airflow through your hive needs a place to come in and a place to leave. By keeping the place to leave (the top entrance) small, you automatically reduce the air into the bottom. If the top air can’t leave, them bottom air can’t get in. Air in has to equal air out—so by restricting one, you restrict the other.

Donna
Reply

Rusty, this is great info. Thanks so much. We are just south of Denver and today has finally brought very cold weather and the girls are in their cluster. Yesterday was 70 degrees and they were busy and the conversation was hot in the front of the hive. My question is should popsicle sticks be placed in the front of the hive for ventilation? I have read that to keep the air flowing in the front of the hive, this is a practical way of doing it. Should they go under the top cover? Thanks!

Rusty
Reply

Donna,

I just spent 20 minutes searching my own site for a photo I have of this, but of course, I can’t find it. Anyway, they usually go just under the top cover to hold it up. I think that is a lot of ventilation for the winter, though. I prefer either an Imirie shim with a 3/4-inch opening, an inner cover with a similar opening in the front, or just a one-inch hole drilled in the top corner of the upper brood box. Alternatively, you can use a quilt box with ventilation ports.

Andrew Hogg
Reply

Rusty, why don’t we use some sort of artificial heat for a hive in cold climates like Alberta? I’m thinking some sort of heating pad like one uses for warming plants that could be on a thermostat so that on warm days it turns off.

Rusty
Reply

Andrew,

I’ve heard of people having good success with terrarium heaters. The critical part is making sure it shuts off at about 40°F (4.5°C) so the bees don’t become too active or break cluster.

Nancy
Reply

Rusty,

Solid! And it answers the question of how bees can eat honey in winter. They keep it warm. Also, the question of why bees starve with honey in the hive…because it’s too far away from the cluster and gets cold.

It’s warm enough here now for robbing, but most of the asters are gone. I wish I HAD a spare full super to set on each hive!! Thanks!

Nan

Mark Luterra
Reply

One idea for Part 6 of this series: condensation in bee hives.

I had an interesting conversation with some folks on BeeSource in this regard last winter. I haven’t done empirical tests on this but based on knowledge of physics:

1. Warm air rises.
2. Air picks up moisture (from bee breath) as it passes through the cluster, raising its dew point.
3. Moisture condenses on any surface cooler than the dew point of the air in contact with it.

Point #3 means that condensation is primarily an issue in the winter, when the walls and roof of the hive are cold relative to the cluster.

In an uninsulated hive with no top ventilation, air in the hive forms a convection current, with warm air rising up from the cluster, cooling in contact with the top, and sinking along the walls. Condensation forms as the warm, moist air reaches the cold top, dripping down on the bees. A good recipe for disease and moldy dead colonies.

There are (at least) three ways to solve this problem.

1. The first is to provide increased top and bottom ventilation. This has the effect of changing the recirculation current into a flow of air from bottom to top, still driven by the heat from the cluster. Because air that passes through the cluster (and picks up moisture) leaves the hive at the top, and because more air is moving through the hive, the dew point remains lower in the hive and moisture does not condense above the bees. The downside is that increased airflow through the cluster means increased heat loss, and the cluster must consume more food to maintain the same temperature (and smaller clusters may be unable to survive). There is fairly good consensus among Midwestern and Northern Plains beekeepers, likely for this reason, that screened bottom boards are not good for overwintering.

2. The second solution is to trap the moisture using a “quilt” of wood chips or other material. This is my present solution, and I am happy with it (and thanks to Rusty for the inspiration and basic design!). Although my quilts have ventilation holes, the chips restrict the rate of airflow, conserving energy but allowing the dew point in the hive to rise. The moisture condenses on the cold underside of the telescoping cover and drips down onto the chips, slowly saturating them from the top. It takes all winter for a 2″ thick layer to be soaked through, and the ventilation holes allow some drying. No moisture drips down on the bees.

3. The third solution, which I have not tried, is to forego upper ventilation or quilts and simply place a thick sheet of nonporous insulation (e.g. 2″-4″ thick styrofoam) above the inner cover. If this works (and others say it does), it does so because the top is no longer colder than the dew point of the air in the hive. Instead, the moisture condenses on the cold walls, where it runs down causing no harm to the bees. Advocates of this approach claim that condensation is actually good for the bees – so long as the condensation does not form above the cluster – as it allows them to obtain water in the winter.

I like my moisture quilts for the time being, but as I expand I might try the insulated-top-only approach. I’m still a bit concerned that in the PNW mold would grow on the sides where the condensation forms. That might not be bad for the bees, but I much dislike mold…

Mark

Rusty
Reply

So, Mark, why not write up part six as a guest post? You’ve already done all the work. It would be a great addition to the series.

Simon Cavill
Reply

You guys should read Tom Seeley’s papers on the size and structures of wild tree nests done pre-Varroa. He analysed hundreds of wild hives noting their dimensions to create a normalised model.

In reading his work you would realise that the inside of a natural honey bee hive is a super-insulated environment that is completely sealed apart from a small gap at the bottom.

One can then use Tom Seeleys data to create a thermodynamic model of a normalised colony and look at the energy requirements of a colony through the winter and just as importantly the amount of energy required to evaporate large quantities of water from harvested honey.

Using this model against the thermodynamics of a standard wooden Langstroth hive is a complete eye opener and shows just how hard the bees have to work to make up for such an unsuitable environment as a wooden bee hive. The amount of energy/stores they have to consume in a wooden hive to keep warm and especially to evaporate water off honey stores (Hint – latent heat of vaporisation) is astounding and we should be surprised that we ever get a harvest in the first place!

So-called natural beekeepers are also way off base with their Warre’s and Top-Bar hives – both utterly unnatural when compared to the real thing!

The conclusion you can come to is that only poly hives start to get close to the physics inside a natural tree hive, and even they are a poor substitute for 9″ of oak or spruce for hive walls! I have a physicist colleague here in the UK building super insulated hives against Tom Seeley’s models and testing them with thermal imaging cameras. The results are quite interesting…

Rusty
Reply

Hi Simon,

You are right, of course. Actually I have read most, if not all, of Seeley’s work over the years.

I touched on the idea that a tree and and hive have nothing in common over three years ago in a post called, “A hive is not a tree.” Although greatly simplified, the purpose of the post was to demonstrate how different a modern hive is from the real thing.

I have also repeatedly expressed my opinion that there is nothing natural about keeping bees in an artificial hive. In “Are you a natural beekeeper?” I write, “And if you are keeping bees . . . you have already crossed the line into unnaturalness.” You have no idea at the amount of “hate mail” these opinions generate!

Anyway, I agree with your comments. Thanks so much for writing.

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