physics for beekeepers

The surprising way honey bees stay warm in winter

To study how bees keep warm in winter, Bill Reynolds installed temperature monitors in three hives. Two hives contained bees and one was empty.

A popular beekeeping myth says honey bees keep the interior of their hives warm all winter. But they don’t. The bees keep the cluster warm while the hive interior rises and falls with the outside air temperature.

Honey bees do not heat their hives the way we heat our homes. Instead, honey bees stay warm by clustering tightly together and vibrating their flight muscles. The center of the cluster is the warmest part of the hive, and the temperature drops as you move out from the center.

The interior of the hive is warmer than the outside air because heat escapes from the cluster and the hive itself offers a small amount of insulation. But the bees do not attempt to keep the entire space warm. In fact, the air inside the hive can be quite cold.

Because hot air rises, the warmest place outside of the cluster is right above the cluster. A beekeeper can help keep the hive slightly warmer by placing insulation above the cluster to capture some of this escaping heat.

Bill’s hive temperature experiment

In order to help explain this phenomenon to new beekeepers, Bill Reynolds of Minnesota decided to monitor the temperature inside his hives as the colonies plunged into winter. According to Bill, he purchased an inexpensive desktop weather forecasting station with three remote wireless sensors for his project. He used a fourth sensor to monitor the ambient outside air.

The weather cooperated for his experiment. Bill says, “Here in Minnesota we are experiencing bone-chilling temps around zero each morning and mid-twenties, if we are lucky, by noon.”

Three hives, two with bees

Bill set up three hives, each with three deeps topped with a quilt box. One hive contained a colony of Carniolans, one a colony of mutts, and one was empty. In each hive he centered the sensor over the third deep but under the quilt box. He did not attempt to place the sensors at the core of the clusters. During the measurement period, the clusters were two deep hive bodies below the sensors.

The hives were not wrapped. All three setups were on the south side of a house with a straw-bale wall blocking northwest winds. According to Bill, “Other than the sensors, there is nothing different between these hives and any other hive one would find in a backyard.”

Partway through the experiment, Bill began recording separate readings for the outside air and empty hive. He made this change because he noticed that the temperatures increased and decreased at different rates inside the empty hive and outside of it. It became apparent that the wooden boxes themselves influenced temperature fluctuations.

It’s warmer inside the hives, but only slightly

The graph below shows temperature readings for each sensor. It is quite clear from this simple experiment that temperatures inside the active hives rose and fell with the outside temperature, but overall the inside remained warmer than the outside.

But far from being cozy, the inside temperatures dropped down into the 30s on the coldest days. It is interesting to see that the two colonies were very consistent with each other, rising and falling in tandem.

It also became clear that the interior of the empty hive box was somewhat warmer than the outside air. I suspect a combination of sun and minimum air movement through the boxes increased the temperature slightly.

Thank you, Bill, for your experiment and awesome graph. Nicely done! Now we can see how honey bees stay warm through a cold winter.

*Ambient Weather WS-10 Wireless Indoor/Outdoor 8-Channel Thermo-Hygrometer with Three Remote Sensors

Honey Bee Suite

Graph showing the temperatures inside the three hives, and beginning November 15, outside the hives.
Graph showing the temperatures inside the three hives, and beginning November 15, outside the hives. © Bill Reynolds.

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  • I find these studies of internal temps fascinating. I work for an HVAC controls manufacturer. We make the computer controllers that monitor sensors and control motors, actuators, heaters, and anything else that either needs an ON/OFF control or variable output control (i.e. 0-10v output). So it’s not been far from my mind to setup a similar experiment with a number of strategically placed thermistor sensors in a hive to do something similar. Perhaps in a few years I might take the time to do something like this and even put access to the temps and trends in real-time on the Internet for others to view. I think it’d be interesting to know how the temps both in winter and summer vary from ambient outside temps for my hives. For now, I’m satisfied just reading about other people’s experiments like this one. If you know of others, please post them.

    Somewhat unrelated, I have been monitoring my cluster by putting the white boards under the hives. Some of that was to help keep the wind out of the hive (I know you say that isn’t necessary). But I also wanted to monitor where the hive cluster was…mostly out of curiosity. And sure enough, you can see exactly where they are by the debris left on the board just after 24hrs. And as usual, you are right, they do still have varroa. I saw 4 dead ones on the board.

    And don’t worry, I won’t leave the boards in there just to continue to collect debris. I know that’s a breeding ground for beatle eggs. Although in the temps we are getting right now, would any eggs actually survive?

    • Chris,

      The beetle eggs won’t survive a good freeze, so it depends on where they are. Eggs on the debris board most likely will freeze. Good riddance.

  • “Because heat rises,”….. Well, heat moves from hot areas to cooler ones. Saying “heat rises” is actually true, but only as much as it also moves sideways or downward in a given medium for all three major means of heat flow, and in the absence of a medium (a vacuum), by radiation. It is better to note that “hot air rises.” That is caused by the buoyancy of heated air, which is less dense than cold air.

    What an annoying comment by me, eh?

    • Rich,

      1) You are right, of course. I will change the phrase to “hot air rises.”

      2) Yes, you are annoying. But after 40 years, I’m used to it, eh?

  • Bill, thank you for the detailed experiments and data! Readers, an indoor-outdoor temperature monitor is available from Oregon Scientific. I would not know what to do without mine for greenhouse, barn and garden.

    About the slightly warmer temperature in the unoccupied hive box: assuming you are using screened bottom boards and no varroa drawer, that warmth could be geothermal. I am reminded every sub-zero day I walk inside the barn (dirt floor and good roof, but no heat source besides 12 goat and 3 horse bodies), that warm air rises from dry ground, which is usually at about 40 degrees.

    Yesterday, anticipating last night’s 13F, I placed the insulation-board shelters around all four hives. I did not add the varroa boards, for the sake of ventilation. Now that Bill’s work has called to mind ground heat, probably just as well. We’ll see.

    Thanks Rusty and Bill!

    Shady Grove Farm
    Corinth, Kentucky

    • Nancy,

      You are absolutely right. I have a garden shed with a concrete floor. It stays about 40 degrees all winter long regardless of the outside temperature. It always amazes me.

  • Very cool experiment! I actually built an electronic sensor board that doubled as a ventilation board for my hive this year. I had one sensor on the outside of the hive, one sensor that I dangled down into the brood area and one sensor inside the vent board on top of the hive. It was very interesting to see the data and how they regulate the temperature in the brood nest to around 95 no matter what. You can see it clear as day in the graphed data here:

  • Along these lines, I noticed some of my bees are flying today as the outdoor temp approaches 40 F. for the first time in what seems like a few weeks. We had a lot of rain right before Thanksgiving, then 4″ of snow, followed by a 4 day-long cold snap that’s just starting to ease. Glad to see them out doing the needful, though I thought they wouldn’t/couldn’t fly in temps much below 50. I sure am glad I got the moisture quilts/boxes on before the weather really set in. Didn’t get the slatted racks in place in time, so I imagine it’s going to wait until spring now.

    • Aaron,

      I’ve seen them fly in cold weather as well, but I think they just make quick cleansing flights and return. They don’t go foraging for anything.

  • Dear Rusty,

    I have a question I hope you might be able to answer: I have a varroa floor and a board with sticky paper underneath. Yesterday, I changed the sticky paper and noticed dead bees on it. I had a look underneath the mesh floor and I noticed a cluster of bees there. I had closed the gap at the back of the hive to try to keep the bees as warm as possible with just enough space for air to come through for ventilation. Still about 30 bees had managed to creep in and clustered there. Although we are in Winter, the temperatures over the last few days have been quite mild, about 10 degrees celsius. Would you know why this happened? Have you encountered it before?

    I left the bees where they were because it was the end of the day. Today, I removed them because they were all dead unfortunately and I completely blocked the gap, just allowing a bit of air to go in and out but no bees.

    Thanks for your reply.

    • Chantal,

      I can only guess. Since 10 degrees is warm enough for some of the bees to fly, they may have left for a cleansing flight and not been able to find their way back into the hive. They followed the scent (perhaps) that led them to the area underneath the varroa floor.

      I haven’t seen this exact scenario, but I have seen confused bees gather on the outside of a hive or underneath it. If your bees were hatched since the colony has been clustering for winter, they may be inexperienced with finding their way back home.

      May sure you leave an entrance for them to come and go. They will need it on those warmish days.

  • I am unclear on ventilation for the hives. How can I tell if there is enough? How do I increase/decrease it? What are the patterns of air flow in Langstroth and in top bar hives?
    Because it is so damp here, I was thinking to put a seeding mat under the back half of my top bars (I have those narrow 6″ wide ones!), hoping it would keep it a bit warmer and therefore drier. Will the screened holes at the top of the body provide enough air circulation, and will the bees really remove the propolis which now seals these holes if they need more?

    • Eddy,

      For more about ventilation and air flow in hives, go the the main menu (top the the page) and click on “Physics” tab.

      Be careful of using artificial heat in your hives. The bees may be fooled into thinking the outside temperature is warmer than it actually is. They may flight out into the cold and be unable to get back.

  • Rusty, The combs from the hive on a limb by my house blew down and had @ 3,000 dead bees with their heads buried deep in cells with their butts barely above the cells in most cases, but some were below the cell tops. Seeley said they did this in a vain attempt to keep the colony warm enough. There was no honey and Seeley said that was because, they eat far more when they’re cold. I will send photos asap. Mike

      • Will send soon–every time they “upgrade” my computer, they downgrade my ability to use it? But a friend who knows computers far better than I, is coming over soon.

              • Rusty, Okay! Even though it seldom reached 50F here after Nov. 7 and dipped to 21, 23, 18, and 22 the next 4 days, yet at least 100 bees survived in this open nest and the last few were seen on Dec. 7. Besides having no honey, this hive had no larvae or unhatched adults. The @ 3,000 bees with their butts sticking up out if cells had about 800 bees standing on top of them. Even though strong winds (gusts of @ 50 mph) blew these combs off of the limbs, they did not blow the bees off of the other bees’ butts. Even though dead their clinging was amazing. The cold preserved the bees quite nicely, so I think you’ll like the photos. I often handled many bees, brushing them off with my hands. It seems these winter bees had so much fat in their abdomens that I never got stung. I know for sure this wouldn’t be the case with yellowjackets, as I’ve been stung twice while handling a dead one. You’d think I’d have learned after the first time?? I was surprised by how soft (cardboard-like), the wet combs were. I had to be careful when pulling the bees out of the combs (using a tweezer to grab their butts). I learned, from collecting hornet nests, that larvae and/or adults stink to high heaven when they rot. Mike

  • Rusty, Once it turned cold, I could see the bees butts sticking out between the combs. All of those bees on the outside kept dying and the process kept repeating until there weren’t enough to keep any alive. But I guess the bees with their butts up in the cells helped keep the living bees alive long after the bees with their butts up died? Mike

    • Mike,

      I’ve always heard that bee butts up meant that the bees were searching for honey in the bottom of the cells. I’ve also heard that the warmth of their bodies in the cells helps to keep adjacent larvae warm. Maybe both are true. I don’t really know.

  • Rusty, Have you or any of your readers ever seen this “butts-up-in-cells” in box hives? I haven’t seen this behavior mentioned anywhere on the net? Mike

    • Mike,

      It’s actually really common. If you search for “heads down” instead of “butts up” I think you will find more.

  • Rusty, It may be common, but it’s not “common knowledge,” as not even Seeley knew about this!? The ONLY place I could find on the net with this information was the famous “Honey Bee Suite,” under, “Why so many starving bees?” The photo there is just like mine and hmm–what you’re saying makes perfect sense because, the honey was all gone. I am going to quote the paragraph right before the photo to Seeley and I’m sure it will change his mind. It must have just been coincidental that the butts probably helped keep some bees warmer and alive a little longer. I’m thinking that the bees on top of the butts, might have been there because, it was the warmest place to be?

  • Who knows how I ended up at this 6 year old post, but it was very timely. I got a BroodMinder temp sensor in January, and when I first put it in, the readings were around 90F in the daytime, dropping to 40/50F at night. OK, it was a mild enough winter that it was plausible that there was brood in progress already, and the sensor was near an upper entrance, so it was also plausible that the local temp would drop at night. Then we had another cold snap, and the inside temp slipped to 40/45F day AND night. Oh, no! but they were still buzzing when I rapped the side. Same pattern every other week as the weather continued to see-saw. Oh, no, oh, no, are they gonna make it? Seeing Bill’s ambient temps drop so low in mid-winter was very reassuring.

    Eventually, the average started to creep up, and for the last 3 weeks the temp hasn’t dropped below 80F (and is peaking at 90/95F). That included a weekend dropping to freezing point again. Woo-hoo! I think she’s back in production for good!

    Waiting for a non-windy/rainy day for a full inspection, but I’m glad to have that temperature tool in the meantime.

  • I am trying to nurse a tiny colony (no more than 1,000 bees) through this Kansas winter so I have wrapped my Layans hive (1.5″ thick plank walls) in heat tape and want to put a wireless, Oregon Scientific sensor in there to monitor the temperature. I have also wrapped the entire hive in R-13 batt insulation. My object is to minimize the work for the bees to get to spring. The last time I checked (a month ago) there were a few larva and eggs so the queen was active. I’m trying to guess what temperature I want that sensor to read. This is really messing with mother nature so I’m sure no matter what I do I’m going to upset something. On the other hand, I think that without some relief they will not make it. It is a local colony trapped in October so I really want to save it if I can. So my questions are where best to place the sensor and what target temperature?

  • The low last night was 9°F so I left the heat tape on all night. The temperature in the hive stayed within a few degrees of 44°F so I think that was a good night. I am still hoping to hear some feedback on what I am trying to do with this tiny colony I trapped late last fall.

    My source for this method is a study done by the Agricultural Extension of Wisconsin which they reported in their technical bulletin 1377 in 1967. I was hoping someone had tried this and had some ideas for me.

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