This question was redirected to me from another website. It is such an interesting topic that I decided to do an entire post on it.
The beekeeper who wrote was concerned that ventilation above the honey supers would cool the hive so much that the bees would not be able to dry (dehydrate) and cap the honey. This is a reasonable thought, but it is based on the mistaken belief that heat is responsible for drying the honey. In fact, it’s not heat that dries the honey, at least not directly. Honey is dried when air with a capacity to hold moisture flows over its surface.
Forget your hive for a moment and think about your clothes dryer. It has a barrel in the center that goes round and round so the clothes don’t lie in a heap. This exposes the clothes to as much air as possible. Your dryer has an air intake, a vent to the outside, a heater, and a fan. Air comes into the machine and is warmed by the heater. Warm air can hold more moisture than cool air, so the warm air absorbs some of the water from the clothes. The fan expels this moist air through the vent as new air comes in. In turn, this new air is warmed so it can hold more moisture, which allows additional evaporation from the clothes. This keeps happening until the clothes are dry.
If you were to block the air intake or the vent to the outside, the clothes would not get dry. They would get even hotter, but they would stay wet.
This may have happened to you. If your vent gets plugged with lint the first thing you probably notice is that your clothes take forever to dry. They go round and round, they get hot, but in the end they are still wet. In fact, the lint may get so hot it ignites. Vents plugged with lint are a common source of house fires.
Now, back to the hive. Instead of wet clothes you have wet honey. It doesn’t need to be rotated because the bees have hung it up to dry in neat rows much like parallel clothes lines. Each honey cell is exposed to the air. Instead of a heater you have heat from the sun (directly or indirectly) and heat from the bees, and instead of a fan you have thousands of bee wings. You have an air intake (front entrance) and, we hope, a vent to the outside, which may be an upper entrance, a vented inner cover, or just a loose-fitting, leaky hive.
Air around the honeycombs is damp just like air around the wet clothes. To dry it, the bees fan their wings and bring in outside air. This new air has a lower relative humidity than air inside the hive and, as it passes through the hive, it gets even warmer due do the many hard-working bees. As a result, this air has lots of capacity to absorb moisture from the honey, which it does. The air current from the bees’ wings expels this humid air to the outside.
With good ventilation through the hive, the bees can dry the honey quickly. Once the moisture level reaches about 18% the honey is capped and the job is done.
However, if the vent to the outside is plugged with something like a lid, the moist air cannot be expelled and the honey cannot get dry. No matter how hot it gets, no matter how hard the bees work, it just stays wet. Once the air around the honey absorbs the maximum amount of moisture, no more can evaporate, and the honey cannot be capped.
Just remember it is air with a capacity to hold moisture that dries things, not heat. The take-home message is that more summer ventilation means the bees can dry more honey faster. In the end, that’s what we all want.
P.S. Special thanks to my M.E. consultant for checking my logic.