Temperature and Humidity Monitors

Hive temperature and humidity sensors measure the heat of the brood area and the humidity inside the hive, showing without opening it whether the colony has brood, what the cluster is doing in winter and whether there is a risk of condensation. They range from simple digital thermometers to wireless data-logging sensors. Placed correctly, they give valuable information, especially for detecting the broodless period and for wintering.

Why measure temperature and humidity inside the hive?

A honey bee colony behaves like a "superorganism" that regulates its own temperature. A breakdown in this regulation is often the first sign of a change inside the colony: brood coming to an end, queen loss, weakening or, in winter, the cluster shifting position.

• Is there brood? A stable, high temperature indicates brood.
• When did the broodless period start? This is the best window for contact varroa treatments.
• Where is the cluster in winter? With several sensors, the cluster's movement towards the stores can be followed.
• Is there condensation? Humidity staying close to saturation suggests that water is collecting on the crown board and dripping onto the cluster.

How do bees regulate hive temperature?

The brood area is kept within a fairly narrow range of about 34–36 °C. Bees that develop from pupae kept outside this range for long periods can show wing deformities and behavioural and learning problems.

• Heating: Worker bees generate heat by shivering their flight muscles without moving their wings. Heater bees sit on top of brood cells or crawl into empty cells to warm their surroundings.
• Cooling: In the heat, bees fan their wings at the entrance to create an air current, and they carry in water and evaporate it. In extreme heat some of the bees gather on the front of the hive; this is called bearding.
• Winter cluster: When there is no brood the bees form a cluster. The centre is warmer and the outer shell cooler; bees in the shell try not to fall below about 8–10 °C. The bees heat the cluster, not the hive; so the empty corners of the hive can be close to outside air temperature.

Types of temperature and humidity sensor

How do you interpret temperature data?

A single reading says very little; the real information is in the trend. Think in terms of a series over several days:

• Constant 34–36 °C with a small day–night difference: There is brood under the sensor.
• Fluctuating temperature that follows outside air: There is no brood under the sensor, or the cluster has moved elsewhere.
• Gradual drop in autumn: Brood rearing is declining; the broodless period may start within a few days.
• Rising again and stabilising at the end of winter: The queen has started laying; store consumption will speed up.
• Sudden drop in mid-season: It could be queen loss, a swarm or serious weakening; open the hive and look.

Why does humidity measurement matter?

Condensation in winter

As the cluster consumes honey it produces water vapour. This vapour condenses on a cold crown board or wall. A well-fed colony can usually withstand the cold; the real danger is water dripping onto the cluster. A humidity sensor gives early warning of a ventilation or roof insulation problem by showing that the humidity inside the hive stays close to saturation for long periods.

Honey ripening in summer

Bees release water from nectar into the air inside the hive and expel it by fanning. In very humid weather this process slows down and capping of the honey is delayed. High humidity readings in this period make a refractometer measurement before harvest all the more important.

Honey room and store

Honey is hygroscopic, meaning it absorbs moisture from the air. Supers left standing in a humid room, or honey in an open container, take up water. Keeping the relative humidity in the honey room below about 60% is a good target; a room thermo-hygrometer is enough for this.

Where and how should the sensor be placed in the hive?

1. Centre of the brood area: The temperature sensor goes between, or just above, the top bars of the two frames in the middle of the brood area.
2. Several sensors in winter: If possible, put one in the centre, one at the side and one on the side with the stores to follow the cluster's movement.
3. Humidity sensor: Slightly above the cluster, near the underside of the crown board, protected from direct contact with wax and propolis.
4. Cable and housing: They must not block bee traffic or get crushed when the lid is closed.
5. Outdoor sensor: For comparison, also place an outside air sensor in the shade, sheltered from rain.

What should you look for when choosing a temperature and humidity sensor?

• Accuracy: About ±0.5 °C for temperature and ±3 points for humidity is enough for a hive.
• Protection: A casing that resists propolis and moisture; a porous protective cap for humidity sensors.
• Logging interval and memory: A reading every 15–60 minutes and memory for at least one season.
• Battery and cold: Battery capacity drops in winter; check the operating range in the cold.
• Data export: Being able to export the data as a table is important for long-term comparisons.

Simple accuracy checks

If you have more than one sensor, leave them all in the same box for a few hours and compare the readings. The salt test is a well-known method for humidity sensors: the air above a saturated table-salt solution in a closed container reaches about 75% relative humidity at room temperature. If the sensor deviates clearly from this value, correct its readings accordingly.

Limitations and common mistakes

• Forgetting that the sensor does not tell you the cause: a temperature drop can mean the end of brood, queen loss or disease.
• Placing the sensor outside the brood area and concluding that "the colony is cold".
• Continuing to trust a humidity sensor coated with propolis; clean and check it at the start of the season.
• Opening the hive often for winter data; the aim is exactly the opposite — to monitor without opening.
• Fitting a sensor to every hive: a few representative colonies are usually enough.