Heat Stress in Bee Colonies: Using the Temperature-Humidity Index
How rising temperature combined with humidity threatens colony cooling capacity, and how the Temperature-Humidity Index helps beekeepers act before brood and bees are harmed.
Why humidity matters as much as temperature
A honey bee colony cools itself primarily through evaporation - fanning air across water droplets that foragers bring in, and spreading a thin film of water across comb surfaces so it can absorb heat as it evaporates. That mechanism depends on the surrounding air having capacity to take up more water vapour, which is exactly what humidity determines. A 35°C day at 30% relative humidity leaves a colony plenty of evaporative cooling capacity to work with; the same 35°C at 80% humidity leaves almost none, because the air is already close to saturated and evaporation slows dramatically.
This is why beekeepers assessing heat risk purely by thermometer reading routinely underestimate danger in humid climates and overestimate it in dry ones. A combined measure that accounts for both variables gives a far more honest picture of how much cooling stress a colony is actually under on a given day.
The Temperature-Humidity Index explained
The Temperature-Humidity Index, or THI, is a single calculated value combining air temperature and relative humidity into one number that tracks how much heat stress a colony is likely experiencing. One common formulation is THI = (1.8 x T + 32) minus [(0.55 minus 0.0055 x RH) x (1.8 x T minus 26)], where T is air temperature in Celsius and RH is relative humidity as a percentage. The formula looks dense but the underlying logic is simple: it starts from a Fahrenheit-scaled temperature and then subtracts a correction that grows smaller as humidity rises, since higher humidity reduces the effectiveness of evaporative cooling and therefore pushes the felt stress level upward relative to temperature alone.
In practice, most beekeepers do not calculate THI by hand every day - many weather stations and hive-monitoring platforms compute it automatically - but understanding the formula helps explain why a muggy 30°C afternoon can be more dangerous to a colony than a dry 34°C one, a pattern that pure air-temperature thresholds miss entirely.
Reading the THI scale
Below roughly 70, colonies operate normally with no meaningful cooling burden. In the 70s, bees increase fanning modestly and forager activity holds steady, representing mild, manageable stress. Once THI climbs into the 75-80 range, bearding - clusters of bees hanging outside the entrance to reduce internal crowding and heat load - can begin, foraging efficiency starts to drop, and brood development may slow slightly as nurse bees redirect effort toward cooling rather than feeding.
Above roughly 81, stress becomes serious: heavy bearding, aggressive fanning, and clustering outside the hive become the norm, brood mortality can rise noticeably, and honey production drops as foragers stay home to help cool the nest instead of collecting nectar. At the most extreme end, THI values at or above the high 80s represent a genuine emergency, where colonies can suffer rapid, serious harm within just a few hours if nothing is done to relieve the heat load.
Practical mitigation steps
Shade is the single most effective and lowest-cost intervention - even partial afternoon shade from a tree, shade cloth or temporary screen measurably reduces the direct solar load a hive box absorbs, which matters because a hive sitting in full sun on a still, humid afternoon experiences a materially higher internal temperature than an identical hive in dappled shade nearby. Reliable water sources placed close to the apiary reduce the distance and time foragers spend collecting water for evaporative cooling, freeing that labour for other tasks and reducing the risk of bees drowning in unsuitable water sources further afield.
Improved ventilation - screened bottom boards, upper entrances, or simply cracking a hive lid slightly during extreme heat - helps hot, humid air escape rather than accumulate inside the box, and reducing colony crowding by adding supers ahead of a heat event gives bees more internal volume to disperse into. In genuinely extreme, sustained conditions, some beekeepers relocate especially vulnerable colonies - nucs, recently split colonies, or those in known heat-trap locations - to cooler sites entirely rather than trying to manage the heat in place.
Why this matters more with a changing climate
Heatwave frequency and intensity have been rising across much of the UK and Europe, and summers that once produced only isolated hot days now more often deliver multi-day stretches of sustained heat, sometimes combined with the kind of still, humid conditions that push THI values highest. Apiary sites chosen decades ago for good forage and shelter from wind may no longer offer adequate shade as surrounding trees are removed or as summer sun angles interact differently with a site's layout, making a periodic reassessment of apiary siting a genuinely useful exercise rather than a one-off decision made at setup.
Tracking THI - even informally, using a simple phone weather app and the threshold table rather than continuous monitoring equipment - gives beekeepers an early warning system that a bare temperature forecast does not, letting shade, water and ventilation measures go in before bearding and brood loss are already underway rather than as a reaction to visible distress.
Frequently Asked Questions
At what THI value should a beekeeper intervene?
Many practical guides treat THI in the mid-70s as the point to start adding water sources and shade, THI in the 80s as a period requiring active mitigation, and THI at or above the high 80s as an emergency requiring immediate intervention such as extra ventilation, shade or relocation.
Can a hot but dry day still stress a colony?
Yes, but generally less than the same temperature combined with high humidity, because dry air lets evaporative cooling work efficiently. THI accounts for this by weighting humidity into the overall stress calculation rather than relying on temperature alone.
Does adding more ventilation always help in heat?
Increased ventilation generally helps hot, humid air escape the hive, but it needs to be balanced against colony size and defensibility - very large openings on a small or weak colony can create other problems, so moderate, adjustable ventilation is usually preferable to maximal ventilation at all times.
Is heat stress a growing concern for UK beekeepers specifically?
Yes. While the UK's climate has historically been temperate, the increasing frequency of prolonged summer heatwaves means beekeepers who previously gave little thought to heat management are now finding it a relevant seasonal risk, particularly for hives in exposed, sun-facing or urban locations with limited natural shade.