Bees cool the hive mainly by evaporation - fanning air across water droplets and comb films. Humid air is already close to saturated, so evaporation slows and the same temperature feels far hotter. The Temperature-Humidity Index (THI) combines both into one stress number.
THI = (1.8·T + 32) - [(0.55 - 0.0055·RH) × (1.8·T - 26)]
T = air temperature (°C), RH = relative humidity (%)
Stress scale:
< 70 normal, no cooling burden
70-75 mild, modest fanning
75-80 moderate, bearding begins
80-88 serious, heavy bearding, brood risk
>= 88 emergency, act immediately
- Temperature / humidity — set the ambient weather; THI recomputes live and drives how many bees beard outside the entrance.
- Shade cloth — cuts direct solar load on the hive box, modelled here as roughly -4 THI of effective stress.
- Ventilation — screened entrances or a cracked lid let hot, humid air escape, modelled as roughly -3 THI.
Both mitigations stack, so a shaded, ventilated hive can stay calm at conditions that would otherwise trigger heavy bearding and falling honey production.