Hot and humid weather is quite possibly the worst weather, as most people will feel sticky and uncomfortable, to the point that it will affect their mood and ability to think. This combination is so terrible that weather forecasts often mention a discomfort index (or temperature-humidity index) to highlight how hot and humid the day will be. Discomfort index is calculated as:
DI = 40.6 + 0.72 (dry-bulb temperature + wet-bulb temperature ).
Here, dry bulb temperature is the “ambient temperature” (not considering humidity), while wet bulb temperature accounts for humidity by looking at how low the temperature can get by evaporating water.
Evaporation absorbs heat but can only happen if the air is not saturated with humidity. Therefore, the more humid it is, the more “discomfort” we feel as we cannot sweat off the heat building up inside our bodies.
When the DI is at 70, about 10% of people experience discomfort. At 75, 50% feel discomfort and at 80, most people will feel extremely uncomfortable. A DI of above 85 is virtually intolerable and anything above this, serious conditions such as heat exhaustion and heat stroke can occur.
As our core body temperature rises and we cannot cool down by sweating, we experience thermal stress. Under thermal stress, our concentration and task performance begins to suffer – a phenomenon people will describe as their brain feeling as if it is melting. This is a well-established phenomenon that has significantly affected how architects design offices and homes to improve air flow and temperature control to create an environment with the least thermal stress possible – for both efficiency and comfort.