Heat
The apparent temperature on your forecast is the output of a regression fitted to a human thermoregulation model from 1979. Knowing its assumptions tells you when to distrust it.
Air temperature alone is a poor predictor of heat stress, because the body's main defence against heat is evaporative cooling and evaporation depends on how much water vapour the surrounding air can still accept. The heat index exists to combine the two.
In 1979 R. G. Steadman published a model of human thermal comfort that solved a heat-balance equation for the vapour pressure required to maintain a defined level of thermal stress. The model carried a long list of stated assumptions, and produced a table of apparent temperatures.
In 1990 Lans Rothfusz fitted a multiple regression to Steadman's table so it could be computed directly from temperature and relative humidity. That polynomial, with two correction terms for low and high humidity extremes, is what the National Weather Service publishes today.
It is therefore a curve fitted to a model, not a direct measurement. It reproduces Steadman's table to within about 1.3 degrees Fahrenheit.
Steadman's reference person was specified in detail, and every one of those specifications is a place where your situation may differ.
Together these mean the forecast heat index systematically understates the stress on someone exercising hard in open sun. A published index of 95°F can correspond to conditions well into the danger category for a runner on an unshaded road at noon.
| Heat index | Category | Effect with prolonged exposure or exertion |
|---|---|---|
| 80–90°F | Caution | Fatigue possible |
| 91–103°F | Extreme caution | Heat cramps and heat exhaustion likely |
| 103–124°F | Danger | Heat exhaustion likely, heat stroke possible |
| 125°F+ | Extreme danger | Heat stroke highly likely |
Note the qualifier attached to every row: prolonged exposure or physical activity. These are not thresholds for sitting still.
Sweat that drips off you has cooled nothing. Only the fraction that evaporates removes heat, and the rate of evaporation depends on the vapour pressure gradient between wet skin and surrounding air.
As relative humidity rises, that gradient narrows. At high enough humidity, evaporative cooling approaches zero regardless of how much you sweat — and sweating that produces no cooling still produces fluid and electrolyte loss. This is the specific reason 90°F at 80 percent humidity is genuinely more dangerous than 100°F at 20 percent, despite the lower number on the thermometer.
Sports governing bodies, the military and industrial safety programmes generally use wet bulb globe temperature rather than heat index. WBGT is a measured composite:
WBGT = 0.7 × natural wet bulb + 0.2 × black globe + 0.1 × dry bulb
The natural wet bulb thermometer captures evaporative potential including wind. The black globe thermometer captures radiant load from sun and hot surfaces. The dry bulb captures air temperature. The weighting reflects their relative contribution to heat strain.
WBGT is superior for exertion decisions precisely because it measures the two things the heat index assumes away: sun and wind. It requires instrumentation, which is why it has not replaced the heat index in public forecasting.
Separate from WBGT is plain wet bulb temperature — the lowest temperature achievable by evaporative cooling in the given air. It represents a hard physiological ceiling.
Human core temperature sits near 37°C. Skin must stay cooler than core for heat to flow outward. Above a wet bulb temperature of about 35°C, that gradient cannot be maintained by any amount of sweating, and core temperature rises regardless of behaviour, hydration or fitness. Recent research has found brief exceedances of this threshold in parts of South Asia and the Persian Gulf, previously thought to be theoretical.
Treat the published heat index as a floor rather than an estimate. Add roughly 15°F for direct sun. Add further for hard exertion, heavy clothing, or being unacclimatised. Subtract nothing for a breeze unless the air is meaningfully drier than your skin.
And remember what the categories describe. A heat index of 95°F is a mild afternoon for someone reading in the shade and a genuine risk for someone running intervals in it.
The heat index and hydration calculator computes the Rothfusz value and pairs it with a sweat-rate estimate scaled by your body mass, your metabolic intensity and your acclimatisation state.