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I’m hoping somebody can answer this one for me. I’ve had no luck finding the answer.

I understand the idea behind the heat index. It combines the effects of temperature and humidity to show what it “feels like” to the body.

The problem is that the temperature it “feels like” itself has a humidity level.

So if it “feels like” 90F, it feels like 90F at what humidity level? There is no such thing as temperature without a corresponding humidity level.

The baseline for heat index is 77 ºF @ 50% humidity.

Heat index calculation contains assumptions about the human body mass and height, clothing, amount of physical activity, thickness of blood, sunlight and ultraviolet radiation exposure, and the wind speed. Significant deviations from these will result in heat index values which do not accurately reflect the perceived temperature. The heat index implies percieved shade temps with stagnant air.

Empirically the baseline heat index (cited above) is derived from actual air temperature given the partial pressure of water vapor to be equal to a baseline value of 1.6 kPa. At standard atmospheric pressure (101.325 kPa), that corresponds to a dew point of 14 °C (57 °F) and a mixing ratio of 0.01, i.e., 10 g of water vapor per kilogram of dry air. Bear in mind that the density of air decreases as temps increase. So a larger volume of air is necessary to contain the same air mass. The vapor pressure of water also increases, and the corresping saturation temp likewise increases.

At high temperatures, the level of relative humidity needed to make the heat index higher than the actual temperature is significantly lower than at cooler temperatures. For example, at approximately 80 °F, the heat index will agree with the actual temperature if the relative humidity is 45%, but at about 110°F, any relative-humidity reading above 17% will make the Heat Index higher than the ambient 110 °F.

A parcel of air with a heat index of 111.7 ºF and mixing ration of 0.01 corresponds to 16% humidity; 111.7 ºF heat index is physicologically congruent to 100 ºF @ 43% humidity OR 77 ºF @ 50% humidity. All three values are congruent as having the same wattage of convective heat transfer per unit time.

Frankly, I prefer the old THI, i.e., nobody is uncomfortable at 70, 1/2 the population is unconfortable at 75, and virtually everybody is uncofortable at 79. The old THI is currently 86.3 here in Detroit.