The Number That Makes 88 Degrees Feel Like 100
On Friday, July 17, the National Weather Service posted a Heat Advisory for Brookings County, warning that the heat index would climb to 100 between 1 and 9 p.m. If you checked a thermometer that afternoon, though, you might not have seen triple digits at all. The actual air temperature that day was likely somewhere in the upper 80s or low 90s. So where did the 100 come from?
That gap between what a thermometer says and what a body feels is the whole point of the heat index, sometimes called the “feels like” temperature. It is not a measurement in the way degrees Fahrenheit is a measurement. It is a calculation, one that tries to capture something thermometers cannot: how humidity interferes with the body's main cooling system.
Here is the mechanism. When it is hot, the body sweats, and as that sweat evaporates off the skin, it carries heat away. This is basic physics, the same principle that cools a glass of water wrapped in a wet towel. Evaporation is what does the work. But evaporation depends on the air's capacity to absorb more moisture. Dry air is thirsty, so sweat evaporates fast and cools efficiently. Humid air is already saturated, so sweat lingers on the skin, doing little. The heat index tries to translate that reduced cooling efficiency back into a temperature number, essentially answering the question: if the air were dry, how hot would it have to be to feel this oppressive?
The formula behind it traces back to work by physicist Robert Steadman in 1979, who modeled a hypothetical person, roughly 5-foot-7, 147 pounds, walking outdoors in the shade, and calculated how various combinations of temperature and humidity would affect that person's perceived heat load. The National Weather Service still uses a regression equation built on Steadman's model today, an approximation refined over decades but still, at its core, an estimate rather than a physical constant like temperature itself.
That is worth sitting with for a second. A heat index of 100 is a modeled judgment, not a direct reading. Two locations at the identical air temperature can carry very different heat index values depending purely on moisture in the air. This is why South Dakota summers can feel deceptively brutal even when the mercury seems unremarkable. A humid air mass moving up from the Gulf, common in mid-July, can push the heat index well above the actual temperature, sometimes by ten or fifteen degrees, even though nothing about the sun's intensity has changed.
It also explains why the advisory had a defined window, 1 to 9 p.m., rather than covering the whole day. Heat index calculations shift constantly as both temperature and humidity move throughout the day. Peak heat typically overlaps with peak humidity in the mid to late afternoon, which is why warnings cluster there rather than at dawn.
There is a broader, more tentative thread here too. Climate researchers have been tracking whether heat index events, as opposed to raw temperature extremes, are becoming more frequent across the Midwest. Warmer air holds more moisture, a straightforward property of physics, which some scientists argue could make humid heat spikes more common even in years when peak temperatures themselves are not record breaking. This remains an active and evolving area of research, not a settled conclusion, and any single advisory like the one Brookings saw in July says nothing on its own about long term trends. One hot, humid afternoon is weather. Patterns across decades are climate, and untangling the two takes careful, ongoing analysis.
What the heat index does offer, reliably, is a translation service between raw meteorological data and lived experience. It is the National Weather Service's attempt to answer a very human question with a number: not just how hot is it, but how hard is your body having to work right now. The next time an advisory mentions a heat index well above the actual air temperature, that gap is not exaggeration. It is humidity, quietly making the case that not all heat is created equal.