What the Heat Index Actually Measures

The number is real, and it is describing the shade

The heat index is not a vibe. It is the Rothfusz regression, a curve the National Weather Service fits to Steadman's human-comfort model, and this calculator runs that equation rather than an approximation of it.

But it was built for one specific situation, and the NWS says so plainly:

Heat index values "were devised for shady, light wind conditions, and exposure to full sunshine can increase heat index values by up to 15°F."

That is not a footnote. It is the difference between one risk band and the next.

Humidity does more than people expect

Hold the temperature at 90°F and change only the humidity:

At 90°F	Feels like
40% humidity	91°F — barely above the air
60% humidity	100°F

Nine degrees, from humidity alone, at an identical thermometer reading.

The reason is that sweating is the entire cooling mechanism. Your body does not cool by producing sweat, it cools by evaporating it — and evaporation slows as the surrounding air fills with water. In humid air the system is throttled, and the heat your muscles produce has nowhere to go.

Which is why dry heat is genuinely different

Run it the other way and the effect inverts:

	Air	Feels like
Humid	84°F at 90%	98°F
Dry	110°F at 10%	104°F

Air that is 26 degrees cooler feels only 6 degrees cooler.

And notice the dry case: 110°F feels like 104°F — six degrees cooler than the thermometer says. That is not an error in the tool. In very dry air your sweat evaporates so readily that you shed heat faster than the raw temperature suggests, and the "hotter than the air temperature" output goes negative to say so.

"But it's a dry heat" is, annoyingly, correct.

The sun figure is the one that changes your afternoon

Take the default case — 90°F at 60% humidity:

In the shade	100°F → Extreme Caution
In full sun, up to	115°F → Danger

Same air, same hour. The shade number and the sun number sit in different NWS bands, and the bands mean different things: Extreme Caution says heat exhaustion is possible with prolonged exposure or activity, while Danger says cramps or exhaustion are likely and heat stroke is possible.

If you are working, playing or exercising outdoors and uncovered, the sun figure is the one describing your day. It is shown as its own output rather than folded into the headline precisely because it is an upper bound, not an average — averaging it in would misrepresent both numbers.

Most calculators implement one third of this

The NWS procedure is three formulas, not one, because a fitted curve misbehaves at its edges:

1. A simple formula first. Averaged with the air temperature; if that average is under 80°F, the simple form is the answer and the regression never runs.
2. A low-humidity subtraction when humidity is under 13% and the temperature is 80–112°F.
3. A high-humidity addition when humidity is over 85% and the temperature is 80–87°F.

All three are implemented here. Both adjustments were verified against the NWS's own published chart — 90°F/60% returns exactly 100, 90°F/40% returns exactly 91, and 100°F/40% returns exactly 109.

A calculator that skips the adjustments is wrong specifically in unusual weather, which is exactly when somebody bothers to look the number up.

The bands, and the words inside them

Band	Heat index	What NWS says
Caution	80–90°F	Fatigue possible
Extreme Caution	90–103°F	Cramps, exhaustion or heat stroke possible
Danger	103–124°F	Cramps or exhaustion likely
Extreme Danger	125°F+	Heat stroke highly likely

Every one of those descriptions carries the same qualifier: "with prolonged exposure and/or physical activity." The bands are about what you are doing in the heat, not about standing still in it. The same 103°F afternoon is a different proposition for someone reading in the shade and someone roofing a house.

⚠️ What this cannot tell you

It describes air, not a person. The equation assumes a notional adult in the shade, and the factors that actually decide who gets into trouble are not inputs: age, heart and lung conditions, many common medications, whether you are acclimatised, whether you are drinking, and how hard you are working.

There is also no wind input, because the NWS heat index assumes light wind and adding one would need a different model rather than a correction to this one. Moving air usually helps — but above roughly body temperature, strong dry wind can heat you faster than evaporation cools you.

And the distinction worth carrying: heat exhaustion looks like heavy sweating, weakness and clammy skin — get cool, sip water, and it usually turns around. Heat stroke looks different: hot dry skin, sweating that has stopped, confusion or collapse. That is an emergency needing cooling and an ambulance, not a rest and a drink, and the change between the two can be fast.

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Equation, adjustments and risk bands from the National Weather Service. General information, not medical advice.