ReckonBoxWhat the heat index tells you
The heat index is the temperature it feels like when humidity is combined with the air temperature, because humid air stops sweat from evaporating, which is the main way your body sheds heat. At 95F with 55% relative humidity, it feels like about 109F.
Sweat cools you only when it evaporates. When the air is already thick with moisture, that evaporation slows, heat builds up, and the same 95F day punishes far harder than a dry one. The heat index puts a number on that, sometimes called the apparent temperature or humiture, and in hot weather more humidity always pushes it above the true reading.
How the National Weather Service computes it
The heat index runs on a two-step procedure: a simpler Steadman average first, and if that lands at 80F or higher, the fuller Rothfusz regression. The regression is a nine-term equation in temperature and relative humidity from a 1990 NWS technical paper.
Two corrections fine-tune the result. When humidity drops below 13% in high heat, a small amount is subtracted, because bone-dry air lets sweat work overtime. When humidity climbs above 85% in the low 80s, a small amount is added. This tool runs the whole procedure, adjustments included, where many sites settle for a rough table lookup. The equation assumes shade, a light breeze, and an average adult, so treat it as a solid general estimate of conditions.
The danger categories
The NWS sorts the heat index into four bands, and naming the band matters more than the raw number when you are deciding whether to send a team outside.
| Heat index | Category | What it means |
|---|---|---|
| 80 to 90F | Caution | Fatigue possible with prolonged activity |
| 90 to 105F | Extreme Caution | Heat cramps and exhaustion possible |
| 105 to 130F | Danger | Cramps and exhaustion likely, heat stroke probable |
| Above 130F | Extreme Danger | Heat stroke imminent |
Our worked default, 95F at 55% humidity, lands at 109F, which is Danger. That is the line where prolonged exertion turns genuinely risky, and where scheduled outdoor work usually shifts to early morning or gets extra water and shade breaks.
The two things it leaves out: sun and very dry air
The heat index assumes you are standing in the shade. Full sunshine can add up to 14F to what you actually feel, so a 105F shaded reading can feel close to 120F in direct sun. That single caveat undoes a lot of "but the app said 105" reasoning.
Dry air cuts the other way. When humidity is low, sweat evaporates freely and cools you well, so the heat index can read below the air temperature. At 108F with 10% humidity it comes out near 102F, six degrees under the thermometer. A dry desert 108F really can be easier on the body than a swampy 95F afternoon, and the low-humidity adjustment is the math admitting it.
These figures estimate environmental heat stress, not your personal risk. Anyone showing signs of heat exhaustion or heat stroke needs shade, water, and prompt medical attention.
Frequently asked questions
What is the heat index? The heat index is the temperature it feels like when humidity is combined with the air temperature, because humid air stops sweat from evaporating, which is how your body sheds heat. At 95F with 55% relative humidity it feels like about 109F. It is sometimes called the apparent temperature or humiture, and higher humidity always pushes it above the true air temperature in hot weather.
How is the heat index calculated? The National Weather Service first tries a simpler Steadman average; if that comes out at 80F or higher, it applies the Rothfusz regression, a nine-term equation in temperature and relative humidity from a 1990 NWS technical paper. Two corrections adjust it: a subtraction when humidity is under 13% in high heat, and an addition when humidity is above 85% in the low 80s. This tool runs the complete procedure, not a rough approximation.
What do the heat index danger categories mean? The NWS sorts the heat index into four bands. Caution runs 80 to 90F, where fatigue is possible with prolonged activity. Extreme Caution is 90 to 105F, where heat cramps and exhaustion become possible. Danger is 105 to 130F, where cramps and exhaustion are likely and heat stroke is probable. Extreme Danger is above 130F, where heat stroke is imminent. This tool names the band for your result.
Does the heat index account for sun? No, the heat index assumes you are in the shade. Standing in full sunshine can raise the value you feel by up to 14F over the shaded figure, so a 105F heat index in the shade can feel like nearly 120F in direct sun. It also assumes a light breeze; still air feels hotter and strong wind over very hot, humid skin can feel hotter still rather than cooler.
Why can very dry heat feel cooler than the air temperature? When humidity is low, sweat evaporates freely and cools you efficiently, so the heat index can read below the actual air temperature. At 108F with 10% humidity the heat index is about 102F, six degrees under the air temperature. That is the low-humidity adjustment at work, and it is why a dry 108F desert day can be more bearable than a muggy 95F afternoon.