What is the heat index
Updated on June 30, 2026 · by Rafael Rossi
The heat index — also called the apparent temperature — combines temperature and relative humidity to estimate how much heat the human body actually perceives. It exists because a thermometer alone does not tell the whole story: on a humid day, 90 °F can feel like 105 °F.
Why humidity changes everything
The human body cools itself mainly by evaporating sweat. Every gram of sweat that evaporates carries roughly 580 calories of energy away from the skin — an efficient refrigeration system, and the main mechanism available above 95 °F.
But evaporation depends on the air having room to take on vapour. At high relative humidity the air is already near saturation, sweat runs off instead of evaporating, and the cooling simply does not happen. You sweat more and cool less.
That is why 95 °F in a dry climate is uncomfortable while 90 °F in a humid one can be dangerous.
The heat index table
Values from the Rothfusz equation, the same one the National Weather Service uses:
| Temperature | 40% humidity | 60% humidity | 80% humidity |
|---|---|---|---|
| 86 °F | 85 °F | 91 °F | 100 °F |
| 90 °F | 91 °F | 100 °F | 113 °F |
| 95 °F | 99 °F | 113 °F | 134 °F |
| 100 °F | 109 °F | 129 °F | — |
Note the behaviour: at 86 °F, raising humidity from 40% to 80% adds 15 degrees to the perceived temperature. At 95 °F, the same humidity change adds 35 degrees. The effect is not linear — it explodes with temperature.
Risk categories
| Heat index | Risk |
|---|---|
| 80 to 90 °F | Caution — fatigue possible with prolonged exposure |
| 90 to 103 °F | Extreme caution — cramps and heat exhaustion possible |
| 103 to 124 °F | Danger — exhaustion likely; heat stroke possible |
| above 125 °F | Extreme danger — heat stroke imminent |
These assume shade and light wind. In full sun, add up to 15 °F to the index — which turns an "extreme caution" day into a "danger" one without the forecast number changing at all.
The limit almost nobody knows: wet-bulb temperature
There is a stricter measure than the heat index: the wet-bulb temperature, the lowest temperature reachable by evaporation alone in that air.
Above a wet-bulb temperature of 95 °F (35 °C), the human body can no longer shed heat by any mechanism — not by sweating, not by convection. Under those conditions even a healthy person, at rest, in shade, with water available, reaches hyperthermia within hours. It is not a matter of acclimatisation or fitness: it is a physical ceiling.
That threshold has been recorded briefly in a few regions of the planet, and it is one of the indicators tracked in climate-habitability research.
The body adapts — but it takes two weeks
Heat acclimatisation is real and measurable: with gradual, repeated exposure the body begins sweating earlier, in greater volume and with less sodium, and plasma volume increases. The result is substantially better tolerance of the same heat index.
The process takes 10 to 14 days of regular exposure, and much of the gain is lost within two to three weeks without it. Two practical implications:
- The first heat wave of the season is the most dangerous — nobody is acclimatised yet, and that is when heat-exhaustion cases cluster.
- Anyone travelling from a cold climate to a hot one should avoid intense exertion in the first days, however fit they are. Fitness and acclimatisation are different things.
Frequently asked questions
Is the heat index the same as wind chill? They are opposite halves of the same idea. In heat, the index uses temperature and humidity; in cold, wind chill uses temperature and wind, because the dominant mechanism there is convective heat loss rather than evaporation.
Why does the index not apply in direct sun? Because the equation was built for shaded conditions. Direct solar radiation adds a thermal load the formula does not model — hence the recommendation to add up to 15 °F.
Does a fan help at any temperature? No. While the air is cooler than skin, a fan speeds evaporation and helps. Once the air exceeds body temperature it starts delivering heat — like a hair dryer. Above roughly 105 °F with high humidity, a fan can worsen dehydration without cooling.
How do I stay safe on a high-index day? Drink steadily and ahead of thirst, stay in shade during peak hours, wear light loose clothing, and postpone hard physical work. Take extra care with older adults, small children, pregnant people and anyone on medication affecting sweating.
Source: National Weather Service (NOAA) — Rothfusz equation and heat index risk table. Informational content — if heat stroke is suspected, seek immediate medical care.