What Is the Heat Index? Why It Is Called the Feels-Like Temperature

The heat index, also called the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature. That is how the National Weather Service describes it. It is reported in degrees Fahrenheit, so it can be read against a normal thermometer: at an air temperature of 94°F and 60% relative humidity, the calculator's formula gives a heat index of 110.2°F, meaning the day puts a heat load on you that a thermometer alone would not show.

Why humidity changes how hot it feels

Your body gets rid of heat largely by sweating, and sweat only cools you when it evaporates. The National Weather Service explains that the rate of evaporation depends on the relative humidity: the more water vapor the air already holds, the slower sweat evaporates, and the less cooling you get from each drop. The heat index translates that loss of cooling into a single number on the temperature scale. Dry air at a given temperature lets sweat work well, so it feels more bearable. Humid air at the same temperature does not, so it feels hotter.

How the number is calculated

The calculator uses a nine-term regression equation of air temperature T in °F and relative humidity R in percent: HI = -42.379 + 2.049T + 10.143R - 0.2248TR - 0.00684T squared - 0.0548R squared + 0.00123T squared R + 0.000853T R squared - 0.00000199T squared R squared. The calculator's documentation traces the idea to R.G. Steadman's 1979 model of human thermoregulation, simplified into a two-variable fit for public use. Below 80°F the calculator simply returns the air temperature. Published charts and weather services may apply extra adjustments in some temperature and humidity ranges, so a chart value can differ slightly from the calculator's output.

Worked example: one thermometer reading, five humidity levels

To isolate the effect of humidity, hold the air temperature at 94°F and change only the relative humidity. Every figure below comes from the calculator's formula, rounded to one decimal place, and assumes shade with light wind.

Going from 40% to 80% humidity at the same 94°F adds 32.2 degrees of felt heat. The 20% row is a reminder that the heat index is not always higher than the air temperature: in very dry air the formula returns a lower number. The calculator applies the raw regression only. The NWS states that the regression is not appropriate when the heat index would fall below about 80°F, and that it is not valid for extreme temperature and humidity conditions beyond the data Steadman considered. The NWS also lists two adjustments for relative humidity below 13% or above 85%, which this calculator does not apply, and neither of them covers the rows in this table.

The four National Weather Service categories

What the heat index leaves out

• Sunshine. The NWS notes that heat index values were devised for shady, light-wind conditions, and that exposure to full sunshine can increase them by up to 15°F. A 92°F afternoon at 65% humidity computes to 108.3°F in the shade; in full sun the felt value could approach 123.3°F.
• Strong wind. The NWS warns that strong winds combined with very hot, dry air can also be hazardous, which the heat index does not capture.
• You. The calculator's own limitations note that age, medications, body size and how used to the heat you are all shift personal risk, and that exertion adds heat the formula does not model.
• The edge of the range. Under 80°F the calculator returns the air temperature, and near that edge the raw formula can land a fraction of a degree under it (80°F at 40% gives 79.9°F).

Common mistakes when reading it

1. Treating it as the air temperature. A heat index of 110°F does not mean the thermometer will read 110°F; it means the body is under a load comparable to that.
2. Forgetting the shade assumption. If you will be working or exercising in direct sun, add a margin rather than reading the shaded value at face value.
3. Using stale or mismatched inputs. Humidity swings through the day; use the temperature and humidity measured at the same time and place.
4. Using it as the only tool for hard work in the sun. The NWS describes wet bulb globe temperature, which also accounts for wind and solar radiation, as a particularly effective indicator of heat stress for active people such as outdoor workers and athletes.

When to check it

The heat index only differs meaningfully from the air temperature once it is warm and reasonably humid, which is why the calculator starts at 80°F. Check it before planning a long walk, yard work, a sports session or an outdoor event, and compare the result with the categories above. Then adjust for sun, wind, how hard you will be working and who is going to be outside with you.