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Heat Index Calculator

Computes the NWS heat index ('feels like' temperature in the shade) from air temperature and relative humidity with the Rothfusz regression, including the NWS low- and high-humidity adjustments, and classifies the NWS risk category.

When to use

You need how hot humid weather feels, the NWS heat-index category (caution to extreme danger) or a heat-safety threshold for temperatures of 80 °F (27 °C) and above.

Do not use when: Temperatures are below about 80 °F (27 °C), where the heat index is not meaningful (for cold weather use wind-chill), or you need the dew point itself (use dew-point).

Formula

T in °F, RH in %. Simple: HI₀ = 0.5·[T + 61 + 1.2·(T − 68) + 0.094·RH]. If HI₀ ≥ 80: HI = −42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² − 0.00000199·T²·RH²; if RH < 13 and 80 ≤ T ≤ 112: HI −= (13 − RH)/4 · √((17 − |T − 95|)/17); if RH > 85 and 80 ≤ T ≤ 87: HI += (RH − 85)/10 · (87 − T)/5

The Rothfusz regression reproduces Steadman's apparent-temperature model within ±1.3 °F for shady, light-wind conditions; full sunshine can add up to 15 °F. NWS categories: 80–90 °F caution, 90–103 extreme caution, 103–124 danger, 125+ extreme danger.

Inputs

ParameterTypeUnitRequiredDescription
temperaturenumberyesAir temperature in the shade, in temperature_unit. The regression is valid for 80–112 °F (27–44 °C). Range: ≥ -50, ≤ 150
temperature_unitenum: c | fdefault fUnit of the temperature input(s).
relative_humidity_percentnumber%yesRelative humidity in percent (70 means 70 %). Range: ≥ 0, ≤ 100

Outputs

OutputTypeUnitDescription
heat_index_fnumber°FApparent temperature in Fahrenheit.
heat_index_cnumber°CApparent temperature in Celsius.
temperature_fnumber°FInput temperature in Fahrenheit (the regression works in °F).
categorystringCaution (80–89 °F), Extreme caution (90–102 °F), Danger (103–124 °F) or Extreme danger (125 °F and above).
effectstringNWS description of heat disorders possible with prolonged exposure or strenuous activity.
method_usedstringRothfusz regression (heat index ≥ 80 °F) or the simple Steadman-average formula below 80 °F.

Example

90 °F, 70 % RH: {"temperature":90,"temperature_unit":"f","relative_humidity_percent":70} → {"heat_index_f":105.9,"heat_index_c":41.1,"temperature_f":90,"category":"Danger","method_used":"Rothfusz regression"}

35 °C, 60 % RH: {"temperature":35,"temperature_unit":"c","relative_humidity_percent":60} → {"heat_index_f":113.1,"heat_index_c":45.1,"category":"Danger"}

GET https://tttkmbb.com/api/v1/calculate/heat-index?temperature=90&temperature_unit=f&relative_humidity_percent=70

Machine access

Sources

FAQ

Why does the chart say 105 °F but the calculator 105.9 °F?

The NWS chart rounds Steadman's original table; the regression used here (and by the NWS online calculator) reproduces it within about ±1.3 °F.

Does it account for sun or wind?

No. The heat index assumes shade and a light breeze; the NWS notes that direct sunshine can raise the apparent temperature by up to 15 °F (8 °C).

What happens below 80 °F?

The simple Steadman-average formula is reported and the category is 'No heat-index warning', because the full regression is only fitted for heat index values of 80 °F and above.

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