Heat Index Calculator (Feels-Like Temperature)

About the Heat Index Calculator (Feels-Like Temperature)

The heat index is the US National Weather Service’s “feels-like” temperature for hot weather. It combines air temperature and humidity. Damp air slows the evaporation of sweat, so a humid 96 °F (36 °C) day stresses the body as much as much hotter dry air would. This calculator uses the exact NWS procedure: Steadman’s simple formula, and the Rothfusz regression with both NWS adjustments above about 80 °F. It also reports the NWS risk category, from Caution to Extreme Danger, and the effect on the body that the NWS gives for it.

For comparison you also get two other national indices: Environment and Climate Change Canada’s humidex and the Australian Bureau of Meteorology’s apparent temperature, which also accounts for wind. An NWS-style chart marks where your conditions fall. You can enter relative humidity or the dew point, in °F or °C.

Common uses

• Checking how dangerous a hot, humid day is before outdoor work, sport or travel.
• Understanding a heat advisory, or the “feels like” figure in a forecast or weather app.
• Comparing the US heat index with the humidex used in Canada and the apparent temperature used in Australia.
• Science and geography lessons on humidity, evaporation and heat stress.

How the heat index is calculated

The NWS first computes a simple formula consistent with Steadman’s tables: HI = 0.5 × [T + 61 + 1.2 × (T − 68) + 0.094 × RH], with T in °F and RH in %. If that comes to 80 °F or more, it uses the Rothfusz regression instead (NWS Technical Attachment SR 90-23):

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.

Two adjustments follow:

• when RH ≤ 13 % and T is 80–112 °F, subtract [(13 − RH) ÷ 4] × √{[17 − |T − 95|] ÷ 17};
• when RH > 85 % and T is 80–87 °F, add [(RH − 85) ÷ 10] × [(87 − T) ÷ 5].

The calculator follows the NWS Weather Prediction Center’s own calculator exactly. That calculator switches to the regression when the simple formula exceeds 79 °F, and gives the air temperature itself at 40 °F or below. The NWS chart is drawn from Steadman’s results; the regression reproduces 125 of its 135 printed values exactly and the other 10 within 2.3 °F.

What the categories mean

These are the NWS classifications, with the NWS wording for the effect on the body:

• Caution, 80–90 °F (26.7–32.2 °C): fatigue possible with prolonged exposure and/or physical activity.
• Extreme Caution, 90–103 °F (32.2–39.4 °C): heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity.
• Danger, 103–124 °F (39.4–51.1 °C): heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity.
• Extreme Danger, 125 °F (51.7 °C) or higher: heat stroke highly likely.

The values are for shady locations with a light wind. The NWS notes that in direct sunlight the heat index can be up to 15 °F (8 °C) higher. The categories are general guidance, not the criteria for official heat alerts.

Humidex and apparent temperature

Environment and Climate Change Canada’s humidex is T + 0.5555 × (e − 10), where e = 6.11 × exp[5417.7530 × (1/273.16 − 1/T_d)] is the vapour pressure in hPa and T_d is the dew point in kelvins. It is a number without units, read against ECCC’s ranges:

• 20–29: little discomfort;
• 30–39: some discomfort;
• 40–45: great discomfort, avoid exertion;
• 46 and over: dangerous, possible heat stroke.

The Australian Bureau of Meteorology’s apparent temperature (after Steadman) is AT = T + 0.33 × e − 0.70 × ws − 4.00. Here T is in °C, e = RH/100 × 6.105 × exp[17.27 × T ÷ (237.7 + T)] hPa, and ws is the wind speed in m/s at 10 m. It is the only one of the three that accounts for wind. The three indices were built for different purposes, so they give different numbers for the same air.

Sources

• NOAA NWS Weather Prediction Center: The Heat Index Equation and its heat index calculator (L. P. Rothfusz, NWS Technical Attachment SR 90-23; R. G. Steadman, J. Appl. Meteor. 18, 861–873).
• NWS Amarillo, What is the heat index? (categories, effects, sunshine), and the NWS heat index chart.
• Environment and Climate Change Canada: glossary (humidex) and How to use the Humidex.
• Australian Bureau of Meteorology, Thermal Comfort observations (R. G. Steadman, Aust. Met. Mag. 43, 1–16).
• NWS, Wet Bulb Globe Temperature informational guide.

Limitations

• The heat index assumes shade and a light wind; it does not include sunshine, clothing or exertion, all of which add heat stress.
• The NWS warns that results outside its chart (80–110 °F, 40–100 % RH) may not be meaningful; the calculator flags those cases. Values above 137 °F are left blank, as on the chart.
• Relative humidity from a dew point uses the WMO Magnus formula. The NWS calculator uses an older formula and rounds RH to a whole percent, so its results can differ by a few tenths of a degree.
• The categories are general NWS guidance; individual risk varies with health, age, fitness and acclimatisation. This is not medical advice.

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