Heat index calculator
How hot it feels when humidity is added to the temperature, worked out with the US National Weather Service heat index formula, in °F or °C. The answer is for shade; in full sun the NWS says it can feel up to 15 °F (about 8 °C) hotter.
Heat index (example)
94.6 °F · 34.8 °C
Extreme caution
NWS: Heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity.
For 90.0 °F (32.2 °C) at 50 % relative humidity, in the shade.
NWS heat index classes
| Class | Heat index | What the NWS says |
|---|---|---|
| Caution | 80 to 90 °F (27 to 32 °C) | Fatigue possible with prolonged exposure and/or physical activity. |
| Extreme caution | 90 to 103 °F (32 to 39 °C) | Heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity. |
| Danger | 103 to 124 °F (39 to 51 °C) | Heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity. |
| Extreme danger | 125 °F or higher (52 °C) | Heat stroke highly likely. |
Ranges as the NWS publishes them; where two meet (90 °F, 103 °F) this calculator uses the higher class. °C values are rounded.
How it is calculated
Exactly as the NWS Weather Prediction Center sets it out. T is the temperature in °F and RH the relative humidity in percent. First the simple formula:
HI = 0.5 × {T + 61.0 + [(T − 68.0) × 1.2] + (RH × 0.094)}
If that comes to 80 °F or more, the full Rothfusz regression is used instead:
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
with two adjustments:
- If RH is below 13 % and T is between 80 and 112 °F, subtract [(13 − RH) ÷ 4] × √{[17 − |T − 95|] ÷ 17}.
- If RH is above 85 % and T is between 80 and 87 °F, add [(RH − 85) ÷ 10] × [(87 − T) ÷ 5].
Temperatures in °C are turned into °F first (°F = °C × 9/5 + 32) and the answer back again. The NWS warns that the calculation may give meaningless results for conditions outside the range of its heat index chart, so very hot, very dry or very cold inputs are best read as rough.
Worked example: 90 °F (32.2 °C) at 50 % humidity
simple formula = 0.5 × {90 + 61 + [(90 − 68) × 1.2] + (50 × 0.094)} = 91.1 °F 91.1 is 80 or more, so use the regression: HI = 94.6 °F (34.8 °C), no adjustment (RH is between 13 and 85 %)
That is about 95 °F, in the NWS Extreme caution class.
This calculator gives an estimate for learning and planning. It is not a forecast or a warning, and it cannot say whether conditions are safe. For decisions, follow your national weather service and local authorities.
Sources
- NWS Weather Prediction Center: The Heat Index Equation: the simple formula, the Rothfusz regression and both adjustments
- NWS Weather Prediction Center: Heat Index Calculator: the note on results outside the chart
- National Weather Service: What is the heat index?: the four classes, their effects, and "up to 15°F" in direct sun
Questions
What is the heat index?
It is how hot it feels when humidity is added to the air temperature, in the shade. The US National Weather Service calculates it with the Rothfusz regression. In full sun the NWS says it can feel up to 15 °F (about 8 °C) hotter.
What heat index is dangerous?
The NWS classes 80 to 90 °F as caution, 90 to 103 °F as extreme caution, 103 to 124 °F as danger and 125 °F or higher as extreme danger. Heat illness can happen below these levels, especially for older people, babies and anyone working hard outdoors.