Heat Index
Calculator
Results
- Heat index (°C)
- 40.409273
- Heat index (°F)
- 104.736692
- Feels hotter by (°C)
- 8.409273
Weather results
| Heat index (°C) | 40.409273 |
| Heat index (°F) | 104.736692 |
| Feels hotter by (°C) | 8.409273 |
formula-map diagram
- Heat index (°C)
- 40.409273
- Heat index (°F)
- 104.736692
- Feels hotter by (°C)
- 8.409273
Weather relationship
Formula
HI = Rothfusz regression on (T °F, RH %), with the NWS dry and humid adjustments= 40.409273679556
Note
This is a simplified model: it applies the published standard formula to the numbers you entered and ignores local terrain, radiation, precipitation type and instrument error. Wind chill is the 2001 NWS/Environment Canada regression, valid at or below 10 °C with wind at or above 4.8 km/h; the heat index is the Rothfusz regression, reliable above about 27 °C; dew point uses the Magnus approximation (a = 17.27, b = 237.7 °C) over liquid water; wet bulb uses Stull's approximation at standard sea-level pressure; pressure and density altitude assume the International Standard Atmosphere and dry air. Do not use these results for aviation, structural or safety decisions without an official forecast or a qualified professional.
More in Weather and climate
See all →Frequently asked questions
What does the heat index measure that temperature alone doesn't?+
The heat index combines air temperature and relative humidity into a single 'feels like' value that reflects how hot the body actually perceives conditions to be, because humidity interferes with the evaporation of sweat, the body's primary cooling mechanism.
Why does high humidity make heat feel so much worse?+
Sweat cools the body only when it evaporates; in humid air, the surrounding air is already close to saturated with water vapor, so sweat evaporates more slowly, leaving the body less able to shed heat even though the air temperature itself hasn't changed.
Why is the heat index usually only reported for warm temperatures?+
The heat index formula was developed specifically for warm-to-hot conditions, typically above about 27°C (80°F); below that threshold, humidity's effect on perceived temperature is minor and the standard formula isn't considered valid or meaningful.
Does the heat index account for wind or direct sunlight?+
The standard heat index formula assumes shade and light wind conditions; it does not add extra adjustment for direct sun exposure, which can make actual conditions feel meaningfully hotter than the reported heat index value, especially in still, sunny conditions.
Why do heat index and wind chill use opposite variables (humidity vs. wind)?+
They model different heat-transfer mechanisms for opposite problems: wind chill measures how fast wind strips heat away from skin in the cold, while heat index measures how much humidity blocks the evaporative cooling the body relies on in the heat. They're not the same calculation run in reverse.