Heat Index (Apparent Temperature)
NWS heat index (Rothfusz regression): apparent temperature from air temperature (F) and relative humidity (%).
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Air temperature
- 90 F
- Relative humidity
- 70 %
Result: Heat index 105.9 F — danger (103-124 F).
The tool opens with these values already filled in. Replace them with your own.
What the result means
- < 80 F
- No elevated heat stress.
- 80-103 F
- Caution to extreme caution.
- >= 103 F
- Danger to extreme danger (heat cramps/exhaustion, then heat stroke risk). A heat-stress value, not a diagnosis.
What you enter
- Air temperature
- Relative humidity
How this is calculated
Rothfusz LP. The heat index equation. National Weather Service Technical Attachment SR 90-23. 1990. (NOAA Weather Prediction Center.) Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.