How the Heat Index Calculator works.
NOAA Rothfusz regression using temperature °F and relative humidity %T is air temperature in Fahrenheit and R is relative humidity percentage; low- and high-humidity adjustments apply in NOAA ranges.
Worked example for the Heat Index Calculator.
At 90 °F and 70% relative humidity, the NOAA Rothfusz regression gives a heat index of approximately 105.9 °F.
How to read this result.
Heat index estimates how hot shaded conditions feel when air temperature and relative humidity are considered together.
How this tool gets the answer
NOAA Rothfusz regression using temperature °F and relative humidity %. T is air temperature in Fahrenheit and R is relative humidity percentage; low- and high-humidity adjustments apply in NOAA ranges. Heat index estimates how hot shaded conditions feel when air temperature and relative humidity are considered together.
A useful example
At 90 °F and 70% relative humidity, the NOAA Rothfusz regression gives a heat index of approximately 105.9 °F. A quick way to verify the calculation is: Normalize temperature to Fahrenheit, apply the Rothfusz polynomial and NOAA humidity adjustments, then convert the result back for a Celsius display.
What the result tells you
Heat index estimates how hot shaded conditions feel when air temperature and relative humidity are considered together. Temperature may be entered in Celsius or Fahrenheit but the NOAA regression operates in Fahrenheit; relative humidity is a percentage.
Where the answer can go wrong
For temperatures of at least 80 °F, NOAA first screens conditions with a simpler Steadman approximation; when that heat-index estimate reaches 80 °F, the Rothfusz regression and its low- or high-humidity adjustments apply. Relative humidity may range from 0% through 100%. Direct sun, wind, clothing, exertion, age, health, hydration, and local warnings can change risk substantially; use official alerts for safety decisions.
Privacy and practical use
The calculation runs locally in your browser and FigureNest does not send the entered values to a calculation service. Keep the source value and its unit with any result you save so you can reproduce the calculation later.
Inputs and edge cases to check.
For temperatures of at least 80 °F, NOAA first screens conditions with a simpler Steadman approximation; when that heat-index estimate reaches 80 °F, the Rothfusz regression and its low- or high-humidity adjustments apply. Relative humidity may range from 0% through 100%.
What this tool does not include.
Direct sun, wind, clothing, exertion, age, health, hydration, and local warnings can change risk substantially; use official alerts for safety decisions.
Reference guidance and methodology.
Heat Index Calculator questions.
What does the Heat Index Calculator do?
Estimate NOAA heat index from air temperature and relative humidity.
How can I check the result?
Normalize temperature to Fahrenheit, apply the Rothfusz polynomial and NOAA humidity adjustments, then convert the result back for a Celsius display.
Which units and assumptions matter?
Temperature may be entered in Celsius or Fahrenheit but the NOAA regression operates in Fahrenheit; relative humidity is a percentage. Direct sun, wind, clothing, exertion, age, health, hydration, and local warnings can change risk substantially; use official alerts for safety decisions.
Are the entered values sent or stored?
No. The calculation runs locally in your browser and FigureNest does not send or store calculator inputs.