When is the heat index formula accurate?
NOAA’s regression is most reliable when temperatures are ≥ 80°F (27°C) and humidity is above ~40%. Outside that range the feels-like value equals the air temperature.
Step-by-step walkthrough
Heat Index Calculator Guide
Use this companion guide beside the interactive calculator to make sure you understand the “why” behind every click. Each section includes the classroom explanation, student-friendly language, and quick practice prompts.
Before you start
Collect the data points listed below and double-check their units. Keeping an organized “givens” list is one of the fastest ways to reduce math errors and impress exam graders.
- heat index
- feels like
- humidity
Detailed procedure
Step 1
Choose Fahrenheit or Celsius and enter the actual air temperature.
Step 2
Enter the relative humidity percentage from your forecast or hygrometer.
Step 3
Review the feels-like value in °F/°C plus the matching caution, extreme caution, or danger guidance.
Teaching & troubleshooting tips
Does wind or direct sun change the result?
The published formula assumes shade with light wind. Direct sunlight or stagnant air can add several more degrees of perceived heat.