🌡️ Heat Index Calculator
What Is the Heat Index?
The Heat Index — also known as the "apparent temperature" or "feels like" temperature — is a measure that combines air temperature with relative humidity to determine how hot it actually feels to the human body. When humidity is high, sweat evaporates more slowly, making it harder for your body to cool itself. This is why a 90°F day with high humidity can feel more like 105°F or higher.
The concept was developed by the U.S. National Weather Service (NWS) and the National Oceanic and Atmospheric Administration (NOAA) based on extensive research into human thermoregulation. The Heat Index is calculated using the Rothfusz regression equation, which models the body's physiological response to combined heat and humidity stress.
Heat Index Categories & Risk Levels
| Category | Heat Index (°F) | Heat Index (°C) | Health Risk |
|---|---|---|---|
| Safe | Below 80°F | Below 27°C | Minimal risk for most people. Normal activity is generally safe. |
| Caution | 80°F – 90°F | 27°C – 32°C | Fatigue possible with prolonged exposure and activity. Stay hydrated. |
| Extreme Caution | 90°F – 103°F | 32°C – 39°C | Heat cramps, heat exhaustion possible. Limit strenuous outdoor activity. |
| Danger | 103°F – 124°F | 39°C – 51°C | Heat exhaustion likely; heatstroke possible with prolonged exposure. Take precautions seriously. |
| Extreme Danger | 125°F or higher | 51°C or higher | Heatstroke highly likely. Outdoor activity extremely dangerous. Seek air-conditioned environments. |
How Is the Heat Index Calculated?
Our calculator uses the official Rothfusz regression equation adopted by NOAA's National Weather Service. The full formula is:
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
Where T = air temperature in °F and RH = relative humidity (0–100). The formula is valid for temperatures above 80°F and relative humidity above 40%. For extreme conditions, NOAA applies additional adjustment factors to ensure accuracy, which our calculator also implements automatically.
Heat Safety Tips
- Stay Hydrated: Drink plenty of water before you feel thirsty. Avoid alcohol and caffeine during extreme heat.
- Limit Outdoor Activity: Schedule strenuous activities for cooler parts of the day (early morning or evening).
- Wear Appropriate Clothing: Choose lightweight, light-colored, loose-fitting clothing that allows sweat to evaporate.
- Seek Shade or AC: Take frequent breaks in shaded or air-conditioned areas, especially when the Heat Index exceeds 90°F.
- Never Leave People or Pets in Parked Cars: Vehicle interiors can reach deadly temperatures within minutes, even with windows cracked.
- Know the Warning Signs: Watch for symptoms of heat exhaustion (heavy sweating, weakness, dizziness, nausea) and heatstroke (hot dry skin, confusion, loss of consciousness).
Frequently Asked Questions
Sources: National Weather Service (NWS) / NOAA — Heat Index documentation; Rothfusz, L.P. (1990). "The Heat Index Equation." NWS Technical Attachment.
