How the Chart Works
The NOAA heat index chart has temperature on the vertical axis and relative humidity on the horizontal axis. Find your current temperature on the left, your humidity percentage across the top, and the intersection is the heat index. The cells are color-coded from yellow to dark purple based on risk level.
Chart Zones and Actions
| Zone | Heat Index | Color | Action |
|---|---|---|---|
| Caution | 80-90°F | Yellow | Fatigue possible. Stay hydrated. |
| Extreme caution | 90-103°F | Orange | Heat cramps/exhaustion possible. Limit strenuous activity. |
| Danger | 103-124°F | Red | Heat stroke possible. Avoid outdoor activity. |
| Extreme danger | 126°F+ | Purple | Heat stroke imminent. Stay indoors. |
Surprising Combinations
Some combinations are deceptively dangerous. 90°F with 60% humidity gives a heat index of 100°F. 85°F with 80% humidity gives a heat index of 97°F. A seemingly mild 85°F day can be dangerously hot if humidity is extreme. This is why the heat index is important — you cannot judge risk by temperature alone.
Check Your Local Heat Index
Calculate the heat index for any temperature and humidity combination.