What Is Heat Index?
The Heat Index (HI) gives a measure of how hot it actually feels due to the combined effect of the air temperature and the relative humidity. Hot, humid air actually feels hotter than hot, dry air.
Heat Index Table
The table below gives the heat index for different values of air temperature and relative humidity.
To determine the Heat Index, locate the actual temperature along the top of the table and find the relative humidity along the left side of the chart. The value at the intersection of the column and row is the Heat Index expressed in degrees Fahrenheit.
| Air Temperature (°F) | ||||||||||
| Relative Humidity |
70° | 75° | 80° | 85° | 90° | 95° | 100° | 105° | 110° | 115° |
| 20% | 66° | 72° | 77° | 82° | 87° | 93° | 99° | 105° | 112° | 120° |
| 25% | 66° | 72° | 77° | 83° | 88° | 94° | 101° | 109° | 117° | 127° |
| 30% | 67° | 73° | 78° | 84° | 90° | 96° | 104° | 113° | 123° | 135° |
| 35% | 67° | 73° | 79° | 85° | 91° | 98° | 107° | 118° | 130° | 143° |
| 40% | 67° | 74° | 79° | 86° | 93° | 101° | 110° | 123° | 137° | 151° |
| 45% | 68° | 74° | 80° | 87° | 95° | 104° | 115° | 129° | 143° | |
| 50% | 69° | 75° | 81° | 88° | 96° | 107° | 120° | 135° | 150° | |
| 55% | 69° | 75° | 81° | 89° | 98° | 110° | 126° | 142° | ||
| 60% | 70° | 76° | 82° | 90° | 100° | 114° | 132° | 149° | ||
| 65% | 70° | 76° | 83° | 91° | 102° | 119° | 138° | |||
| 70% | 70° | 77° | 85° | 93° | 106° | 124° | 144° | |||
| 75% | 70° | 77° | 86° | 95° | 109° | 130° | ||||
| 80% | 71° | 78° | 86° | 97° | 113° | 136° | ||||
| 85% | 71° | 78° | 87° | 99° | 114° | |||||
| 90% | 71° | 79° | 88° | 102° | 122° | |||||
| 95% | 71° | 79° | 90° | 105° | ||||||
| 100% | 72° | 80° | 91° | 106° | ||||||
Key
| Category | Heat Index | Possible effects |
| Caution | 80 – 90°F (27 – 32°C) |
Fatigue possible with prolonged exposure and/or physical activity. |
| Extreme Caution | 90 – 105°F (32 – 41°C) |
Sunstroke, muscle cramps, and/or heat exhaustion possible with prolonged exposure and/or physical activity. |
| Danger | 105 – 129°F (41 – 54°C) |
Sunstroke, muscle cramps, and/or heat exhaustion likely. Heatstroke possible with prolonged exposure and/or physical activity. |
| Extreme Danger | 130°F or higher (54°C or higher) |
Heat stroke or sunstroke likely. |