Heat Index Explained: What It Is, Chart & Danger Levels
What is the heat index?
The heat index (sometimes called the “apparent temperature”) is a measure of how hot it really feels when relative humidity is combined with the actual air temperature. Your body cools itself by evaporating sweat. When the air is already loaded with moisture, that evaporation slows down, so the same 90°F (32°C) afternoon feels far more punishing in New Orleans than it does in Phoenix.
The National Weather Service computes the heat index with the Rothfusz regression, an equation fitted to human physiology studies. It assumes shady, light-wind conditions - direct sunshine can add up to 15°F to the values below.
Heat index chart
| Temp \ RH | 40% | 50% | 60% | 70% | 80% | 90% | 100% |
|---|---|---|---|---|---|---|---|
| 80°F | 80 | 81 | 82 | 83 | 84 | 86 | 87 |
| 84°F | 83 | 85 | 88 | 90 | 94 | 98 | 103 |
| 88°F | 88 | 91 | 95 | 100 | 106 | 113 | 121 |
| 92°F | 94 | 99 | 105 | 112 | 121 | 131 | 143 |
| 96°F | 101 | 108 | 116 | 126 | 138 | 152 | 168 |
| 100°F | 109 | 118 | 129 | 143 | 158 | 176 | 195 |
| 104°F | 119 | 131 | 145 | 161 | 181 | 202 | 226 |
| 108°F | 130 | 144 | 162 | 182 | 205 | 231 | 260 |
Heat index danger levels
| Heat index | Category | What it means |
|---|---|---|
| 80–90°F (27–32°C) | Caution | Fatigue possible with prolonged exposure and activity |
| 90–103°F (32–39°C) | Extreme caution | Heat cramps and heat exhaustion possible |
| 103–124°F (39–51°C) | Danger | Heat exhaustion likely; heat stroke possible with continued exposure |
| 125°F+ (52°C+) | Extreme danger | Heat stroke highly likely |
How to stay safe in high heat index conditions
When the heat index climbs above 100°F, small habits make the biggest difference:
- Shift your schedule. Move hard work and exercise to early morning, the coolest hours of the day.
- Drink before you feel thirsty. Thirst lags behind what your body has already lost.
- Take air-conditioned breaks. Even short ones let your core temperature reset.
- Never leave kids or pets in a car. Cabin temperatures can rise 20°F in ten minutes.
- Check on older neighbors. Heat illness hits seniors first and hardest.
Know the warning signs
Heat exhaustion shows up as heavy sweating, weakness and nausea. Heat stroke - confusion, hot dry skin, a body temperature above 103°F - is a medical emergency: call for help immediately.
See the heat index in action on today's conditions in Miami, Houston or Dubai.
Heat index vs. WBGT: which number protects workers and athletes
The heat index assumes you are resting in the shade. For anyone working or training in the sun, safety bodies use wet-bulb globe temperature (WBGT), which folds in sunshine and wind as well. A shaded heat index of 95°F can coexist with a WBGT that already demands rest breaks every 15 minutes.
Rule of thumb: plan your own comfort with the heat index, but follow team and workplace rules keyed to WBGT - our feels-like guide compares all the systems.
Why hot nights are the silent danger
Heat waves kill mostly through nights that stay hot. When overnight lows hold above about 80°F (27°C), the body never gets its recovery window, and risk compounds day after day - especially in cities, where concrete releases stored daytime heat (the urban heat island effect adds several degrees to nights).
During multi-day heat, treat consecutive hot nights as the warning sign: check the overnight lows on the hottest cities ranking or any city forecast, hydrate before bed, and use the coolest room in the home for sleep.