What is the heat index
Updated on June 30, 2026 · by Rafael Rossi
The heat index is the "feels-like" for heat: it combines air temperature with humidity to estimate how hot the body actually feels.
Why humidity matters
The body cools by evaporating sweat. In very humid air, sweat barely evaporates, cooling drops and you feel hotter than the thermometer reads.
How to read it
At 32°C with 70% humidity, the feels-like tops 40°C — a danger zone, with risk of exhaustion and heatstroke during prolonged effort.
Risk bands
| Heat index | Risk |
|---|---|
| 27–32 °C (80–90 °F) | Caution — fatigue on long exposure |
| 32–41 °C (90–105 °F) | Extreme caution — cramps and exhaustion possible |
| 41–54 °C (105–130 °F) | Danger — heatstroke likely on exertion |
| above 54 °C (130 °F) | Extreme danger — imminent heatstroke risk |
Precautions
At high values, hydrate often, avoid strong midday sun, wear light clothing and cut back on intense activity. Infants and the elderly are most vulnerable.
FAQ
Heat index vs wind chill? In heat it's humidity + temperature; in cold, "feels-like" uses wind (wind chill), not humidity.
Why does shade feel cooler? The index measures shaded air; in direct sun, it can feel up to ~8 °C hotter.