Enter air temperature and relative humidity to find the real-feel temperature, heat stress category and personalised safety advice — instantly.
Temperature unit
Note: the Rothfusz heat index formula is accurate for temperatures ≥ 80°F (27°C) and relative humidity ≥ 40%.
The heat index — also called the apparent temperature or feels-like temperature — is a measure of how hot it actually feels to the human body when relative humidity is factored in with air temperature. It was developed by Robert G. Steadman in 1979 and adopted by the U.S. National Weather Service (NWS) as a public heat hazard communication tool.
The fundamental mechanism is straightforward: human bodies cool themselves primarily by sweating. When sweat evaporates from the skin it carries heat away. But when the air is already saturated with moisture (high humidity), sweat evaporates much more slowly, reducing the body's ability to shed heat. The result is that you feel significantly hotter than the thermometer reads.
The NWS uses the Rothfusz multivariate regression equation (1990), derived from Steadman's original work:
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 = dry-bulb temperature (°F), RH = relative humidity (%)
Two adjustments are applied: if RH < 13% and 80°F < T < 112°F, the result is corrected downward (dry air feels less oppressive); if RH > 85% and 80°F < T < 87°F, the result is corrected upward. For values below 80°F, a simplified linear formula is used instead.
| Heat Index (°F) | Heat Index (°C) | Category | Possible Effects |
|---|---|---|---|
| Below 80°F | Below 27°C | Comfortable | No significant risk |
| 80 – 90°F | 27 – 32°C | Caution | Fatigue with prolonged activity |
| 91 – 103°F | 33 – 39°C | Extreme Caution | Heat cramps and exhaustion possible |
| 103 – 124°F | 40 – 51°C | Danger | Heat cramps, exhaustion likely; stroke possible |
| 125°F and above | 52°C and above | Extreme Danger | Heat stroke highly likely — life-threatening |
Source: U.S. National Weather Service. Categories assume shaded conditions and light wind. Direct sunlight can add 10–15°F (6–8°C) to the perceived heat index.
Consider a summer afternoon at 95°F (35°C). At 40% humidity the heat index is around 101°F (38°C) — uncomfortable but manageable. At 75% humidity that same temperature feels like 130°F (54°C), placing conditions firmly in the Extreme Danger zone. That 35-degree jump in perceived temperature comes entirely from moisture in the air.
A second example: 88°F (31°C) at 90% humidity — a common tropical morning — produces a heat index of approximately 110°F (43°C), squarely in the Danger category, even though the thermometer reads a seemingly moderate temperature. This is why monitoring humidity is just as important as monitoring temperature during summer heat advisories.
The heat index is an average calculated for a healthy adult at rest in the shade. Several groups face significantly greater risk at the same heat index reading:
The heat index combines temperature and humidity into a single apparent temperature — intuitive and easy to communicate to the public. The dew point gives an absolute measure of atmospheric moisture independent of temperature, making it more consistent across different air temperatures. Meteorologists often prefer dew point; emergency managers and the public generally find heat index easier to act on. Both tools are complementary: use the heat index to assess immediate outdoor conditions and comfort, and the dew point to understand the underlying atmospheric moisture load.
The heat index is the apparent temperature — what the air actually feels like — when relative humidity is combined with air temperature. It matters because high humidity slows sweat evaporation, the body's primary cooling mechanism, making hot weather significantly more dangerous than temperature alone would suggest. A heat index of 103°F (39°C) or above puts healthy adults at risk of heat cramps and heat exhaustion.
Any combination that produces a heat index above 103°F (39°C) falls in the NWS Danger category. Common dangerous combinations include: 95°F at 65% humidity, 100°F at 50% humidity, or 90°F at 90% humidity. Above 125°F (52°C) heat index, conditions are Extreme Danger — heat stroke becomes highly likely without immediate cooling.
They are closely related but not identical. The heat index is the specific NWS formulation based on Steadman's work, calculated in the shade with a light breeze. "Feels like" or "apparent temperature" is a broader term sometimes incorporating wind speed, solar radiation and other factors. Weather apps often use slightly different models, which is why a feels-like reading may differ marginally from a strict heat index calculation.
Yes. Warm nights with high humidity are dangerous precisely because the body cannot cool down during sleep. Heat stroke risk accumulates over multiple hours of heat exposure without a recovery period overnight. A heat index above 80°F (27°C) at night means core body temperature stays elevated — this is one reason heat waves over consecutive nights cause higher mortality than a single hot day.
The standard heat index assumes a light, steady wind. Stronger winds can make conditions feel slightly cooler by improving sweat evaporation — the opposite of wind chill in winter. However, wind provides little relief when temperatures are above body temperature (approximately 98.6°F / 37°C), because the wind is then blowing hot air over the skin, which can add heat rather than remove it.
Heat exhaustion is characterised by heavy sweating, weakness, cold or pale skin, fast or weak pulse, nausea and muscle cramps. The body is still able to cool itself but is struggling. Heat stroke is a medical emergency: the core temperature exceeds 104°F (40°C), sweating may stop, the person may become confused or lose consciousness. Heat stroke causes organ damage and can be fatal within minutes without immediate cooling and emergency care.
Yes, significantly. At 96°F (36°C) and 100% humidity the heat index reaches approximately 148°F (64°C) — 52 degrees above the measured temperature. In practice, saturations near 100% at high temperatures are rare, but tropical and coastal regions regularly produce heat index readings 20–35°F above actual temperature during summer heatwaves.
Call emergency services (911 / 112) immediately. Move the person to a cool environment. Cool them rapidly — the fastest method is cold water immersion (bathtub or any large container). If immersion is not possible, apply ice packs to the neck, armpits and groin while fanning them. Do not give water to drink if the person is confused or unconscious. Rapid cooling in the first 30 minutes is the single most important factor in survival.