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Calculate the heat index (feels-like temperature) from air temperature and relative humidity. The heat index tells you how hot it actually feels to your body — which can be dramatically higher than the thermometer reading when humidity is high.
High humidity prevents sweat from evaporating efficiently, reducing your body ability to cool itself. On a 95°F (35°C) day with 60% humidity, the heat index reaches 114°F (46°C) — dangerously hot. Understanding the heat index helps you make smart decisions about outdoor activity and heat safety.
The National Weather Service (NWS) uses the heat index to issue heat advisories and warnings. The heat index is calculated using temperature and relative humidity and represents the apparent temperature felt by the human body in shaded conditions. Direct sunlight can add an additional 10-15°F to the heat index.
| Heat Index | Danger Level | Risk |
|---|---|---|
| 80-90°F (27-32°C) | Caution | Fatigue possible with prolonged exposure and physical activity |
| 90-103°F (32-39°C) | Extreme Caution | Heat cramps and heat exhaustion possible |
| 103-124°F (39-51°C) | Danger | Heat cramps and exhaustion likely; heatstroke possible |
| 125°F+ (52°C+) | Extreme Danger | Heatstroke highly likely with continued exposure |
At 100°F (38°C): 15% humidity feels like 99°F, 40% humidity feels like 109°F, 65% humidity feels like 124°F. Humidity prevents sweat evaporation — your body primary cooling mechanism. In dry desert heat, the body can cool itself effectively even at high temperatures. In humid conditions, the same temperature is far more dangerous.
When heat index exceeds 90°F: stay in air-conditioned spaces during peak hours (10am-4pm), drink water before you feel thirsty (minimum 8 oz per hour in heat), wear light loose-fitting clothing, never leave children or pets in parked cars, and check on elderly neighbors. Heat is the deadliest weather phenomenon in the US — killing more people than hurricanes, tornadoes, and floods combined.
The NWS heat index formula uses temperature in °F and relative humidity. It is most accurate between 80-112°F and humidity 40-100%. The Australian Bureau of Meteorology uses the Apparent Temperature formula which also includes wind speed. The two formulas give different results — this calculator uses the official NWS Rothfusz regression formula.