Calculate the real feel temperature using heat index. Understand heat danger levels and precautions. Free heat index calculator. No signup.
90°F at 30% humidity feels completely different from 90°F at 90% humidity — the heat index can push the apparent temperature to over 103°F in high humidity. Heat index is what matters for health and safety decisions, outdoor work planning, and exercise safety. Our calculator shows the apparent temperature and associated health risk level.
National Weather Service heat index categories: 80-90°F apparent temperature: Caution — fatigue possible with prolonged exposure. 90-103°F: Extreme Caution — heat cramps and exhaustion possible. 103-124°F: Danger — heat cramps, exhaustion likely, heat stroke possible. Over 125°F: Extreme Danger — heat stroke highly likely with continued exposure. Context: Phoenix Arizona with high humidity events can reach apparent temperatures of 115-120°F — life-threatening for outdoor workers.
Why humidity makes heat dangerous: Human cooling mechanism is sweating and evaporation. High humidity prevents sweat evaporation — body cannot cool effectively. At 100% humidity: sweating provides almost no cooling benefit. At 30% humidity: evaporation is very effective — desert heat feels more bearable than humid heat at same temperature. This is why Phoenix at 110°F often feels more manageable than Miami at 95°F — different effective heat index.
Heat index combines air temperature and relative humidity to calculate apparent temperature — what it feels like to the human body. The NWS uses the Rothfusz regression equation based on heat balance research. Accurate when temperature is above 80°F and relative humidity is above 40%. Below these thresholds: humidity has less impact on perceived heat and simple temperature is adequate measure.
Outdoor safety guidelines: Under 91°F apparent: generally safe for most activities. 91-103°F: limit strenuous activity, take breaks in shade, stay hydrated. Over 103°F: avoid strenuous outdoor work for vulnerable populations (elderly, children, medical conditions). Over 115°F: extremely dangerous for anyone — occupational safety guidelines typically require suspension of outdoor work at this level.
Heat safety essentials: Hydration: drink water before thirsty — thirst signals mild dehydration that impairs performance. 16-24 oz per hour during physical activity in heat. Timing: avoid outdoor activity 10am-4pm peak heat hours. Clothing: light colored, loose fitting, moisture-wicking. Acclimatization: takes 7-14 days to adapt to heat — start slow. Warning signs of heat stroke: confusion, hot dry skin, rapid breathing — medical emergency, call 911.
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