This calculator provides an estimate for informational purposes only and is not a medical device. Heat-related illness risks vary by individual fitness level, acclimatization, medications, age, and health conditions. This is not medical advice. Always consult a healthcare professional before exercising in hot conditions. Seek immediate medical attention for heat exhaustion or heat stroke symptoms.
Calculate the "apparent temperature" or "feels like" temperature when humidity is factored into air temperature.
Last updated: March 2026
Heat Index is the "apparent temperature" or "feels-like" temperature, calculated by combining air temperature and relative humidity. It represents how hot it actually feels to your body because high humidity prevents effective sweating and cooling.
Example: 85°F with 70% humidity feels like approximately 93°F to your body.
Drink water regularly before, during, and after activity. Do not wait until you feel thirsty.
Exercise early morning or evening when temperatures are cooler. Avoid peak heat hours (11am–3pm).
Wear light, loose-fitting clothing that allows sweat to evaporate. Use a hat or visor for sun protection.
Take frequent breaks in shade or air-conditioned spaces. Allow your body to cool down.
Gradually increase activity duration in heat. Your body adapts to heat over 1–2 weeks.
Heat exhaustion: heavy sweating, weakness, dizziness. Heat stroke: high temp, no sweating, confusion. Seek help immediately.
Your body cools itself by sweating and evaporation. High humidity reduces evaporation efficiency, so your sweat doesn't cool you effectively.
Yes. At very high humidity (85–100%), the heat index can be 10–20°F higher than the actual air temperature.
They're the same thing. Heat index is the official term used by meteorologists to describe apparent temperature.
80°F is the threshold where heat stress begins. For healthy people doing light activity, it's fine. For vulnerable groups (elderly, children, those with health conditions), risk increases earlier.
Very accurate near standard conditions. It uses the Rothfusz regression formula adopted by the National Weather Service. Accuracy is ±1–2°F in most cases.
This occurs at low humidity and cool air temperatures, where evaporative cooling is very efficient. The calculator is working correctly.
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