Heat Index Climate Calculator
Calculate the apparent 'feels like' temperature by combining air temperature, humidity, and wind speed. Useful for athletes, event organisers, and public health officials assessing outdoor heat risk.
About this calculator
The heat index (apparent temperature) estimates how hot the air actually feels to the human body, accounting for humidity and wind. High humidity slows sweat evaporation, trapping body heat, while wind accelerates it. The formula used here (Steadman-derived) for Celsius input is: apparentTemp = T + (0.33 × (RH/100 × 6.105 × e^(17.27T/(237.7+T)))) − (0.70 × windSpeed) − 4.00, where T is air temperature (°C), RH is relative humidity (%), and windSpeed is in km/h. The exponential term estimates actual vapour pressure. For Fahrenheit inputs, temperature is first converted to Celsius before applying the same formula. The result is then converted back if needed. Altitude can additionally reduce effective temperature due to lower air pressure and cooler ambient conditions. A heat index above 40 °C is considered dangerous; above 54 °C is life-threatening.
How to use
It is 35 °C outside with 70% relative humidity and a wind speed of 10 km/h. Using the Celsius formula: vapour pressure term = 0.33 × (0.70 × 6.105 × e^(17.27×35/(237.7+35))) = 0.33 × (0.70 × 6.105 × e^(2.208)) = 0.33 × (0.70 × 6.105 × 9.096) ≈ 0.33 × 38.86 ≈ 12.82. Then: apparentTemp = 35 + 12.82 − (0.70 × 10) − 4.00 = 35 + 12.82 − 7.00 − 4.00 = 36.82 °C. The air feels about 37 °C — hot but not yet in the danger zone.
Frequently asked questions
Why does humidity make hot weather feel so much hotter than the actual temperature?
Your body cools itself primarily through sweat evaporation. When relative humidity is high, the air is already saturated with water vapour, so sweat evaporates more slowly and less heat is removed from your skin. The result is that body temperature rises faster than on a dry day at the same air temperature. At 35 °C and 90% humidity, the apparent temperature can exceed 50 °C — a potentially fatal combination with prolonged exposure.
How is the heat index different from the humidex or wet-bulb temperature?
All three measure heat stress but use different methods. The heat index (used in the US and this calculator) combines temperature and humidity via an empirical regression equation. The humidex (used in Canada) uses a simpler formula based on dew point. Wet-bulb temperature is a physical measurement — the lowest temperature achievable by evaporative cooling — and is considered the most accurate predictor of heat-related mortality risk. They agree broadly but diverge at extremes.
At what heat index value does outdoor activity become dangerous for healthy adults?
Health agencies generally classify a heat index of 27–32 °C as 'caution', 32–41 °C as 'extreme caution', 41–54 °C as 'danger', and above 54 °C as 'extreme danger'. Healthy adults can work outdoors at caution levels with regular hydration breaks, but strenuous activity should be curtailed above 40 °C. Elderly people, children, and those with cardiovascular conditions face elevated risk at lower thresholds. Acclimatisation over 7–14 days significantly improves tolerance.