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.
Last updated: September 2026
Formula below · 2 sources (usgs.gov, Wikipedia) · Updated Sep 2026
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About this calculator
The 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. This calculator uses Steadman's apparent temperature as published by the Australian Bureau of Meteorology: AT = T + 0.33 × e − 0.70 × ws − 4.00, where T is air temperature (°C), e is the water vapour pressure in hPa, estimated as e = (RH/100) × 6.105 × exp(17.27 × T / (237.7 + T)), and ws is wind speed in metres per second — the km/h value you enter is divided by 3.6 first. For Fahrenheit inputs the temperature is converted to Celsius before applying the formula; the result is always reported in °C. This is not the same equation as the US National Weather Service heat index, which ignores wind, but the two agree within a few degrees in still air. An apparent temperature above about 40 °C is dangerous, and 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. Step 1 — Vapour pressure: e = 0.70 × 6.105 × e^(17.27 × 35 / 272.7) = 0.70 × 6.105 × 9.176 ≈ 39.21 hPa, so the humidity term is 0.33 × 39.21 ≈ 12.94. Step 2 — Wind: 10 km/h ÷ 3.6 = 2.78 m/s, so the wind term is 0.70 × 2.78 ≈ 1.94. Step 3 — apparentTemp = 35 + 12.94 − 1.94 − 4.00 ≈ 42.0 °C. The air feels about 42 °C — in the danger band, where heat exhaustion is likely with prolonged exertion. Entering 95 °F with the Fahrenheit unit gives the same 42.0 °C.
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 US heat index combines temperature and humidity via an empirical regression equation; this calculator uses the closely related Steadman apparent temperature, which also includes wind. 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.