Heat Index Calculator
Compute heat index from temperature (°C) and relative humidity (NOAA, valid for T ≥ 27°C).
Heat Index ≈ — °C
Heat index: how heat and humidity combine
The heat index combines air temperature and relative humidity to estimate the "feels-like" temperature on hot days, because humid air slows sweat evaporation and impairs the body's cooling. The NWS formula (Rothfusz regression) is a multinomial polynomial in T (°F) and RH (%); a simpler approximation for T > 27 °C and RH > 40% is HI ≈ T + 0.5·(T − 26)·(RH − 40)/100. Example: 30 °C with RH = 80% feels like ~35 °C. Risk thresholds: caution above 27 °C, extreme caution above 32 °C, danger above 40 °C, and extreme danger above 54 °C — heatstroke can occur within minutes. Brazil's NR-15 (Annex 3) uses the WBGT instead to assess occupational heat stress.
Applications
Weather forecasting and heatwave alerts, sports event safety (road races and matches are postponed when HI is high), public health (elderly, children, and athletes are the most vulnerable), occupational health (work-rest cycles and hydration breaks), and school PE class planning during summer.
FAQ
Why does humidity make heat feel worse? Because sweat cools the body only when it evaporates. Humid air is already loaded with water vapor, so evaporation slows down and body heat builds up.
Does heat index apply in the shade? Yes — the formula assumes shade and light wind. Direct sun can add another 5 to 8 °C to the perceived temperature.
Heat index or WBGT for workplace safety? WBGT (used in Brazil's NR-15) is more complete because it also accounts for radiant heat and wind, but the heat index is simpler and works well for everyday public alerts.
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.