The thermometer measures one thing: the temperature of the air molecules. But human comfort and human heat stress depend on what the body experiences as it tries to dump or conserve heat. Two derived numbers translate dry-bulb temperature into something closer to felt temperature: heat index (warm conditions) and wind chill (cold conditions).
Heat index combines temperature with humidity. At 32 °C and 50% RH, heat index is about 35 °C; at 32 °C and 80% RH, heat index climbs to 41 °C. The physics: high humidity reduces sweat evaporation, the body's main cooling mechanism, so the same dry-bulb temperature feels, and is, physiologically, much hotter. NWS issues heat advisories on heat-index thresholds, not temperature alone.
Wind chill combines temperature with wind speed. At -10 °C with 30 km/h wind, wind chill is about -19 °C. The physics: wind disrupts the thin warm boundary layer the body builds around skin and clothing, accelerating heat loss. Wind chill below -25 °C produces frostbite on exposed skin within 30 minutes; below -40 °C within 10 minutes.
Open-Meteo exposes apparent temperature directly as a forecast field: the dashboard uses it both for outdoor advisories and for cross-checks against indoor temperature. A 26 °C indoor temperature feels cool when outdoor apparent is 35 °C; the same 26 °C feels warm when outdoor apparent is 12 °C. Context matters for comfort tier interpretation.
References
- NOAA NWS - Heat index www.weather.gov
- NOAA NWS - Wind chill chart www.weather.gov
- OSHA - Heat exposure www.osha.gov
- Open-Meteo - Forecast API documentation open-meteo.com
