The wind chill (or apparent temperature in cold conditions) quantifies how cold the air feels on exposed human skin when wind is factored in. Moving air strips away the thin insulating layer of warm air next to the skin far more efficiently than calm air, accelerating heat loss. The Wind Chill Index, developed by the US and Canadian meteorological services, combines air temperature and wind speed into a single equivalent temperature.
The formula used since 2001 is based on a model of heat loss from the face, the body part most exposed in winter. For example, an air temperature of −10 °C with a wind of 30 km/h produces a wind chill of approximately −20 °C. At this level, exposed skin can develop frostbite in 30 minutes. At wind chills below −35 °C, frostbite can occur in under 10 minutes. The index only applies when the temperature is below 10 °C and wind speed exceeds 4.8 km/h.
In summer, the reverse concept is the heat index, which combines air temperature and relative humidity to express how hot it feels. High humidity hampers the body's ability to cool through evaporation of sweat, making 35 °C at 70 % RH feel like 50+ °C. Both indices are critical for public health warnings — cold-wave alerts in winter and heatwave alerts in summer. Weather services like AEMET and Meteo.es routinely display these adjusted temperatures alongside the actual air temperature and dew point.