Apparent temperature is what our body actually perceives, as opposed to what the thermometer reads. Two main factors modify it: wind (in cold) and humidity (in heat). At 5 °C with 40 km/h wind, it can feel like −5 °C. At 32 °C with 80% humidity, it can feel like 42 °C.
In cold conditions, the wind chill index measures how wind accelerates body heat loss. With wind chill below −27 °C, exposed skin can suffer frostbite within 10–15 minutes. In hot conditions, the heat index combines temperature and relative humidity: high humidity impedes sweat evaporation, the body's primary cooling mechanism.