Wind chill
Temperature & humidity wind chill temperature
Wind chill describes the extra heat lost from exposed skin in cold wind. The comparison temperature can be much lower than the measured air temperature.
Why wind feels colder
Skin warms the air immediately next to it. Wind carries that warmer air away and increases heat exchange. Wind chill expresses this effect as an apparent temperature calculated from air temperature and wind speed.
An example and the formula’s limits
At an air temperature of 0 °C and a wind speed of 30 km/h, the widely used North American formula gives a wind chill of about −6.5 °C. Exposed skin loses substantially more heat than it would in calm conditions.
This formula is intended for temperatures at or below 10 °C and wind above about 4.8 km/h. Sunshine, wet clothing and physical activity are not fully represented. It is therefore not a suitable measure for a summer breeze.
Does it make water freeze above zero?
Wind alone cannot cool a dry object below the air temperature. It makes the object approach that temperature faster. A wind chill of −6.5 °C with air at 0 °C does not mean that a dry road reaches −6.5 °C. Evaporation and heat loss by radiation are separate processes.
People continuously generate heat, but wind can still greatly increase their heat loss. A comparison value cannot replace consideration of clothing and exposure time.