Often times in Fall or Winter, meteorologists will report an outside temperature on the news, as well as the wind chill temperature or factor if the forecast predicts a windy day. Wind chill is the decrease in air temperature due to the wind. But what does wind chill temperature really mean?
Wind chill is an attempt to quantify the perceived decrease in ambient air temperature that the human body will feel as a result of wind. To understand this with concepts of heat transfer and mechanical engineering, the human body would be analogous to a house. Typically the human body would have its thermostat set to approximately 97 to 99 degrees F. The human body is constantly burning energy or fuel from food in order to maintain the body’s temperature, because heat is constantly being lost primarily through conduction (through our clothes) and convection (the ambient air temperature). Similarly, if the thermostat is set to a certain temperature in the house, the heating system will come on and off in order to heat the home and maintain it at or about the thermostat setting, while energy is constantly being lost to the outside air.
In the presence of wind, the human body experiences a higher coefficient of convection and therefore higher heat loss to the ambient air. As a result, the human body has to burn more fuel or energy in order to maintain the body’s temperature. Similarly in a house, during a cold and windy day, the home heating system will consume more fuel in order to maintain the house’s temperature.
Ultimately, the human body doesn’t actually feel temperature. The human body senses thermal heat transfer and interprets the gain or loss of thermal energy as hot or cold. If the human body can comfortably burn enough energy to maintain body temperature, then the person will feel comfortable. For example, a person wearing cold weather gear with good insulation (reduction in conductive heat transfer) can feel warm outside in cold weather, whereas a person who is underdressed can feel cold, despite the fact that both are in the same outside air temperature.
In that manner, the wind chill temperature attempts to quantify the difference between the temperature a person would feel if it were a calm day with a normal rate of convective heat transfer as opposed to the temperature a person would feel with additional convective heat transfer loss.