Felt Air Temperature - Explanation

Explanation

The human body loses heat through convection, evaporation, conduction, and radiation. The rate of heat loss by a surface through convection depends on the wind speed above that surface. As a surface heats the air around it, an insulating boundary layer of warm air forms against the surface. Moving air disrupts the boundary layer, allowing for new, cooler air to replace the warm air against the surface. The faster the wind speed, the more readily the surface cools.

The speed of cooling has different effects on inanimate objects and biological organisms. For inanimate objects, the effect of wind chill is to reduce any warmer objects to the ambient temperature more quickly. It cannot, however, reduce the temperature of these objects below the ambient temperature, no matter how great the wind velocity (however a wet surface can become cooler than ambient temperature, due to loss of latent heat, for example, water sticking to a wet brick can freeze while the air temperature is above freezing point). For most biological organisms, the physiological response is to maintain surface temperature in an acceptable range so as to avoid adverse effects. Thus, the attempt to maintain a given surface temperature in an environment of faster heat loss results in both the perception of lower temperatures and an actual greater heat loss increasing the risk of adverse effects such as frostbite, hypothermia, and death.

A surface that is wet, such as a person wearing wet clothes, will lose heat quickly as the moisture evaporates and therefore feels colder. Conversely, humid air slows evaporation and makes a surface feel warmer, and this is incorporated into longer wind chill formulas. During warm months, this effect can be described in the heat index or humidex.