Windchill Factor

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.

What does wind chill mean?

The wind chill factor is a temperature that represents the “feels like” temperature of wind on exposed skin in terms of an equivalent temperature in calm conditions. Wind speed is the most important factor in calculating wind chill, but humidity and pressure are also sometimes included. The wind chill effect arises because as wind blows over the skin, it evaporates moisture, leading to a cooling effect.

Wind chill factors are generally only considered for temperatures below 20C. Examples of some wind chill factors are:

20C with no wind = 20C wind chill

20C with 30kph wind = 15C wind chill

15C with 20kph wind = 10C wind chill

15C with 60kph wind = 5C wind chill

5C with 60kph wind = -10C wind chill

0C with 40kph wind = -15C wind chill

Wind chill is not an exact measurement. Direct sunlight can provide some warming effect to counteract it. Different people may feel wind chill in different ways, such as thin people who will lose heat faster than larger people. However, it is a useful measurement. Wind chill below -50C can lead to unconsciousness and eventually death. Generally, in Australian conditions, wind chill factors will not fall far below -15C as seen above. While this may be unpleasant, it will have no impact generally on human health.

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