Wind-chill, windchill, wind chill factor, wind chill index is an estimate of how cold you will feel at a given air temperature when there is a wind blowing. It is a popular tool used by weather presenters to make you feel worse about going outside when it’s cold and windy! Seriously, if, for example, the reported air temperature (as measured by a thermometer housed in one of those white boxes with the grills, a Stevenson screen, or shelter) is -7 Celsius and the wind is blowing at a steady 8 km/h, then it will “feel” like it’s -11 Celsius. But why and how does one get from -7 to -11, 4 degress C cooler?
Here’s the physics and formula courtesy of Wikipedia: “A surface loses heat through conduction, convection, and radiation. The rate of convection depends on the difference in temperature between the surface and its surroundings. As convection from a warm surface heats the air around it, an insulating boundary layer of warm air forms against the surface. Moving air disrupts this boundary layer, or epiclimate, allowing for cooler air to replace the warm air against the surface. The faster the wind speed, the more readily the surface cools.” Additionally, skin sweats so air blowing on exposed sweaty skin will cause cooling by evaporation, but the likelihood of you sweating and then exposing your skin when you’re outside and it’s -7 Celsius is quite low…
The calculation used by the US and UK weather forecasters and meteoroligists for wind chill is quite complicated looking but relatively easy to implement with a modern calculator, although different regions use different units F vs C, mph vs kmh, so you have to make sure you’re using the appropriate formula. A neat chart (by RicHard-59 on Wiki) saves you the trouble of doing the calculations yourself, if you’re a common or garden weather presenter.
And, here’s a nice video description of wind chill that I first saw mentioned on the Free Technology for Teachers blog:
For pedants in Australia, it’s rare that a wind chill is observed, you might want to try “heat index” instead… ;-)