MarkB_MI said: (quoted from post at 12:16:34 12/11/09) Wind chill is a calculated number. Just because it's generally used to quantify the perception of temperature by humans doesn't mean it doesn't affect inanimate objects. You don't have to send a person outside to calculate wind chill, you only need to know wind velocity and air temperature.
........a bare face in the wind............
In November 2001 the National Weather Service implemented the new wind chill index, used by the U.S. and Canadian weather services, which is determined by iterating a model of skin temperature under various wind speeds and temperatures. The model used standard engineering correlations of wind speed and heat transfer rate. Heat transfer was calculated for a bare face in wind, facing the wind, while walking into it at 1.4 metres per second (3.1 mph). The model corrects the officially measured wind speed to the wind speed at face height, assuming the person is in an open field.[7] The results of this model may be approximated, to within one degree, from the following formula:
where T_{wc}\,\! is the wind chill index based on the Celsius scale, T_a\,\! is the air temperature in °C, and V\,\! is the air speed in km/h measured at 10 metres (33 ft), standard anemometer height).[8]
where T_{wc}\,\! and T_a\,\! are measured in °F, and V\,\! in mph.
Windchill Temperature is only defined for temperatures at or below 10 °C (50 °F) and wind speeds above 4.8 kilometres per hour (3.0 mph).[9]
As the air temperature falls, the chilling effect of any wind that is present increases. For example, a 16 km/h (9.9 mph) wind will lower the apparent temperature by a wider margin at an air temperature of −20 °C (−4.0 °F), than a wind of the same speed would if the air temperature were −10 °C (14.0 °F).
Celsius Wind Chill Chart w/Frostbite information
Comparison of old and new values of Wind Chill at −15 °C (5.0 °F)
The method for calculating wind chill has been controversial because experts[who?] disagree on whether it should be based on whole body cooling either while naked or while wearing appropriate clothing, or if it should be based instead on local cooling of the most exposed skin, such as the face. The internal thermal resistance is also a point of contention. It varies widely from person to person. Had the average value for the subjects been used, calculated WCET's would be a few degrees more severe.
The 2001 WCET is a steady state calculation (except for the time to frostbite estimates[10]) There are significant time-dependent aspects to wind chill because cooling is most rapid at the start of any exposure, when the skin is still warm.
The exposure to wind depends on the surroundings and wind speeds can vary widely depending on exposure and obstructions to wind flow.
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