|
We have all experienced the difference in comfort between a cold day and a cold, windy day. Wind dramatically increases the heat transfer coefficient, increasing the effectiveness of heat flow out of your body. To quantify this effect, in 1941, explorers Paul Siple and Charles Passel, of the U.S. Antarctic Service Expedition, performed a very simple experiment during the frigid Antarctic winter. Siple and Passel measured how long it took for a gallon of water to freeze at different air temperatures and wind speeds. For example, they found a temperature of 0°F with a 30 mph wind froze the water in the same time as a temperature of −26°F with no wind. 
|
|
|
To illustrate their findings, Siple and Passel created the wind chill index. A wind chill of −26°F corresponds to an air temperature of 0ºF with a 30 mph wind. This means that a body loses heat at the same rate as if the air temperature were actually −26°F without any wind. The chart below shows the equivalent wind chill temperatures for different temperatures and wind speeds.
|
|
|
The wind chill temperature represents an imaginary temperature at which there is an equivalent heat loss rate between free convection with no wind and forced convection at the actual temperature and wind speed. Wind chill does not mean that the actual temperature goes down with the wind speed! Many people make this mistake when interpreting the meaning of wind chill. For example, the antifreeze in a car radiator will freeze at −30ºF. Will the radiator freeze if the air temperature is −10ºF with a 30 mph wind? The answer is no! The radiator will lose heat as if the temperature was −39ºF according to the wind chill index but the lowest temperature the radiator will reach is the same as the air temperature, or −10ºF, so it will not freeze.
|
Which of the following statements best describes the wind chill temperature? - Wind chill describes how temperature decreases as the velocity of air increases.
- Wind chill is equal to the air temperature minus the wind speed in meters per second.
- Wind chill is the imaginary temperature at which heat loss in still air is equal to heat loss by forced convection.
 |
Choice c is correct. Wind chill is the imaginary temperature at which an object loses heat in still air at the same rate as at the actual temperature, after including the effects of enhanced convection from the wind. 
|
