Atmosphere And Weather

# How is Wind Chill Calculated

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Just what is meant by wind chill factor? Put simply, this is how cold exposed human skin will feel outside when the temperature of the air is affected by wind. It should be noted that the wind chill index is only used when the temperature is 50 degrees Fahrenheit ( 10 degrees Celsius) or lower and the wind speed is greater than 3 miles per hour (about 5 km per hour). This scale further assumes the speed of wind at an elevation of 5 feet (about 152 cm), for this is approximately the average height above ground of a human’s face. Finally, sunshine and its apparent heat are not taken into consideration. As a consequence, the lowest wind chill indexes usually (but not always) occur during the nighttime hours. Thanks to this index, weather services can implement advisories as to how to dress and how quickly exposed skin will suffer from the effects of frostbite.

The idea for this relative index was first coined in 1939 by an Antarctic explorer named Paul Siple. In the following years, Siple and Charles Passel determined how long it took water to freeze in a plastic cylinder in various temperatures and wind conditions. It was found that depending on how strong the outside wind, the water would turn to ice at a quicker rate. Thus, a mathematical formula was devised to calculate the apparent temperature at a given wind velocity.

The formula was revised in 2001 by both the US and Canadian National Weather Services. Up until this time, wind velocity was measured at a height of 33 feet (10 M); the typical elevation of instruments used in weather observations. Well, it just so happens that average wind speed at that height is about three times greater than at the height of the typical person’s face. This in turn raised the old wind chill factors to far worse numbers than what was actually being experienced (that is, unless you happened to be outside on a third-floor balcony!)

So when you hear your parents or grandparents talking about that brutal winter with wind chills of 80 degrees below zero, it must be remembered that this was before the scales were revised. Moreover, during those ensuing six decades, we learned much more about how wind depletes heat from the bodies of humans and animals.

The formula itself is rather complex, and as a result, it is far easier to refer to conversion charts. At any rate, here is how it works, both in Fahrenheit and Celsius temperature scales:

Fahrenheit

35.74 + .6215 (temperature in degrees Fahrenheit) minus 35.75 (wind velocity in mph to the .16 power) +.4275 (temperature) x wind velocity to the .16 power

Celsius

13.112 + .6215 (temperature in degrees Celsius) minus 11.37 (wind velocity in kilometers per hour to the .16 power) + .3965 (temperature) x wind velocity to the .16 power

Just as brilliant sunshine can make temperatures feel warmer, winds can make the temperature feel much colder than what that thermometer may read. In climates that experience cold winter weather, wind chill factors serve as a very useful tool for anyone who must spend any appreciable time outdoors.

Sources:

http://www.nws.noaa.gov/om/windchill/

http://www.usatoday.com/weather/resources/basics/windchill/wind-chill-formulas.htm

http://www.islandnet.com/~see/weather/life/windchill.htm#c

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