Wednesday, December 8, 2010
We toss around the term "wind chill" without hesitation during cold spells like the one we are seeing right now. If you sit and think about it for just a second wind chill is a pretty complicated indicator. How cold does the temperature combined with the wind feel on your face? At what temperature, wind speed, and exposure time will frostbite occur? The first index used by the United States and Canada was based on research done by Antarctica explorers Siple and Passel in 1945. In 2001 the Office of the Federal Coordinator for Meteorological Services and Supporting Research decided that the index set forth in 1945 needed revising because it underestimated the time to freezing and overestimated the chilling effect of the wind. A group called the Joint Action Group for Temperature Indices or JAG/TI was organized by the U.S. and Canadian governments to upgrade and standardize the Wind Chill Index. Human trials were conducted in Toronto, Canada. Twelve volunteers were placed in a chilled wind tunnel with thermal transducers on their faces to measure heat flow from the cheeks, forehead, nose, and chin while walking 3 mph on a treadmill. Each was exposed to varying wind speeds and temperatures. A new wind chill formula was created using the information obtained along with advances in science, technology, and computer modeling to provide a more accurate formula for computing the Wind Chill Index. And, for the first time the new Wind Chill Chart includes a frostbite indicator showing where temperature, wind speed , and exposure time will produce frostbite on humans.
This new Wind Chill Index now being used by the U. S. and Canada does the following according the the National Weather Service:
-Calculates wind speed at an average height of five feet, typical height of an adult human face, based on readings from the national standard height of 33 feet, typical height of an anemometer
-Is based on a human face model
-Incorporates heat transfer theory, heat loss from the body to its surroundings, during cold and breezy/windy days
-Lowers the calm wind threshold to 3 mph
-Uses a consistent standard for skin tissue resistance
-Assumes no impact from the sun (i.e., clear night sky).
The following is the current Wind Chill formula:
where and are measured in °F, and in mph
The graph below indicates the difference between the old and new Wind Chill indicies.