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The Leader

New Wind Chill Index Implemented

By Matt Wendling
NOLS Intern
Reprinted from The Leader, Spring 2002, Vol. 17, No. 2

Anyone who's spent a gusty day hiking in the mountains knows that wind means colder temperatures. The sensation of wind chill is nothing new. Recent studies, however, have shed new light on the subject.

Wind chill is difficult to study because it can't be measured by an instrument. In calm weather conditions the human body is protected from external temperatures by a layer of warm air next to the skin. The feeling we call wind chill is caused by the wind whisking this warm layer away and replacing it with cold air. As a result, the body must work harder to heat up the new air. If this layer of warm air continues to be blown away, the body's skin temperature drops, causing frost bite in temperatures below freezing.

On November 1, 2001, the National Weather Service (NWS) and Meteorological Services of Canada (MSC) implemented a new Wind Chill Temperature (WCT) index. Using advances in science and new technologies such as computer modeling, weather experts were able to improve upon the old WCT index. The original index was based on the Siple and Passel Formula devised by Antarctic explorers in the 1940s. The formula was determined through a series of observations on freeze times of plastic cylinders filled with water and placed in differing temperatures and wind speeds. This old system was flawed in that the human body, unlike plastic, produces its own heat. In addition, weather stations measure wind speed, the basis for the WCT index, at a height of 33 feet where it is often blowing stronger. This lead to meteorologists dispensing wind chill readings much lower than actuality.

The new WCT index accepted last November is based upon studies using volunteers placed in a wind tunnel with various combinations of wind and temperature. For this new index, wind speed is measured at a height of five feet, the average height of a human face. The index takes into account heat loss from a body to its surroundings. Researchers were also able to develop a standard figure for skin tissue resistance to wind. The end result from these studies: it's not quite as cold as before. According to the old system of wind chill measurement, an air temperature of 20 degrees Fahrenheit combined with 15 mile per hour winds creates a WCT index of five degrees below zero. These same conditions measured by the new system equal an index of six above. So, as the mercury plunges and the winds pick up, take heart, it's not nearly as cold as last year.



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