How cold is it outside? Simply knowing the temperature doesn't tell you enough about the conditions to enable you to dress sensibly for all winter weather. Other factors including wind speed, relative humidity and sunshine play important roles in determining how cold you feel outside. A description of the character of weather known as "coldness" was proposed about 1940 by scientists working in the Antarctic. The "wind chill index" was developed to describe the relative discomfort/danger resulting from the combination of wind and temperature.
On November 1, 2001, the National Weather Service began using a new wind chill index. The reason for the change is to improve upon the current index, which is based on the 1945 Siple and Passel Index. During the Fall of 2000, the Office of the Federal Coordinator for Meteorological Services and Supporting Research (OFCM) formed a special group consisting of several Federal agencies, MSC, the academic research community (Indiana University-Purdue University in Indianapolis (IUPUI), University of Delaware, and University of Missouri), and the International Society of Biometeorology. Their job was to evaluate the existing wind chill formula and make necessary changes to improve upon it. The group is called the Joint Action Group for temperature Indices (JAG/TI) and is chaired by the NWS. Weird name, but very important work. The goal of JAG/TI is to internationally upgrade and standardize the index for temperature extremes (a.k.a. Wind Chill Index). They ultimately reached an agreement on a new wind chill formula. It will make use of the advances in science, technology, and computer modeling to provide a more accurate, understandable, and useful formula for calculating wind chill. Lots of time and energy was put into coming up with the new formula and what it does differently. Specifically, the new wind chill index will use wind speed calculated at the average height (5 feet) of the human body's face instead of 33 feet (the standard anemometer height); be based on a human face model; incorporate modern heat transfer theory (heat loss from the body to its surroundings, during cold and breezy/windy days); lower the calm wind threshold to 3 mph; use a consistent standard for skin tissue resistance; and assume the worst case scenario for solar radiation (clear night sky). In 2002, adjustments for solar radiation (i.e., the impact of sun) for a variety of sky conditions (sunny, partly sunny and cloudy) will be added to the calculation model
Wind chill does not affect your car's antifreeze protection. It will have an impact on how quickly your home's exposed water pipes freeze, but has little impact on whether they would freeze or not. The importance of the wind chill index is as an indicator of how to dress properly for winter weather. In dressing for cold weather an important factor to remember is that entrapped insulating air warmed by body heat is the best protection against the cold. Consequently, wear loose-fitting, lightweight, warm clothing in several layers. Outer garments should be tightly- woven, water-repellant and hooded. Mittens snug at the wrist are better protection than fingered gloves.
To use the chart, find the approximate temperature on the top of the chart. Read down until you are opposite the appropriate wind speed. The number which appears at the intersection of the temperature and wind speed is the wind chill index.