Winter Weather Topics

Week 2: Wind Chill

Wind chill is the temperature a person perceives based on the actual air temperature and the wind speed.  The human body creates a natural heat barrier that serves to protect itself from the cooling effects of the surrounding environment.  When conditions are calm, this barrier remains in place around the body and is able to effectively keep the body warm.  However...when the wind blows, the barrier becomes disturbed and results in a net loss of heat from the body.  Wind chill is a numeric description of the loss of heat from the body caused by the prevailing wind speed at a specific air temperature. 

Undisturbed heat barrier                               Disturbed heat barrier
under calm conditions                                   due to wind

For many years the National Weather Service used a wind chill formula based upon research conducted in 1945 by Antarctic explorers Siple and Passel.  They measured the cooling rate of water in a container hanging outside: however, since water freezes faster than flesh, their results underestimated the time to freezing and overestimated the cooling effects of the wind.  Below is the previous wind chill chart developed as a result of the 1945 research.

In 2000, a joint effort between the U.S. and Canadian governments sought to test and implement a new wind chill formula based on modern scientific theories and techniques.  Human trials consisting of 6 male and 6 female volunteers were conducted, with each subject being placed in a wind tunnel and exposed to varying wind speeds and temperatures.  Special thermal transducers measured heat flow away from their cheeks, forehead, nose, and chin.  The data collected from this experiment was then used to create a more accurate wind chill formula based on modern heat flow theory.  Below is the updated wind chill chart currently used by the NWS.

When potentially dangerous wind-chill readings are expected, the NWS will issue either a Wind Chill Advisory or a Wind Chill Warning. 

For the NWS Central Illinois County Warning Area (CWA), the threshold for an advisory is -15F or colder, while a warning will be issued for wind chill values of -25F or colder.  As you can see from the two maps below, each weather service office has specific local criteria for both products.  For example, the St. Louis, Central Illinois, and Indianapolis offices use -15F as a threshold for a Wind Chill Advisory...while offices further north such as Davenport and Chicago use slightly colder values of -20F...and Minneapolis and Duluth go even colder with -25F.

Week 1: Winter Precipitation Types