Fact Sheet - The New Wind Chill Index
Shawn Crowe Research Assistant and Stuart A. Foster
State Climatologist for Kentucky
 
Wind Chill is the term used to describe the rate of heat loss on the human body resulting from the combined effect of low temperature and wind. As winds increase, heat is carried away from the body at a faster rate, driving down both the skin temperature and internal body temperature. This effect is only present for humans and animals, not for other inanimate objects made of metals, plastics, etc. Meteorologists with the National Weather Service have recently devised a new way to calculate wind chill that should prove more accurate than the system that has been in use for years.

The wind chill system used until this year has been in use in the U.S. and Canada since 1945. This system relies on observed wind speeds at 33 feet above the surface and the amount of time it takes the combination of wind and temperature to make water freeze. The formula used in this calculation is:

Old Wind Chill = 0.081 * (3.71 * sqrt (V) + 5.81 - 0.25 * V) * (T - 91.4) + 91.4
Where T = temperature in degrees Fahrenheit and V = wind speed in MPH


The new wind chill system will be much more detailed, accounting for wind effects at face-level. This should result in more accurate calculations of human body heat loss. An updated calculation of heat transfer (from the body to its surroundings), a standard for skin tissue resistance, clear night sky conditions, and a lowered calm wind threshold from 4 to 3 MPH are also new additions to the wind chill calculation and should result in more accuracy.

As an example of the difference between the two systems, if the air temperature is 35°F, and the wind is blowing at 15 MPH, the old wind chill value would have been 16°F. On the new system, the wind chill value will be 25°F, a considerable difference. In general, the new index will be warmer than you would have expected with the old index. The new more conservative wind chill system will be put into use beginning with the 2001-2002 winter season.

The new formula for calculating wind chill is considerably different than the previous one:

New Wind Chill = 35.74 + 0.6125T - 35.75 (V0.16) + 0.4275T (V0.16)
Where T = temperature in degrees Fahrenheit and V = wind speed in MPH


This new formula has been reviewed and tested by scientists from all over the world. The hope of the National Weather Service is to create a standard wind chill index that will be used by all countries.

Wind Chill Temperature Comparison Graph

REFERENCES:
Wind Chill. National Weather Service Sioux Falls, SD website. 2001. URL: www.crh.noaa.gov/fsd/windchill.htm.
Just in Time for Winter, Nation Gets New Wind Chill Index Formula. National Oceanic and Atmospheric Administration website. 2001.

www.noaanews.noaa.gov/stories/s720.htm