WIND CHILL Weather data – Derived Variables
Parameters Used: Outside Air Temperature and Wind Speed
What is it?
Wind chill takes into account how the speed of the wind affects our perception of the air temperature. Our bodies warm the surrounding air molecules by transferring heat from the skin. If there’s no air movement, this insulating layer of warm air molecules stays next to the body and offers some protection from cooler air molecules.
However, wind sweeps that comfy warm air surrounding the body away. The faster the wind blows, the faster heat is carried away and the colder the environment feels.
The formula below was adopted by both Environment Canada and the U.S. National Weather Service to ensure a uniform wind chill standard in North America.
The formula is supposed to more closely emulate the response of the human body when exposed to conditions of wind and cold than the previous formula did:
35.74 + 0.6215T – 35.75 * (V 0.16 ) + 0.4275T * (V 0.16 )
This relationship takes into account the fact that wind speeds are measured “officially” at 10 meters (33 feet) above the ground, but the human is typically only 5 to 6 feet (2 meters) above the ground.
So, anemometers still need to be mounted as high as possible (e.g., rooftop mast) to register comparable wind speed readings and wind chill values. See Application Note 30 for siting recommendations and guidelines.
This newer version of the formula addresses the fact that the latest National Weather Service (NWS) formula was not designed for use above 40°F. The result of the straight NWS implementation was little or no chilling effect at mild temperatures. This updated version provides for reasonable chilling effect at mild temperatures based on the effects determined by Steadman (1979) (see THSW Index section), but as with the new NWS formula, no upper limit where chilling has no additional effect. No chilling affect occurs at wind speeds of 0 mph or temperatures at or above 93.2°F (34°C). As per Steadman (1979), 93.2 F (34°C) is the average temperature of skin at mild temperatures, thus temperatures above this value will actually create an apparent warming effect (see THSW Index section).
The Vantage Pro2, Vantage VUE, and Vantage Pro consoles use the “10-minute average wind speed” to determine wind chill, which is updated once per minute. When 10-minute of wind speed data is unavailable, it uses a running average until 10-minutes worth of data is collected. The WeatherLink® software uses the 10-minute average wind speed also. If it is unavailable, it uses the current wind speed (which updates every 2.5 to 3 seconds).
The reason an average wind speed is employed in the Vantage Pro2, Vantage VUE and Vantage Pro to calculate wind chill is as follows: The human body has a high heat capacity, thus wind gusts have no effect on the body’s thermal equilibrium. So, an average wind speed provides a more accurate representation of the body’s response than an instantaneous reading. Also, “official” weather reports (from which wind chill is calculated) provide average wind speed, so using an average wind speed more closely matches the results that are seen in weather reports.
“Media Guide to NWS Products and Services”, National Weather Service Forecast Office, Monterey, CA, 1995.
“New Wind Chill Temperature Index”, Office of Climate, Water and Weather Services, Washington, DC, 2001.
Siple, P. and C. Passel, 1945. Measurements of Dry Atmospheric Cooling in Subfreezing Temperatures. Proc. Amer. Philos. Soc.
Steadman, R.G., 1979: The Assessment of Sultriness, Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science. Journal of Applied Meteorology, July 1979
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