DECEMBER 1998 NOTES AND CORRESPONDENCE 1187 NOTES AND CORRESPONDENCE The Steadman Wind Chill: An Improvement over Present Scales ROBERT G. QUAYLE National Climatic Data Center, Asheville, North Carolina ROBERT G. STEADMAN School of Agricultural Science, La Trobe University, Bundoora, Victoria, Australia 19 February 1998 and 24 June 1998 ABSTRACT Because of shortcomings in the current wind chill formulation, which did not consider the metabolic heat generation of the human body, a new formula is proposed for operational implementation. This formula, referred to as the Steadman wind chill, is based on peer-reviewed research including a heat generation and exchange model of an appropriately dressed person for a range of low temperatures and wind speeds. The Steadman wind chill produces more realistic wind chill equivalents than the current NWS formulation. It is easy to determine from tables (calculated by application of a quadratic ®t in both U.S. and metric units) with values accurate to within 18C. 1. Introduction freezing water to actually freeze under various wind and temperature conditions rather than any human physio- Many scientists have noted de®ciencies in the current logical model of heat loss and gain under equivalent wind chill scale (e.g., Court 1992; Dixon and Prior 1987; conditions. The major consequence is that water, not Driscoll 1985, 1992, 1994; Kessler 1993, 1994, 1995; having a metabolic heat source and not being appro- Horstmeyer 1994; Osczevski 1995; Schwerdt 1995; priately attired in human clothing, produces colder wind Steadman 1995). There was a session partly devoted to chills (for given wind and temperature conditions) than this topic at the 22d Annual Workshop on Hazards Re- those experienced by live, clothed humans. Further- search and Applications (13±16 July 1997) sponsored more, the traditional wind chill adaptation makes two by the Natural Hazards Research and Applications In- other assumptions that are not entirely correct. formation Center, Boulder, Colorado. Unquestionably the current wind chill scale has served us well in high- 1) The base wind speed at which the wind chill equiv- lighting the need for extra clothing and protection in alent temperature equals the ambient air temperature cold, windy conditions. But it may have also made us is shifted from 0 to 4 mph, which implies that winds complacent by exaggerating the chilling effect of the less than 5 mph have a warming effect at a given wind at very cold temperatures; lulling us, by experi- temperature. ence, into the notion that temperatures well below 2) Winds above 40 mph are assumed to have no ad- 2358C(2318F) are safe, when in fact, they can be ditional chilling effect. dangerous. Pioneering work by Steadman (1971) and his sub- The wind chill concept was pioneered by Siple and sequent re®nements (1979a,b, 1984, 1994; and 1996, Passel (1945), extrapolated for wind speeds above 12 21 1997, unpublished manuscripts) offer an improved wind ms , and promulgated by the National Oceanic and chill scale based on peer-reviewed research employing Atmospheric Administration (NOAA) and the media a biophysical energy balance model of the human body. (NWS 1992a,b; Kuhl 1992). The original wind chill is The Steadman model of heat balance of the human body, based on the length of time it took a vessel of near- originally conceived as a tool for textiles and clothing science, explicitly and implicitly includes all the major heat sources and sinks when estimating heat loss or gain to ascertain the comfort level of a person. Figure 1 sche- Corresponding author address: Dr. Robert G. Quayle, Chief, Glob- al Climate Lab., National Climatic Data Center, DOC/NOAA/NES- matically highlights the various heat exchange mecha- DIS, 151 Patton Avenue, Asheville, NC 28801-5001. nisms [German Klima±Michel Model (WMO 1997)] E-mail: [email protected] that are representative of the Steadman approach. While q 1998 American Meteorological Society Unauthenticated | Downloaded 10/03/21 04:00 PM UTC 1188 WEATHER AND FORECASTING VOLUME 13 FIG. 1. The heat balance of a human being: M 5 metabolic heat production, QH 5 sensible heat ¯ux, QSW 5 latent heat ¯ux, QL FIG. 2. Steadman wind chill vs air temperature and wind speed 5 moisture heat ¯ux, QRe 5 respiration heat ¯ux (sensible and from the NCDC model. latent). Radiation budget (including extra radiation received from sources external to the body): I 5 direct solar radiation, D 5 diffuse solar radiation, R 5 re¯ected radiation, A 5 atmospheric reradiation, E 5 radiation from the surroundings, EKM 5 radiation from human surfaces (after WMO 1997). 1 (Steadman 1971, 1984, 1994, 1995; and 1996, 1997, unpublished manuscripts). Although apparent temper- ature is robust with respect to most physiological and clothing variables, such as body temperature, moisture similar, the Klima±Michel model was not used in this permeability of clothing, skin resistances, etc., it is sen- study. The Steadman apparent temperature is operative sitive to the extent to which wind penetration lowers for both heat stress and cold stress (called wind chill). the clothing resistances to heat and moisture transfer. Its input variables include air temperature, humidity, This sensitivity derives partly from the fact that the wind speed, and external solar radiation loading, and it clothing resistance predominates, relative to skin and can be applied to various modeled levels of clothing. boundary layer, in cold conditions. The effects of hu- The National Weather Service (NWS 1992a) employs midity are relatively small at low temperatures and can a version of the Steadman apparent temperature scale be neglected. The warming effect of full midday sun as its heat index, using temperature and humidity input can be highly signi®cant but is not considered here, as only, and assumes appropriate clothing and a very light we are addressing only cloudy or shaded conditions and breeze. At cold temperatures the Steadman apparent are considering wind speed and temperature only. temperature scale, used to update the present wind chill The Steadman wind chill matrix from Table 1 (not formulation, offers a replacement for the wind chill for- the long series of Steadman equations) is converted from mula, tables, and graphs used by the NWS and others. metric to U.S. units. Then both metric and U.S. wind The revised scale is simple and provides accurate equa- chill tabulations are ®tted to regression surfaces using tions for both metric and U.S. units. The resulting Stead- second-order multiple-regression analysis. The equa- man wind chill includes heat gained by metabolism; heat tions for these smooth three-dimensional surfaces are retained by clothing; and heat lost by respiration and then used as the Steadman wind chill equations. The conduction, as exacerbated by low temperatures and the regression equations specify the wind chill as the de- wind. For a given temperature and wind speed, it pro- pendent variable, and air temperature and wind speed vides the equivalent temperature at which net heat loss as the independent variables. The resulting surfaces ®t would be the same as actual conditions but with calm the base data with an rmse of 0.318C and 0.558F, re- winds. spectively, and an r 2 of 0.9999. The T 2 term in both equations is negligible and is omitted. The resulting equations are as follows. 2. Wind chill calculations The Steadman wind chill equation in metric units is As our basic starting point we employ the complete wind chill 5 1.41 2 1.162V 1 0.980T Steadman tables including marginal notations for the SC effects of humidity and extra radiation as shown in Table 1 0.0124V 2 1 0.0185(VT), (1) Unauthenticated | Downloaded 10/03/21 04:00 PM UTC D TABLE 1. Steadman (1996) apparent temperature (8C,ms21): Wind effects at low temperatures including humidity and added radiation effects. These are the input data used to derive the ECEMBER Steadman wind chill equations. Danger: Bare skin freezes at wind chill of 2358C and colder. Additionally, prolonged exposure of skin to any freezing temperatures can cause injury. Metric units 1998 Vapor pres- Approx Additive Wind speed (m s21) sure (hPa) RH at base humidity saturation Base VP % Dry bulb (8C) 2.0 4.0 6.0 8.0 10.0 12.0 15.0 20.0 effect* (8C) 95.82 17.50 18 45.0 44.8 44.6 44.4 44.4 44.4 44.2 44.3 44.2 73.77 17.26 23 40.0 39.8 39.3 39.0 38.8 38.6 38.4 38.1 37.7 56.22 16.98 30 35.0 34.7 34.0 33.4 32.9 32.4 32.0 31.4 30.3 42.44 16.71 39 30.0 29.6 28.7 27.8 27.0 26.3 25.7 24.8 23.4 37.80 16.60 44 28.0 27.6 26.6 25.6 24.7 24.0 23.3 22.3 20.8 33.62 16.40 49 26.0 25.6 24.4 23.4 22.4 21.6 20.9 19.8 18.2 29.84 15.60 52 24.0 23.6 22.3 21.1 20.1 19.2 18.4 17.2 15.4 26.44 14.80 56 22.0 21.6 20.1 18.8 17.7 16.8 15.9 14.6 12.7 23.38 14.00 60 20.0 19.4 18.0 16.6 15.4 14.4 13.4 12.1 10.0 0.491 NOTES AND CORRESPONDENCE 20.62 13.20 64 18.0 17.4 15.9 14.4 13.1 12.0 11.0 9.6 7.3 0.423 18.18 12.40 68 16.0 15.4 13.8 12.2 10.8 9.7 8.4 6.9 4.6 0.369 16.00 11.60 73 14.0 13.4 11.6 10.0 8.4 7.3 5.6 3.9 1.2 0.310 14.13 10.80 76 12.0 11.4 9.4 7.6 6.0 4.6 3.2 1.4 21.4 0.260 12.28 10.00 81 10.0 9.3 7.3 5.4 3.7 2.2 0.9 21.1 24.0 0.221 10.73 9.20 86 8.0 7.3 5.2 3.2 1.4 0.0 21.6 23.6 26.6 0.188 9.35 8.40 90 6.0 5.3 3.1 1.0 20.8 22.4 23.9 26.0 29.2 0.159 8.13 7.60 93 4.0 3.3 1.0 21.1 23.0 24.7 26.3 28.4 211.9 0.134 7.06 6.80 96 2.0 1.2 21.1 23.3 25.3 27.0 28.6 210.9 214.4 0.112 6.11 6.00 98 0.0 20..8 23.2 25.6 27.6 29.3 211.0 213.4 217.2 0.097 5.18 5.18 100 22.0 22.9 25.3 27.7 29.8 211.7 213.4 215.9 219.8 4.37 4.37 100 24.0 25.0 27.4 29.9 212.0