Wind Chill Equivalent Temperature Research Articles

The Wind-Chill Index.

This Lessons from History article about the wind-chill index (WCI) explores the historical polar and meteorologic literature relevant to the topic and presents unpublished work from 1939. Geographer Paul Siple (1908-1968) was a 6-time Antarctic explorer and scientist who invented and named the WCI in his doctoral dissertation at Clark University. Siple and Charles Passel (1915-2002) performed studies in Antarctica in 1940 that led to publication in 1945. This paper is often credited as the beginning of the WCI. Through years of critiques and revisions by others, these efforts evolved into the wind-chill equivalent temperatures (WCTs) used today. This essay explores the history, the science, and the overlooked originality, simplicity, and details of Siple's unpublished work. The remarkable similarity of the original chart to a current chart is shown by adapting and overlaying the 1939 WCI onto a current WCT chart with its times-to-frostbite data. The writings of Siple, Passel, and others provide an evocative supporting narrative to illustrate some of the problems of living in cold environmental conditions.

Death Zone Weather Extremes Mountaineers Have Experienced in Successful Ascents.

BackgroundFew data are available on mountaineers’ survival prospects in extreme weather above 8000 m (the Death Zone). We aimed to assess Death Zone weather extremes experienced in climbing-season ascents of Everest and K2, all winter ascents of 8000 m peaks (8K) in the Himalayas and Karakoram, environmental records of human survival, and weather extremes experienced with and without oxygen support.Materials and MethodsWe analyzed 528 ascents of 8K peaks: 423 non-winter ascents without supplemental oxygen (Everest–210, K2–213), 76 ascents in winter without oxygen, and 29 in winter with oxygen. We assessed environmental conditions using the ERA5 dataset (1978–2021): barometric pressure (BP), temperature (Temp), wind speed (Wind), wind chill equivalent temperature (WCT), and facial frostbite time (FFT).ResultsThe most extreme conditions that climbers have experienced with and without supplemental oxygen were: BP 320 hPa (winter Everest) vs. 329 hPa (non-winter Everest); Temp –41°C (winter Everest) vs. –45°C (winter Nanga Parbat); Wind 46 m⋅s–1 (winter Everest) vs. 48 m⋅s–1 (winter Kangchenjunga). The most extreme combined conditions of BP ≤ 333 hPa, Temp ≤ −30°C, Wind ≥ 25 m⋅s–1, WCT ≤ −54°C and FFT ≤ 3 min were encountered in 14 ascents of Everest, two without oxygen (late autumn and winter) and 12 oxygen-supported in winter. The average extreme conditions experienced in ascents with and without oxygen were: BP 326 ± 3 hPa (winter Everest) vs. 335 ± 2 hPa (non-winter Everest); Temp −40 ± 0°C (winter K2) vs. −38 ± 5°C (winter low Karakoram 8K peaks); Wind 36 ± 7 m⋅s–1 (winter Everest) vs. 41 ± 9 m⋅s–1 (winter high Himalayan 8K peaks).Conclusions1.The most extreme combined environmental BP, Temp and Wind were experienced in winter and off-season ascents of Everest.2.Mountaineers using supplemental oxygen endured more extreme conditions than climbers without oxygen.3.Climbing-season weather extremes in the Death Zone were more severe on Everest than on K2.4.Extreme wind speed characterized winter ascents of Himalayan peaks, but severely low temperatures marked winter climbs in Karakoram.

Comparison of Environmental Conditions on Summits of Mount Everest and K2 in Climbing and Midwinter Seasons.

(1) Background: Today’s elite alpinists target K2 and Everest in midwinter. This study aimed to asses and compare weather at the summits of both peaks in the climbing season (Everest, May; K2, July) and the midwinter season (January and February). (2) Methods: We assessed environmental conditions using the ERA5 dataset (1979–2019). Analyses examined barometric pressure (BP), temperature (Temp), wind speed (Wind), perceived altitude (Alt), maximal oxygen uptake (VO2max), vertical climbing speed (Speed), wind chill equivalent temperature (WCT), and facial frostbite time (FFT). (3) Results: Most climbing-season parameters were found to be more severe (p < 0.05) on Everest than on K2: BP (333 ± 1 vs. 347 ± 1 hPa), Alt (8925 ± 20 vs. 8640 ± 20 m), VO2max (16.2 ± 0.1 vs. 17.8 ± 0.1 ml·kg−1·min−1), Speed (190 ± 2 vs. 223 ± 2 m·h−1), Temp (−26 ± 1 vs. −21 ± 1°C), WCT (−45 ± 2 vs. −37 ± 2 °C), and FFT (6 ± 1 vs. 11 ± 2 min). Wind was found to be similar (16 ± 3 vs. 15 ± 3 m·s−1). Most midwinter parameters were found to be worse (p < 0.05) on Everest vs. K2: BP (324 ± 2 vs. 326 ± 2 hPa), Alt (9134 ± 40 vs. 9095 ± 48 m), VO2max (15.1 ± 0.2 vs. 15.3 ± 0.3 ml·kg−1·min−1), Speed (165 ± 5 vs. 170 ± 6 m·h−1), Wind (41 ± 6 vs. 27 ± 4 m·s−1), and FFT (<1 min vs. 1 min). Everest’s Temp of −36 ± 2 °C and WCT −66 ± 3 °C were found to be less extreme than K2’s Temp of −45 ± 1 °C and WCT −76 ± 2 °C. (4) Conclusions: Everest presents more extreme conditions in the climbing and midwinter seasons than K2. K2’s 8° higher latitude makes its midwinter BP similar and Temp lower than Everest’s. K2’s midwinter conditions are more severe than Everest’s in the climbing season.

HERMAL COMFORT FROM RURAL WORKERS DURING TOBACCO HARVEST

A cultura do tabaco é desenvolvida principalmente em propriedade familiar por necessitar de mão-de-obra intensiva e pouco mecanizada. O presente trabalho tem como objetivo investigar o conforto térmico de trabalhadores rurais durante a colheita do tabaco, utilizando os índices de conforto térmico: Índice de temperatura e umidade (ITU) e Temperatura Equivalente de Windchill (Twc), na região do Vale do Rio Pardo – RS. Para isso, primeiramente realizou-se a coleta de informações por meio de um questionário respondido por 80 produtores rurais e, posteriormente, foi realizada uma análise bioclimática por meio do cálculo e interpretação dos índices. A análise dos índices e os tipos de desconfortos sentidos durante a colheita do tabaco relatados pelos produtores rurais, são indicativos de estresse calórico no turno de trabalho da tarde (14:00 às 19:00 h) para os meses de dezembro e janeiro. Já, para o turno de trabalho da manhã, a sensação térmica é de frio (&lt; 15ºC), em função da velocidade do vento existente na região (&gt;2,0 m/s). Pausas na atividade de colheita devem ser realizadas no período da tarde, em que o estresse calórico existe em função das condições meteorológicas e pode ser potencializado com estresse físico acumulado ao longo do dia de trabalho.

Development of the Adjusted Wind Chill Equivalent Temperature (AWCET) for cold mortality assessment across a subtropical city: validation and comparison with a spatially-controlled time-stratified approach

BackgroundGlobal warming has reduced the adaptability of the people living in subtropical regions to cope up with cold stress due to lengthening of hot days and shortening of transition period from hot to cold weather. However, existing studies on measuring cold stress are based on biometeorological indices designed for temperate regions. This may overestimate the impact of wind chill on mortality risk in subtropical cities.MethodsThis study developed an Adjusted Wind Chill Equivalent Temperature (AWCET) index. A spatially-controlled time-stratified approach was applied to evaluate the ability of AWCET for estimating cold mortality in subtropical cities, based on a mortality dataset (2008–2012) in Hong Kong.ResultsThe use of AWCET could indicate increase in all-cause, cardiovascular, respiratory, and cancer-related mortality risk during the days with average temperature < = 1st [11.0 °C], <= 3rd [12.6 °C] and < = 5th [13.4 °C] percentiles. The results were stable and consistent based on both log-linear and curve-linear relationships between AWCET and mortality risk. AWCET was also compared with the New Wind Chill Equivalent Temperature (NWCET) designed for temperate regions, and has found that higher magnitude of mortality risk would be found when using AWCET for assessing all-cause and cause-specific mortality in Hong Kong, for days with average temperature < = 1st, <= 3rd and < = 5th percentiles.ConclusionsAWCET is validated to be effective to access cold mortality in the context of subtropical cities. The use of AWCET may enhance the cold weather warning system in subtropical cities, as a supplementary tool to help demonstrating small administrative-level perceived temperature with volunteered geographic information.

Spatiotemporal Prediction of Increasing Winter Perceived Temperature across a Sub-Tropical City for Sustainable Planning and Climate Change Mitigation.

Climate variability has been documented as being key to influencing human wellbeing across cities as it is linked to mortality and illness due to changes in the perceived weather cycle. Many studies have investigated the impact of summer temperature on human health and have proposed mitigation strategies for summer heat waves. However, sub-tropical cities are still experiencing winter temperature variations. Increasing winter perceived temperature through the decades may soon affect city wellbeing, due to a larger temperature change between normal winter days and extreme cold events, which may cause higher health risk due to lack of adaptation and self-preparedness. Therefore, winter perceived temperature should also be considered and integrated in urban sustainable planning. This study has integrated the increasing winter perceived temperature as a factor for developing spatiotemporal protocols for mitigating the adverse impact of climate change. Land surface temperature (LST) derived from satellite images and building data extracted from aerial photographs were used to simulate the adjusted wind chill equivalent temperature (AWCET) particularly for sub-tropical scenarios between 1990 and 2010 of the Kowloon Peninsula, Hong Kong. Compared with perceived temperature based on the representative station located at the headquarters of the Hong Kong Observatory, the temperature of half the study area in the Kowloon Peninsula has raised by 1.5 °C. The areas with less green space and less public open space in 2010 show higher relative temperatures. Socioeconomically deprived areas (e.g., areas with lower median monthly income) may suffer more from this scenario, but not all types of socioeconomic disparities are associated with poor sustainable planning. Based on our results and the “no-one left behind” guideline from the United Nations, climate change mitigation should be conducted by targeting socioeconomic neighborhoods more than just aging communities.

Investigation of Pedestrian Comfort with Wind Chill during Winter

Two types of methods are used to evaluate pedestrian comfort: pedestrian wind comfort and outdoor thermal comfort. To accurately ascertain the outdoor wind environment, wind speed is the only parameter considered. However, pedestrians may still feel discomfort when the perceived temperature is low, even though the wind comfort criterion has been satisfactorily fulfilled. The purpose of this study is, therefore, to investigate pedestrian comfort when the perceived temperature is low, especially in winter conditions. To achieve this, a pedestrian survey was conducted, and 588 respondents completed a questionnaire. The results show that pedestrians feel discomfort when the WCET (Wind Chill Equivalent Temperature) is low, with almost 40 percent of respondents answering that they feel discomfort in these conditions. In conclusion, the threshold wind speed of the winter season could be determined to be lower than that of the existing comfort criteria by applying the WCET.

Directional distribution of chilling winds in Estonia.

Wind chill equivalent temperature (WCET) is used to define thermal discomfort in winter months. Directional distributions of winds, which are associated with uncomfortable weather, were composed of three climatologically different Estonian locations: Vilsandi, Kuusiku, and Jõhvi. Cases with wind chill equivalent temperature <-10°C, which could be classified as "uncomfortable or worse," were investigated. Additional thresholds were used to measure weather risk. The 25th percentile of daily minimum WCET was tested to measure classical prevalent wind directions in Estonia: W, SW, and NW bring warm air in winter from the North Atlantic, while winds from the East-European plain (NE, E, and SE) are associated with cold air. The eastern prevalence was stronger when a lower threshold was used. A directional approach may find several applications, such as building, agricultural, landscape, or settlement planning.

Low temperatures and wind; challenges, applicability and limitations from an industrial perspective

The effect of low temperature and wind on the cooling of exposed skin has been a field of interest since the first study by Siple and Passel in 1945[1]. This resulted in the Wind Chill Index (WCI), which expressed the rate of heat loss of a cylinder per unit surface mass. The WCI remained unchanged for many years, and is still regarded as an established and well-defined framework for assessing cold stress. However, in recent years, the Wind Chill Equivalent Temperature (WCT) has supplanted the WCI. WCT is a calculated air temperature based on dry-bulb temperature and wind speed. In the last number of years, these equations have undergone several revisions, in an attempt to develop a tool that is easy to understand and use, and is scientifically valid. The rise in industrial activity in the High North has brought new challenges and a need for better winterisation standards in installations and vessels designed to operate in extremely cold climates. Reducing the need for winterisation without compromising the health and safety of the workers will reduce both building costs and the weight of offshore installations. The aim of this study was to compare the wind-chill recommendations in ISO 11079 and NORSOK S-002, Working Environment.

Changes in thermal discomfort indices in Romania and their connections with large-scale mechanisms

We analysed changes in variability characteristics for 2 indices quantifying winter (WCT: wind chill equivalent temperature chart index) and summer (THI: temperaturehumidity index) thermal stresses at a high spatial resolution across Romania for the period 1962� 2010. The linear trends, and shifts in the mean and main modes of variability were examined using nonpara- metric tests (Mann-Kendall and Pettitt) and empirical orthogonal function (EOF) analysis. The physical mechanisms responsible for the identified characteristics of variability were examined through canonical correlation analysis (CCA). The results showed a significant upward trend for both indices over the entire country, except for some areas for WCT, with a significant upward shift around the year 1987 for winter and 1985 for summer. A strong increase in the frequency of extremely high THI values was revealed, indicating an increased risk to human health during summers in Romania after 1985. However, the upward trend in WCT would indicate a decreased risk of human health during winters in Romania after 1987. The main large-scale mechanism responsible for the spatial and temporal behaviour of the 2 stress indices was given by the first CCA pair, showing that the increasing trend was mainly explained by the increasing trend in the temperature at 850 hPa covering Romania simultaneously with an increasing trend in zonal circu- lation (WCT) and an increasing trend in specific humidity at 700 hPa (THI). On a decadal/multi- decadal time scale, the variations in the 2 indices are modulated by the North Atlantic Oscillation (winter) and the Atlantic Multidecadal Oscillation (summer).

Comments on "Modified wind chill temperatures determined by a whole body thermoregulation model and human-based convective coefficients" by Ben Shabat, Shitzer and Fiala (2013) and "Facial convective heat exchange coefficients in cold and windy environments estimated from human experiments" by Ben Shabat and Shitzer (2012).

Ben Shabat et al. (Int J Biometeorol 56(4):639-51, 2013) present revised charts for wind chill equivalent temperatures (WCET) and facial skin temperatures (FST) that differ significantly from currently accepted charts. They credit these differences to their more sophisticated calculation model and to the human-based equation that it used for finding the convective heat transfer coefficient (Ben Shabat and Shitzer, Int J Biometeorol 56:639-651, 2012). Because a version of the simple model that was used to create the current charts accurately reproduces their results when it uses the human-based equation, the differences that they found must be entirely due to this equation. In deriving it, Ben Shabat and Shitzer assumed that all of the heat transfer from the surface of their cylindrical model was due to forced convection alone. Because several modes of heat transfer were occurring in the human experiments they were attempting to simulate, notably radiation, their coefficients are actually total external heat transfer coefficients, not purely convective ones, as the calculation models assume. Data from the one human experiment that used heat flux sensors supports this conclusion and exposes the hazard of using a numerical model with several adjustable parameters that cannot be measured. Because the human-based equation is faulty, the values in the proposed charts are not correct. The equation that Ben Shabat et al. (Int J Biometeorol 56(4):639-51, 2013) propose to calculate WCET should not be used.

Martian Windchill in Terrestrial Terms

With an average temperature of −63°C and winter lows of −120°C, Mars sounds far too cold for humans. However, thermometer readings from Mars are highly misleading to terrestrials who base their expectations of thermal comfort on their experience in Earth's much thicker atmosphere. The two-planet model of windchill described here suggests that Martian weather is much less dangerous than it sounds because in the meager atmosphere of Mars, convection is a comparatively feeble heat transfer mechanism. The windchill on Mars is expressed as the air temperature on Earth that produces the same cooling rate in still air, in Earth's much denser atmosphere. Because Earth equivalent temperature (EET) is identical to the familiar wind chill equivalent temperature (WCET) that is broadcast across much of North America in winter, it provides a familiar context for gauging the rigors of weather on another planet. On Earth, WCET is always lower than the air temperature, but on Mars the equivalent temperature can be 100°C higher than the thermometer reading. Mars is much colder for thermometers than for people. Some frontier areas of Earth are at least as cold as midlatitude Mars is, year round. Summer afternoons in the tropics of Mars might even feel as comfortable as an average winter day in the south of England. Sunshine on Mars should be about as warm as it is on Earth. Heat balance and clothing emissivity are also briefly discussed.

Modified wind chill temperatures determined by a whole body thermoregulation model and human-based facial convective coefficients

Wind chill equivalent temperatures (WCETs) were estimated by a modified Fiala's whole body thermoregulation model of a clothed person. Facial convective heat exchange coefficients applied in the computations concurrently with environmental radiation effects were taken from a recently derived human-based correlation. Apart from these, the analysis followed the methodology used in the derivation of the currently used wind chill charts. WCET values are summarized by the following equation:[Formula: see text]Results indicate consistently lower estimated facial skin temperatures and consequently higher WCETs than those listed in the literature and used by the North American weather services. Calculated dynamic facial skin temperatures were additionally applied in the estimation of probabilities for the occurrence of risks of frostbite. Predicted weather combinations for probabilities of "Practically no risk of frostbite for most people," for less than 5 % risk at wind speeds above 40 km h(-1), were shown to occur at air temperatures above -10 °C compared to the currently published air temperature of -15 °C. At air temperatures below -35 °C, the presently calculated weather combination of 40 km h(-1)/-35 °C, at which the transition for risks to incur a frostbite in less than 2 min, is less conservative than that published: 60 km h(-1)/-40 °C. The present results introduce a fundamentally improved scientific basis for estimating facial skin temperatures, wind chill temperatures and risk probabilities for frostbites over those currently practiced.

Environmental conditions at the South Col of Mount Everest and their impact on hypoxia and hypothermia experienced by mountaineers

BackgroundHypoxia and hypothermia are acknowledged risk factors for those who venture into high-altitude regions. There is, however, little in situ data that can be used to quantify these risks. Here, we use 7 months of continuous meteorological data collected at the South Col of Mount Everest (elevation 7,896 m above sea level) to provide the first in situ characterization of these risks near the summit of Mount Everest.MethodsThis is accomplished through the analysis of barometric pressure, temperature and wind speed data collected by an automatic weather station installed at the South Col. These data were also used as inputs to parameterizations of wind chill equivalent temperature (WCT) and facial frostbite time (FFT).ResultsThe meteorological data show clear evidence of seasonality, with evidence of pre-monsoon, monsoon and post-monsoon conditions. Low pressures, cold temperatures and high wind speeds characterize the pre- and post-monsoon periods with significant variability associated with the passage of large-scale weather systems. In contrast, the monsoon period is characterized by higher pressures, warmer temperatures and lower wind speeds with a pronounced reduction in variability. These environmental conditions are reflected in WCTs as low as −50°C and FFTs as short as 2 min during the pre- and post-monsoon periods. During the monsoon, the risk of cold injury is reduced with WCTs of order −20°C and FFTs longer than 60 min. The daily cycle in the various parameters is also investigated in order to assess the changes in conditions that would be experienced during a typical summit day. The post-monsoon period in particular shows a muted daily cycle in most parameters that is proposed to be the result of the random timing of large-scale weather systems.ConclusionsOur results provide the first in situ characterization of the risk of hypoxia and hypothermia on Mount Everest on daily, weekly and seasonal timescales, and provide additional confirmation as to the extreme environment experienced by those attempting to summit Mount Everest and other high Himalayan mountains.

Freezing and Frostbite on Mount Everest: New Insights into Wind Chill and Freezing Times at Extreme Altitude

Cold injury is an acknowledged risk factor for those who venture into high altitude regions. There is, however, little quantitative information on this risk that can be used to implement mitigation strategies. Here we provide the first characterization of the risk of cold injury near the summit of Mount Everest. This is accomplished through the application of a meteorological dataset that has been demonstrated to characterize conditions in the region as inputs to new parameterizations of wind chill equivalent temperature (WCT) and facial frostbite time (FFT). Throughout the year, the typical WCT near the summit of Everest is always <-30°C, and the typical FFT is always less than 20 min. During the spring climbing season, WCTs of -50°C and FFTs of 5 min are typical; during severe storms, they approach -60°C and 1 min, respectively; values typically found during the winter. Further, we show that the summit barometric pressure is an excellent predictor of summit WCT and FFT. Our results provide the first quantitative characterization of the risk of cold injury on Mount Everest and also allow for the possibility of using barometric pressure, an easily observed parameter, in real time to characterize this risk and to implement mitigation strategies. The results also provide additional confirmation as to the extreme environment experienced by those attempting to summit Mount Everest and other high mountains.

Manual Performance Deterioration in the Cold Estimated Using the Wind Chill Equivalent Temperature

Manual performance during work in cold and windy climates is severely hampered by decreased dexterity, but valid dexterity decrease predictors based on climatic factors are scarce. Therefore, this study investigated the decrease in finger- and hand dexterity and grip force for nine combinations of ambient temperature (-20, -10 and 0 degrees C) and wind speeds (0.2, 4 and 8 m x s(2)), controlled in a climatic chamber. Finger dexterity was determined by the Purdue pegboard test, hand dexterity by the Minnesota manual dexterity test and grip force by a hand dynamometer. Twelve subjects with average to low fat percentage were exposed to cold air for one hour with and without extra insulation by a parka. The subjects were clothed in standard work clothing of the Royal Netherlands Air Force for cold conditions. Extra insulation did affect cold sensation but not manual performance. The deterioration in manual performance appeared to be strongly dependent upon Wind Chill Equivalent Temperature (WCET) and the square root of exposure time (r=0.93 for group average). These simple models may be valuable to assess problems with work in the cold, but more work should be done to determine critical values in dexterity for a wide variety of operational tasks.

Comments on “Inconsistencies in the ‘New’ Windchill Chart at Low Wind Speeds”

Shitzer and de Dear (2006) used the regression equation that accompanies our 2001 windchill model (Bluestein and Osczevski 2002) to calculate windchill equivalent temperatures (WCET) at low wind speeds. They were surprised to find that WCET was not equal to the air temperature when the air was “still,” according to their definition of calm. They also found an abrupt transition in WCET above a minimum wind speed. To circumvent these inconsistencies, they proposed a correction; however, these inconsistencies do not exist. The problem is not in our model of windchill, but in their understanding of it. Their definition of still air as “any wind speed that is equal to, or below, the assumed threshold of 1.34 m s ” (Shitzer and de Dear 2006, p. 788) is the only real inconsistency. That is not how we defined it in the 2001 model of windchill, where “still air” or “calm” means no wind at all. There is no minimum applicable wind speed for calculating WCET; it can be found for any wind, right down to dead calm (Fig. 1). Because they used a wind speed of 1.34 m s 1 (4.8 km h ) in our regression equation, the WCET they thought they were calculating for someone walking in still air was actually for a person walking into a light wind. It is therefore neither inconsistent nor surprising that they calculated a value that was colder than the air temperature. The other “inconsistency” they found, the abrupt transition in WCET values, also results from their misinterpretation of what we meant by still air. WCET actually varies smoothly with wind speed (Fig. 1). Even if Shitzer and de Dear (2006) had used the regression equation properly, it would not have worked well at very low wind speeds. The regression equation was only intended for operational use. As Fig. 1 shows, it is a good approximation to the model output at wind speeds above 3 km h 1 (0.8 m s ). Lower wind speeds are not easily measured and are never reported anyway. We regret that the regression equations were published without an explicit warning of this limitation in our early paper. We did point it out in a later, more detailed paper (Osczevski and Bluestein 2005). The full model must be used to calculate WCET in this range. At wind speeds close to zero, WCET may be slightly warmer than the air temperature. This is due to the greater radiant heat transfer in the reference still-air condition, which assumes that there is no water vapor in the air. Any factor that increases the severity of the reference condition, such as greater radiant heat transfer, increases WCET. WCET is equal to air temperature at zero wind speed if the same relative humidity, that is, 50%, is assumed for the still-air condition as for other combinations of wind and temperature. Especially when there is no wind, radiant heat loss is significant: about one-half as large as the convective heat loss caused by walking in cold, still air. The supposed problems reported by Shitzer and de Dear (2006) do not exist and do not demonstrate any need to correct the windchill calculation method or the existing charts. We could make the model more elegant by increasing the relative humidity in the still-air condition to 50%, making WCET exactly equal to air temperature at zero wind speed. However, this adjustment would have no little or no operational impact. Corresponding author address: Randall Osczevski, DRDC Toronto, P.O. Box 2000, 1133 Sheppard Ave. W., Toronto, ON, Canada M3M 3B9. E-mail: randall.osczevski@drdc-rddc.gc.ca OCTOBER 2008 N O T E S A N D C O R R E S P O N D E N C E 2737

Transient Analysis of Cold Winds on Exposed Skin: Reflections on the Assessment of Wind Chill Equivalent Temperatures

Abstract A transient analysis of the human–environment thermal interaction in cold and windy environments is presented. The site selected to represent this interaction is the head–face, which is depicted as a hollow cylinder wherein heat is conducted in the radial direction. Environmental radiation effects, which are small relative to the wind-driven convection effects, as well as metabolic heat and blood perfusion effects, are not accounted for at this stage. The model is solved analytically by an integral transform. Results are used to calculate the wind chill equivalent temperatures (WCETs) for different environmental temperatures and wind speeds. The temporal variations of the calculated WCETs are contrasted against the steady-state values that are published by the weather services in North America. The inherently transient nature of this parameter is clearly demonstrated. This observation should form a basis for modifying the heretofore employed steady-state calculation method of this useful weather predictor.

Assessment of the effects of environmental radiation on wind chill equivalent temperatures

Combinations of wind-driven convection and environmental radiation in cold weather, make the environment "feel" colder. The relative contributions of these mechanisms, which form the basis for estimating wind chill equivalent temperatures (WCETs), are studied over a wide range of environmental conditions. Distinction is made between direct solar radiation and environmental radiation. Solar radiation, which is not included in the analysis, has beneficial effects, as it counters and offsets some of the effects due to wind and low air temperatures. Environmental radiation effects, which are included, have detrimental effects in enhancing heat loss from the human body, thus affecting the overall thermal sensation due to the environment. The analysis is performed by a simple, steady-state analytical model of human-environment thermal interaction using upper and lower bounds of environmental radiation heat exchange. It is shown that, over a wide range of relevant air temperatures and reported wind speeds, convection heat losses dominate over environmental radiation. At low wind speeds radiation contributes up to about 23% of the overall heat loss from exposed skin areas. Its relative contributions reduce considerably as the time of the exposure prolongs and exposed skin temperatures drop. At still higher wind speeds, environmental radiation effects become much smaller contributing about 5% of the total heat loss. These values fall well within the uncertainties associated with the parameter values assumed in the computation of WCETs. It is also shown that environmental radiation effects may be accommodated by adjusting reported wind speeds slightly above their reported values.

Paradox: increased blood perfusion to the face enhances protection against frostbite while it lowers wind chill equivalent temperatures

A model of facial heat exchange in cold and windy environments is presented. The tissue is depicted as a hollow cylinder and the model includes heat conduction and heat transport by blood circulation from the warmer core. A steady-state solution facilitating the estimation of wind chill equivalent temperature (WCET) as a function of the effective wind velocity, air temperature and blood perfusion rate was obtained. The results quantify and demonstrate the elevation of skin temperatures caused by increased flow of warmer blood from the inner core to the face. Elevated facial temperatures, while enhancing protection against frostbite and other cold-related injuries, also increase heat loss to the colder environment. Paradoxically, such elevated facial temperatures cause WCETs, as estimated by the prevailing definition, to attain lower rather than higher values, indicating, in fact, increased risk of frostbite. The results of this study should be useful in understanding and quantifying the effects of blood perfusion in protection against cold-related injuries. They should also be considered in the re-evaluation and re-formulation of the concept of wind chill, which has been a useful cold weather indicator for decades.