Reprint 444 Application of a Weather Stress Index for Alerting the Public to Stressful Weather in Hong Kong P.W. Li & S.T. Chan Meteorological Applications, Volume 7, p.p. 369-375, 2000 Copyright of Royal Meteorological Society Meteorol. Appl. 7, 369-375 (2000) Application of a weather stress index for alerting the public to stressful weather in Hong Kong P W Li and S T Chan, Hong Kong Observatory, 134A Nathan Road, Kowloon, Hong Kong An operational procedure for alerting the public of stressful weather (exceptionally hot and cold situations) in Hong Kong has been implemented since late 1997. From a 28-year (1968-1995) climatological database, criteria based on a weather stress index (WSI) was established to define stressful weather conditions. A special bulletin is issued to the public when exceptionally high (hot, i.e. ≥ 97.5%) or low (cold, i.e. ≤ 2.5%) WSI is forecast. The bulletin provides information on current or predicted stressful weather conditions as well as statements on precautionary action to take. A study was also carried out to examine the effects of WSI on mortality rates on days with extreme WSI. Preliminary results indicate that there was a relationship between low WSI and mortality rates in winter. In summer, however, the results were not conclusive. 1. Introduction 2. Background Climate and weather play important roles in the well- Most of the operational systems currently in use can be being of mankind. Of the various meteorological fac- broadly divided into three categories: (a) simple physi- tors directly or indirectly affecting human health, cal indices or comfort indices, (b) the human energy extreme heat and cold conditions have been shown to balance approach, and (c) the synoptic climatological be the most significant ones in terms of human mor- classification approach. bidity and mortality (e.g. WHO/WMO/UNEP, 1996a, 1996b; Collier & Hardaker, 1995). Prolonged exposure For the physical type, the most common wintertime to hot and humid weather can cause illnesses ranging indices are the Siple-Passel wind-chill index and its from depression, hypotension, fatigue, hypothermia, variants (WMO, 1972), which measure the quantity of tachycardia and inappetence, to fatal conditions such as heat that the atmosphere is capable of absorbing from heat stroke and acute heart failure. On the other hand, an exposed surface. Taking it one step further, the US prolonged exposure in an extremely cold environment National Weather Service (NWS) employs the can lead to depression, indolence, asthma, respiratory Steadman wind-chill formula (Steadman, 1984) to mea- infections, hyperthermia, hypertension, stroke, heart sure the net heat loss of a healthy adult with an appro- failure and even frostbite (WHO/WMO/UNEP, priate amount of outdoor clothing in winter. For 1987). extreme heat, the NWS, on the basis of the apparent temperature (Steadman, 1979), uses a heat stress index To provide health-related weather services to the gen- (HSI) to alert the public to excessively hot weather. eral public, many weather centres around the globe The apparent temperature measures human perception have developed various kinds of operational system for to heat by adjusting the ambient temperature with rel- alerting the public to the danger of extreme weather ative humidity. Corrections due to the effects of mois- conditions. The objectives of this study are to explore ture and wind can also be included in the calculations the methodologies available, to adopt an operational of the Steadman wind-chill formula and HSI respec- index suitable to Hong Kong and to implement a warn- tively. ing service based on the index. In Germany, the Deutscher Wetterdienst adopts the Section 2 of this paper is a review of the more popular human balance approach and runs a more sophisticated approaches to the problem. Section 3 presents the cli- model, called the Klima-Michel Model, to take into matological results in terms of the net effective temper- account virtually all aspects of heat exchange processes ature (NET) computed for Hong Kong. Section 4 between a human body and the environment. The heat reports on how weather stress index (WSI) is derived exchanges considered include human heat production, and the criteria chosen. The relationship between WSI sensible heat flux, latent heat flux, moisture heat flux, and local mortality rates and the warning system since and respiration heat flux, as well as direct/diffuse/ implemented are summarized in sections 5 and 6 reflected solar radiation entering and re-emitting from respectively. Some concluding remarks are given in human surfaces. This model is considered to be a valu- section 7. able tool in bioclimatic assessments, providing addi- 369 P W Li and S T Chan tional comprehensive information not given by simple general public of Hong Kong of potentially stressful comfort indices. weather. The WSI parameter used was based on the cal- culation of the net effective temperature (NET). A dis- Climatic and synoptic classifications are done mainly by tribution of NET from a 28-year database was pre- analysing a number of meteorological elements, includ- pared for the determination of WSI. To test the corre- ing temperature, dew point, atmospheric pressure, lation of the WSI during extreme weather situations cloud cover and wind. The approach was adopted, for with human health, daily WSI figures during cold win- example, in Philadelphia, Pennsylvania, USA in the ter and hot summer were checked against the mortality form of a Hot Weather-Health Watch/Warning System rates from 1986 to 1995, the period when data were to alert local residents of oppressive weather situations readily available. potentially harmful to human health. Kalkstein et al. (1996) defined 11 airmass categories and correlated them with mean daily mortality rate so as to identify the 3. Net effective temperature most oppressive conditions. On the basis of weather forecasts from NWS, the Philadelphia Department of Among a number of indices tested for use as the Public Health, in cooperation with other government WSI parameter, NET is found to be the most suitable departments, issues alerts or even warnings via the for operational purposes. Effective temperature, the media whenever oppressive weather is expected. predecessor of NET first introduced by Missenard in 1937 (Hentschel, 1987) to include the effect of These operational systems often assume some sort of relative humidity, was limited to hot weather situa- standard adult human model which is obviously not tions. Modifications by Gregorczuk (WMO, 1972; applicable to people of different ages and different Hentschel, 1987) to include the effect of winds physical conditions. They generally require input of a extended its use to cold situations. The resulting for- host of meteorological variables. Some of them even mula for NET is: require input parameters, such as radiation values, 37 − T which are difficult to forecast. Whether the output war- NET = 37- −−0.29TRH (1 0.01 ) 0.75 rants further action would normally depend on certain 0.68−++ 0.0014RH 1/(1.76 1.4v ) criteria, such as HSI greater than 80 in NWS's system, or a cold stress index below -3 in the Klima-Michel where T is the ambient temperature (in °C), v the wind Model. It is, however, accepted that such criteria speed (in m s-1) and RH the relative humidity (in %). should be 'relative' rather than 'absolute', as a result of acclimatization. In other words, to what extent a The advantages of using NET are: weather condition should be treated as stressful at a particular locale is not universal. For instance, a tem- (a) It is relatively simple to compute and easy to inter- perature of 10 °C may be quite pleasant to people liv- pret. A large positive value implies exceptionally ing in mid-latitudes but rather uncomfortable for those high heat load, while a large negative value repre- used to tropical heat. In contrast, a warm day of 30 °C sents large heat loss. will be quite acceptable to people living in the tropics (b) It is applicable in both hot and cold situations. but intolerable to people from high latitudes. (c) It displays similar sensitivity as 'wind-chill' and 'apparent temperature', the two commonly used To take into account the acclimatization effect, indices for cold and hot weather respectively. Kalkstein et al. (1986) advocated the use of a relative (d) It has a predictive value since the meteorological climatological index. The index so derived is called the inputs required are routinely forecast. Other weather stress index (WSI). In his study, the WSI on a indices involving solar radiation or cloud amount particular day is derived by calculating the apparent are more difficult to forecast. temperature and determining to what extent that (e) It is consistent with common human perception: apparent temperature deviates from the mean value for (i) in hot weather, NET increases as temperature that locale. In other words, a WSI is defined as the pro- and/or RH increases, but decreases with portion of days with apparent temperature lower than increasing winds; the day under consideration. While a WSI of 99% is (ii) in cold weather, NET decreases with tempera- considered to be stressful in Kalkstein's scheme, other ture, and with increasing RH and winds. studies such as WHO/WMO/UNEP (1996a) consider that a WSI of 95% at some locale would already pose a Figure 1 shows the behaviour of NET as a function of threat to human health. It should be noted that the rel- wind and temperature. It can be seen that NET reflects ative climatological index approach is not confined to the common observations that people tend to feel more apparent temperature. Any other appropriate parame- stressful on calm, hot and humid days in summer and ters can be used instead.