There is robust literature (Kovats and Koppe 2005) associating what is generally termed "oppressive" heat with some negative health consequence. However, the means by which "oppressive" is defined varies widely (Watts and Kalkstein 2004); accordingly, the HHWWS that have been developed across the world in recent years have utilized a diversity of methods. Each of these methods has their respective strengths and weaknesses.
The utilization of a temperature threshold is perhaps the simplest of all methods. However, as outdoor temperature alone is significantly correlated with human mortality during excessive heat events (EHEs), temperature is considered by some to be a fairly reliable indicator. Moreover, the sole utilization of temperature has a further advantage in that it is the most commonly measured of all meteorological variables and thus is available for more locations. A number of nations, including Spain (Ministero de Sanidad y Consumo 2005), France (Pascal et al. 2006), the United Kingdom (UK Department of Health 2005), and Portugal (Paixao and Nogueira 2002), utilize maximum and/or minimum temperature thresholds in determining heat stress (Fig. 3.1).
An extension of the temperature threshold is the utilization of an "apparent temperature" that takes into account humidity (and wind speed in certain cases) as well as temperature. Several different formulations of the apparent temperature exist, including the Heat Index (Steadman 1984), used widely in the USA and Australia, and the Humidex (Masterton and Richardson 1979), developed in Canada. These indices are especially useful in locations where summer absolute humidity levels can vary widely, hence their widespread use in North America. Thresholds can then be developed as with temperature; the 40.6°C threshold of heat index across much of the USA is a prime example (Watts and Kalkstein 2004).
Another method of assessing meteorological conditions for application to the heat-health issue involves the classification of weather types, or air masses. The philosophy behind this "synoptic" methodology is to classify an entire suite of meteorological variables and thus holistically categorize the atmospheric situation at a given moment for a particular location or region (Yarnal 1993). This categorization when applied to heat is usually based upon surface weather variables, although upper atmospheric variables may also be incorporated. By categorizing the atmosphere into one of several internally homogeneous groups, other factors, such as solar radiation, wind speed, and cloud cover are inherently accounted for. For example, as a building's "heat load", as expressed by solar radiation income, has been associated with variability in human mortality, cloud cover or a some direct measure of solar radiation can be an important inclusion (Koppe and Jendritzky 2005). In synoptic approaches, discrete categories are created rather than a meteorological threshold
30 20 10 0
180 160 140 120
60 40 20 0
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20 25 30
Fig. 3.1 Mean daily mortality in relation to 1700hr temperature for Dallas and Boston, USA
value along the continuum of a continuous variable (e.g., temperature); the result is a determination of "oppressive" synoptic categories that are historically associated with negative health outcomes. The synoptic-based systems generally require meteorological data that is more comprehensive than the temperature- or apparent temperature-based models, including hourly surface data for a number of variables.
A number of systems employ the synoptic methodology. Most notable are around 20 of the newer HHWWS across the USA (Sheridan and Kalkstein 2004), that incorporate the Spatial Synoptic Classification (SSC, Sheridan 2002). Several systems in Italy (Michelozzi and Nogueira 2004), Canada, South Korea, and China (Tan et al. 2003) also utilize the SSC.
A more physiologically based approach by which heat stress is evaluated includes those that are based on modeling the response in the human ihermoregulatory system to ambient weather conditions. Rather than rely on proxy indicators, these methods aim to provide a direct assessment based on radiative fluxes to and from a typical human being. In the HeRATE system (Koppe and Jendritzky 2005), the thermal stress of ambient conditions is combined with an evaluation of short-term adaptation in assessing the overall level of heat stress upon the average individual. While thorough, the thermoregulatory system does require the most detailed array of meteorological conditions: in order to correctly model radiative fluxes, detailed information on temperature, humidity, wind, and cloud type and cloud cover at different levels must be assessed. The German HHWWS is the foremost advocate of the thermoregula-tory system, and utilizes the HeRATE system as the foundation for its warning system structure (Koppe and Jendritzky 2005).
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