ORIGINAL PAPER A simple heat alert system for Melbourne, Australia Neville Nicholls & Carol Skinner & Margaret Loughnan & Nigel Tapper Received: 16 May 2007 / Revised: 25 September 2007 / Accepted: 15 October 2007 / Published online: 6 December 2007 # ISB 2007 Abstract A simple heat alert system, based solely on predicted maximum and minimum daily temperatures, has been developed for the city of Melbourne in southeast Australia. The system is based upon a demonstration that, when mean daily temperature exceeds a threshold of 30°C (mean of today’ s maximum temperature and tonight’ s minimum temperature), the average daily mortality of people aged 65 years or more is about 15–17% greater than usual. Similar numbers of excess deaths also occur when daily minimum temperatures exceed 24°C (increases of 19–21% over expected death rate), so a heat alert system based solely on this widely available weather forecast variable is also feasible. No strong signal of excess heat- related deaths appears when the data are stratified using daily maximum temperatures. This may be because in Melbourne some days with very high maximum temper- atures will be affected by the passage of cool changes and cold fronts in the afternoon, leading to a rapid drop in temperature (i.e., some days with high maximum temper- atures will not continue to be hot throughout the day and into the evening). A single day with temperatures exceed- ing the thresholds noted above is sufficient to cause this increase in mortality, rather than requiring an extended heat wave. The increased daily mortality does not appear to represent a short-term advancement of mortality. Keywords Australia . Heatwave . Human health . Mortality . Climate change Introduction Although there have been previous studies relating excess mortality to high temperatures in Australia (e.g., Rankin 1959; Guest et al. 1999; Tong et al., submitted) and elsewhere (e.g., Semenza et al. 1996; Rooney et al. 1998; Curriero et al. 2000; Hajat et al. 2002), the high mortality associated with the European heatwave of August 2003 has led to an increased focus on this subject (e.g., Filleul et al. 2006; Fouillet et al. 2006; Le Tertre et al. 2006; Kovats and Ebi 2006). In various places, heat wave alert systems have been developed, either based on studies prior to the European heatwave (e.g., Kalkstein et al. 1996), or subsequently (Pascal et al. 2006; Kovats and Ebi 2006). No such system is available for Melbourne, Australia. The intention of this study was to determine if it was possible to prepare a simple system that uses only publicly available forecasts of daily maximum and minimum temperatures to determine whether a heat alert should be issued for a specific day. The system derived by Pascal et al. (2006) for France also uses maximum and minimum temperatures. Although it is common knowledge that there is a tendency for increased mortality and morbidity on hot days, especially amongst the elderly (e.g., Ashcroft 2001; Flynn et al. 2005; Brücker 2005; Kosatsky 2005; Nogueira et al. 2005; Pirard et al. 2005; Simón et al. 2005; Michelozzi et al. 2005; Garssen et al. 2005; Johnson et al. 2005), there have been few studies examining whether there is a threshold temperature above which mortality increases rapidly, rather than a gradual increase as temper- ature increases. Such a threshold phenomenon, if it were Int J Biometeorol (2008) 52:375–384 DOI 10.1007/s00484-007-0132-5 N. Nicholls (*) : M. Loughnan : N. Tapper Monash Climate, School of Geography and Environmental Science, Monash University, Melbourne, Australia e-mail: Neville.Nicholls@arts.monash.edu.au C. Skinner Formerly Bureau of Meteorology, Melbourne, Australia