**Konstantia Tolika**

Department of Meteorology and Climatology, School of Geology, Aristotle University of Thessaloniki, Thessaloniki, 54124, Greece; diatol@geo.auth.gr; Tel.: +30-2310-998404

Received: 23 November 2018; Accepted: 29 December 2018; Published: 7 January 2019

**Abstract:** Heat waves are considered one of the most noteworthy extreme events all over the world due to their crucial impacts on both society and the environment. For the present article, a relatively new heat wave index, which was primarily introduced for the study of extreme warming conditions over Australia (Excess Heat Factor (EHF, hereafter)), was applied over Greece (eastern Mediterranean) for a 55-year period in order to examine its applicability to a region with different climatic characteristics (compared to Australia) and its ability to define previous exceptional heat waves. The computation of the EHF index for the period 1958–2012 demonstrated that, during the warm period of the year (June, July, August, and September (JJAS)), Greece experiences approximately 20 days per year with positive anomalous conditions (EHF > 0) with positive statistically significant trends for all stations under study. Moreover, an average of 128 spells with a duration of 3 to 10 consecutive days with positive EHF values were found during the examined 55-year period. As the duration of the spell was extended, their frequency lessened. Finally, it was found that the EHF index not only detected, identified, and described efficiently the characteristics of the heat waves, but it also provided additional useful information regarding the impact of these abnormal warming conditions on the human ability to adapt to them.

**Keywords:** temperature; heat wave; excess heat factor; acclimatization; Greece

### **1. Introduction**

Heat waves have been a phenomenon of great worldwide interest due to their substantial societal and environmental impacts. These impacts intensify the necessity of measuring, studying, and even predicting these extreme hot conditions especially in the impacted communities and the affected regions [1] because remarkably warmer weather can have a direct negative effect on health, especially for the vulnerable elderly population [2–5]. There is also a global demonstration that extreme temperatures are highly correlated with human mortality [6–8], making heat waves one of the natural hazards with the greatest percentage of casualties [3,9]. Langlois et al. [10] mention that even though people tend to adapt and acclimatize themselves to temperature changes, if this change is sudden and abrupt, it can then cause certain heat-related diseases or even death.

Nevertheless, there is no single and standard definition of the physical nature of a heat wave and their overall description remains quite broad [11]. Heat waves are usually described as periods of exceptionally hot weather. However, the intensity of this temperature rise as well as the duration of the extreme warm consecutive days and the time of year that they occur are important aspects necessary to categorize a hot event as a heat wave. In general, a heat wave is an acute period of extreme warmth during the summer months, whereas the respective hot periods during winter are referred to as warm spells [12].

In order to define suitable metrics for waves, scientists have instituted either absolute or relative approaches [13]. Even though experts differ in the selection of thresholds and duration, the first approach is based on the meteorological/climatological values of certain parameters, such as daily mean temperature, maximum and minimum temperatures, temperature indices, duration, and relative humidity, whereas the relative approach also incorporates human acclimatization to weather and uses more human-related bioclimatic indices (e.g., [14–17]). Thus, the diverse definitions of a heat wave mainly depend on the scope that is being studied. If the climatological-statistical characteristics of these extreme hot events are of primary interest to the researcher, then straightforward metrics are being used. On the other hand, if the study is more human-centered, then the impact of the heat wave on people's health is the main drive and different approaches are used [11,18]. It should also be mentioned that due to the fact that most of the heat wave indices are developed for a specific use and a specific target group or sector, they are most of the time not flexible and cannot be applied to different regions or for different purposes [1].

Moreover, since temperature is increasing on a global scale, the interest concerning heat waves is also increasing as they are expected to become more frequent, more intense, and of longer duration [19,20]. Especially with respect to the Mediterranean region, which will probably experience a much larger number of heat waves in the future, particularly during the summer months (e.g., [21–25]), the need to define these extreme events efficiently becomes more and more urgent, due to their severe impacts on several aspects of human lives [26–28]. In Greece, which is the center of interest in the present study, heat waves have been analyzed by several researchers using different approaches, methodologies, and metrics either from a statistical or a more bioclimatological point of view (e.g., [24,29–33]).

However, in this study, an attempt was made to carry out an in-depth analysis of Greece's heat waves with a relatively new index, developed primarily for assessing heat waves in Australia [34] but which had recently been applied to the Czech Republic [35] and the Balkan Peninsula (Romania), where Greece is also located [3]. This index, defined as the excess heat factor (EHF) and described in detail in the next paragraph, is actually a set of indices, whose major advantage is that it combines both the statistical and the human-impact aspects of the heat wave. Moreover, with respect to temperature, not only maximum but also minimum temperatures were used for their definition. Adding Tmin on a heat wave index is not only climatologically tempting [1], but high minimum temperature values intensify the heat wave conditions, also increasing the degree of heat stress [34,36]. In addition, Karl and Knight [37] underlined that no relief from high minimum temperatures, for more than three consecutive days, could have crucial impacts on human health. Finally, another advantage of the excess heat factor index is that it takes into consideration not only the temperature conditions of the specific day but also of the previous two ones, which can intensify or reduce the heat wave's magnitude [1].

In the next section of this study, the methodology for the EHF index computation is analyzed as well as the data that are being used. Moreover, the statistical characteristics and results of the index are presented for the stations under study as well as the assessment of the EHF's ability to define and describe two representative heat waves (July 1987 and July 2007) that occurred in Greece during the past few years. Finally, the conclusions derived from the study as well as a literature discussion of them can be found in the last station of this research article.
