**4. Discussion and Conclusions**

The statistical characteristics of the combined heat wave index (excess heat factor (EHF)), as well as its ability to identify and describe efficiently previously recorded heat waves over Greece, was the main objective of the present study. Daily maximum and minimum temperature time series from 14 stations distributed over the geographical Greek region were employed for a 55-year period (1958–2018). This index introduced relatively recently [34] had, to the author's knowledge, never been used before in Greece. Although it was primarily proposed for the monitoring of heat waves in Australia [10,12,34], it was also used during 2018 for heat wave detection in another Balkan country, Romania [3].

Apart from the fact that it is the first time that the EHF index is being used over the center of interest, the main reasons for its selection are the following:


The computation of the EHF index for the period 1958–2012 demonstrated that, during the warm period of the year (JJAS), Greece experienced approximately 20 days per year with positive anomalous conditions (EHF > 0) with no discrepancies worth mentioning among the stations. In addition, these days tended to become more frequent (with positive statistically significant trends for all stations under study), agreeing with the general consideration of an increase in extreme hot events in the future over the Mediterranean [20]. The years that had the largest number of days with positive EHF values were the last ones of the time period used (2007 to 2012). Up to 89 days in Naxos, for 2010, were observed, where it seemed that most days of the summer season were characterized as abnormally hot.

However, since, according to Perkins and Alexander [1] a heat wave is defined when at least three or more consecutive days have positive anomalies, the spells of these days were also computed according to the index values. The selected stations presented an average of 128 spells with a duration of 3 to 10 days during the examined 55-year period. As the duration of the spell was extended, their frequency lessened. Four stations, mainly island ones, did not present any heat waves longer than 30 consecutive days, whereas others such as Kozani and Kerkira which are located at the northwestern part of Greece experienced an intense heat wave in 2012 lasting 58 and 57 days, respectively.

Apart from the significant insight into the heat wave statistical characteristics obtained from the EHF, an attempt was made in the study to assess its ability to record previous heat waves since the index was primarily introduced for another area with very different climatic characteristics from Greece. After its application to the daily temperature data for July 1987 (one of the most intense heat waves recorded in Greece), it was found that the index identified very efficiently both the duration and the intensity of the heat wave. In addition, the acclimatization index showed that the heating conditions were quite rapid, not allowing people to adapt to them a few days earlier than the defined start of the heat wave. The index was also able to capture the fact that the heat wave was more intense in the northern parts of the country where the EHF values were higher. Conversely, regarding the second heat wave case of July 2007, the computation of the EHF index made it clear that the heating conditions had a different intensity level over Greece since its values differed substantially among the stations. Moreover, the lack of human acclimatization started in this case on the same dates as the beginning of the main heat wave event (a few days of positive value index were also observed during the first days of July). However, the EHIaccl turned negative, for several stations, before the end of the heat wave, meaning that people started to adapt to these extreme conditions due to the longer duration of this specific heat wave.

Overall, the main conclusion of this study is that the EHF index applies not only to the detection and analysis of heat waves in Greece, but it also provides information about the conditions that may or may not have an impact on human health and well-being. Future work includes plans to examine other years which, according to the EHF, seemed to have been extremely hot with extended heat waves such as in the year 2012.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The author declares no conflict of interest.
