3.2.2. The Heat Wave of the Year 2007

Occurring twenty years after the previous heat wave, the extreme hot conditions experienced during the summer of 2007 have been the subject of several climatological studies. Greece has experienced record-breaking temperatures in most of its regions [33] and the most extreme maximum temperatures appeared during the last days of each of the summer months [24], mainly between days 21 and 29. Theoharatos et al. [42] mentioned that especially in July, the daily Tmax values repeatedly exceeded 40 ◦C, while the regions where the population experienced discomfort (high discomfort index (DI) values) were Thessalia, Sterea Ellada, and western Pelloponisos. The impacts of this heat wave were substantial with an increase in forest fires, changes in the hydrological balance, and large losses in the agricultural and energy sectors [24,43]. Similarly to the previous paragraph, the three examined indices were computed on a daily basis during July 2007 in order to evaluate their ability to capture and describe the extreme temperatures in Greece during that year (Figure 5).

In most of the stations used in this study, the extreme hot conditions seemed to have started from days 15 to 17 of the month when the daily Tmean exceeded the 95th percentile. However, in most of the stations, the days approximately from 7 to 10 July also surpassed the Tmean95. According to the actual index values, the onset of the heat wave was determined one or two days later from the dates of the Tmean > 95th percentile, 16–18 July, and continued until the end of the month. This was the starting point where the EHIsig was positive, a necessary condition for the determination of a heat wave. Regarding the acclimatization index, it started to present positive values on the same dates as the EHIsig. Yet, in some stations (Souda, Skiros, Naxos, Mitilini, Alexandroupoli, Heraklio, Kalamata), it went negative sooner than the actual ending of the heat wave. This means, according to Nairn and Fawcett [12] that in some cases of longer heat waves, there may be some human adaptation to the extreme hot conditions, decreasing their impact on human health. For example, in the case of Skiros, EHIaccl dropped below zero on 28 July, whereas the end of the heat wave occurred two days later. This characteristic of the heat wave, due to its duration, was not observed in the previous heat wave case (July 1987).

Finally, one of the main issues observed in the computation of the EHF index was its magnitude which differed substantially from one station to the other. This means that even though all of Greece experienced heat wave conditions, its intensity varied considerably (Figure 5). The maximum index values were found for the stations of Kozani, Kerkira, Larissa, and Samos and the lowest ones were found in Rodos. On the other hand, the findings from all stations were in agreement that the peak (the highest EHF value) was found either on day 25 or 26 of the month and that the heat wave then started to wane.
