**2. Materials and Methods**

The identification and determination of heat waves over the study area (Greece) was achieved using the computation of the excess heat factor (EHF) index, which provides a measure of the environmental temperature load [10] and the intensity of a potential heat wave [12]. The EHF is the product of the multiplication of two other excess heat indices (EHIs), namely, EHIaccl and EHIsig.

$$\text{EHF} = \text{EHI}\_{\text{sig}} \times \max\{1, \text{EHI}\_{\text{acccl}}\} \tag{1}$$

EHIaccl is defined as:

$$EHIaclcl = \left[\frac{T\_i + T\_{i-1} + T\_{i-2}}{3}\right] - \left[\frac{T\_{i-3} + \dots \, T\_{i-32}}{30}\right],\tag{2}$$

where the first term of Equation (2) is the average daily Tmean for a three-day period and the second term is the average daily Tmean of the preceding thirty days. As proposed by Nairn et al. [34], this is an acclimatization index and its positive (negative) values are related to hot (cold) weather conditions. It determines a period of heat that is warmer than the recent past [10], and it should be highlighted that this index is not influenced by the potential general warming trend [34]. This index describes an important factor of the influence of heat to the population because, even though humans tend to acclimatize themselves to their environmental local climate according to the temperature variations throughout the year, they may be unprepared to an abrupt temperature rise above that of the recent past [12]. Thus, positive EHIaccl values indicate a lack of acclimatization to the warmer temperatures which may result in negative health impacts.

The second term of the EHF equation is the significance index (EHIsig) defined by the following equation:

$$EHIsig = \left[\frac{T\_i + T\_{i-1} + T\_{i-2}}{3}\right] - T\_{\mathfrak{F}\mathfrak{F}\_{\prime}}\tag{3}$$

which is calculated by the difference of the average daily Tmean for a three-day period minus the 95th percentile of the daily Tmean. The percentile is computed for a reference period of 30 years (1971–2000) using the daily values of the mean temperatures for all days throughout the year. A heat wave occurs when EHIsig is positive, while the comparison with the 95th percentile measures the statistical significance of the heat event [30]. The authors also underline the fact that, since T95 is computed for a fixed climatological period, the EHIsig (contrary to the EHIaccl) is expected to become more extreme under a general warming trend.

Therefore, the excess heat factor expresses the long-term temperature anomalies amplified by the short-term ones [10], and the days with positive EHF values indicate heat wave conditions, while the higher the values of the index, the more intense is the heat wave. However, according to Perking and Alexander [1], a heat wave episode will be defined when at least three consecutive days present EHF values above zero.

Finally, daily Tmean in all the above indices should be computed by averaging the Tmin and Tmax daily values since the diurnal temperature variation is highly associated with the ability of the biological systems to recover from high heat load. Hence, for this study, the daily Tmin and Tmax time series, derived from 14 meteorological stations, were used for a 55-year period starting from 1958 to 2012, for the computation of the Tmean values. Except for the data for the Thessaloniki station which were available from the meteorological station of the Aristotle University of Thessaloniki (AUTh), the remaining station data were provided by the Hellenic National Meteorological Service (HNMS). These data were proven to be homogenous according to the Alexandersson test [38] and had no gaps. The geographical distribution of the station locations is presented in Figure 1.

It is worth mentioning at this point that although abnormally warm conditions may occur even during the winter months, it was decided in this case to compute the EHF index for the hottest period of the year, that is, June, July, August, and September (the JJAS period) since summer heat waves tend to be more intense with severe impacts for humans during these months. Finally, adopting the definition used by Perkins and Alexander [1] who mentioned that a heat wave occurs when the abnormally hot conditions (EHF > 0) persist for more than three consecutive days, the duration of spells longer than 3 days was calculated for the 14 stations under study during the 55-year time period.

**Figure 1.** Geographical distribution of the stations under study. For each station, the altitude where it is located can be found on the right of the map. For each station, the average Tmin, Tmax, and Tmean values for the period 1958–2012 are provided. The sign of the trend of these time series is found in the brackets (). The asterisk indicates the statistical significance of the trends at the 95% level of significance.
