## 2 Materials and Methods
### Climate Data Description
#### 2.3.2. Heatwaves in the Summer of 2018 and Future Heatwave (HWS2018, HWS2050-RCP4.5)  
HWS2018 represents a recent April-to-September period with severe heatwaves in summer. Here a heatwave is defined as a period of days with the daily maximum outdoor temperature above 25 \({}^{\circ}\)C [38]. In turn, if the nighttime (9 p.m. to 9 a.m.) minimum temperature exceeds 20 \({}^{\circ}\)C, the night is defined as a tropical one [39]. The year 2018 was in Finland, in terms of the national monthly mean temperature, a year with record warm May and July [40]. At Helsinki-Vantaa in 2018, the months from April to September were all unusually warm, with two heatwaves, one in May and the other in July-August. The first lasted for four days (from 13 th to 16 th), with the highest hourly temperature of 28.8 \({}^{\circ}\)C but no tropical nights. During the latter heat wave, from the 12 th of July to the fifth of August, daily maximum temperature exceeded 25 \({}^{\circ}\)C on 24 days (out of 25), with the highest hourly temperature of 30.8 \({}^{\circ}\)C and five tropical nights. At Helsinki-Vantaa in 2018, the months  
Figure 4: Monthly mean (**a**) temperature and (**b**) global horizontal irradiance at Helsinki-Vantaa in the test reference year (TRY2020), in test reference year projected for the year 2050 under the RCP4.5 scenario (TRY2050), during the year 2018 (HWS 2018), and year 2018 transformed to represent climate conditions around the year 2050 (HWS2050). from April to September were all unusually warm, and the monthly mean temperatures of May and July were respectively 4.6 \({}^{\circ}\)C and 3.8 \({}^{\circ}\)C higher than in TRY2020. Moreover, the 95 th (99 th) percentile of the daily mean temperature, having a value of 23.8 \({}^{\circ}\)C (25.4 \({}^{\circ}\)C) during Julys in 1989-2018 [34], was exceeded on 12 (2) days in July 2018. The monthly mean temperatures of May and July were respectively 4.6 \({}^{\circ}\)C and 3.8 \({}^{\circ}\)C higher than in TRY2020. Furthermore, due to unusually clear skies, the monthly mean global horizontal irradiance in May 2018 was about 24% higher than in the TRY2020. Both heatwaves were associated with anomalous high-pressure areas over Fennoscandia [41] and led to droughs, crop failures, forest fires [42], and about 380 additional deaths in Finland [43]. In Helsinki, the mortality attributable to the heatwave in 2018 was about 2.2 times higher than in its surroundings [6]. Noteworthy, due to unusually clear skies, the monthly mean global horizontal irradiance in May 2018 was about 24% higher than in the TRY2020. An important aim of this study is to assess how the severity of a heatwave like that in 2018 would be different in the climate of the mid-century. Therefore, hourly weather observations during the year 2018 were modified to represent future heatwaves (HWS2050) with the aid of climate change projections. In the transformation, the delta-change method and the RCP4.5 scenario were again used, and it was assumed that the hot summer could have occurred in any year of the period 1989-2018, without taking into account any potential temporal trends in the magnitude or probability of the occurrence of heatwaves during that period. Besides changes in the mean temperature, the delta-change method applied for the outdoor air temperature took into account changes in the standard deviation of daily mean temperature. In the study area, the day-to-day temperature fluctuations are projected to slightly increase in June and July but attenuate in other seasons. The diurnal temperature range, i.e., the difference between the daily maximum and minimum temperatures, is projected to alter little in other seasons but winter [36]. For May and July, in this HWS2050 data set, the monthly mean temperature was projected to increase by 1.5 \({}^{\circ}\)C and the monthly mean global horizontal irradiance by 2% relative to the year 2018 (HWS2018) (Figure 4).