## 2 Background  
Increased temperature variations during the summer months and associated transformations in microclimates are among the most identifiable effects of climate change in central European areas. In Vienna, weather statistics over 253 years have shown that among the 10 hottest summers, 9 have been in the recent past. The summer of 2019 confirmed this trend, because it was the second hottest summer measured in history [8]. In the Austrian capital, the number of \"heat days\", i.e., days with a maximum temperature of more than 30 \({}^{\circ}\)C, has significantly increased by about 153% over a period of around 40 years. The value rose from 8.9 days (mean value for the period 1955-1964) to 22.5 days (mean value for the period 2008-2017) (see Table 1) [9]. Figure 1 shows that while \"frost days\" and \"ice days\" have steadily declined over the last 63 years, \"summer days\" and \"heat days\" have increased exponentially. In one study undertaken for the city of Vienna, the number of summer days over the next two decades was simulated, with the scenarios based on regional climate models and simulations. The results showed that a moderate increase (up to 25 days per year) of summer days is expected for the period 2021-2050, and a strong increase (20-50 days per year) is expected for the period 2071-2100. Within this context, summer days were defined as days when the outside air temperature exceeds 25 \({}^{\circ}\)C [10]. Addressing the UHI effect is of particular importance when it comes to building development, as a series of factors increasing the UHI effect can be associated with the construction and retrofitting of the built environment. Lower evaporative cooling due to sealed surfaces; reduced air circulation; lower wind speeds; larger heat-absorbing surfaces; inadequate shading of buildings due to a lack of vegetation; diffused solar radiation due to reflection from other buildings; as well as waste heat from air-conditioning systems, industrial processes, and traffic can all be summarized as effects that predominately occur in densely populated and heavily surface-sealed environments [3]. Waste heat, especially, has a strongly counteractive effect: the hotter it gets, the more people are inclined to install power intensive air-conditioning systems, which in turn emit heat into the atmosphere. In Austria, where air-conditioning systems are not yet regularly installed in residential buildings, the electricity consumption for air conditioning and mechanical ventilation increased in the period 1995-2012 from 27.1 GWh/year to 210.3 GWh/year [11]. In order to avoid this additional heat burden, the city of Vienna has published recommendations for planners to avoid summertime overheating by implementing adequate passive design measures into their building designs. Simulations with future climate data have shown that in residential buildings, air conditioning can be avoided with a combination of high thermal mass, shading, and night cooling [12]. \begin{table}
\begin{tabular}{c c c c c} \hline
**Period** & **Frost Days** & **Ice Days** & **Summer Days** & **Heat Days** \\ \hline
1955–1964 & 82.3 & 31.3 & 37.5 & 8.9 \\
2008–2017 & 56.5 & 16.2 & 74.6 & 22.5 \\ Alternation & \(-\)31.35\% & \(-\)48.24\% & \(+\)98.93\% & \(+\)152.81\% \\ \hline \end{tabular}
\end{table} 
Table 1: Alternation of ice days and heat days in Vienna [9]. Figure 1: Development of heat days between 1955 and 2017 in Vienna [9]. The UHI effect is also dependent on the overall thermal mass prevailing in urban environments. Heavily sealed areas constitute large heat-absorbing surfaces that also reduce evaporative cooling effects. With an increasing demand in housing due to the still rising move toward urban environments, cities must balance new construction with green infrastructure to counteract the adverse effects of sealed surfaces. In Vienna, the development rate of new apartments is about 1% each year [13]. This also means that the majority of buildings already exist.