“…Such non-traditional, opportunistic sources of data are, e.g., smartphones (e.g., Overeem et al, 2013b;Mass and Madaus 2014;Droste et al, 2017), smart wearable devices (Nazarian et al, 2021), cars (e.g., Haberlandt and Sester 2010;Bartos et al, 2019), commercial microwave links (e.g., Messer et al, 2006;Overeem et al, 2013a;Chwala and Kunstmann 2019), and privately-owned weather stations, called citizen weather stations (CWS) in the following (e.g., Steeneveld et al, 2011;Wolters and Brandsma 2012;Bell et al, 2013;Madaus et al, 2014;Chapman et al, 2017;de Vos et al, 2017;Venter et al, 2021). Each type of these data sources alone or multiple combined can be used in different meteorological and climatological applications, such as weather forecast (e.g., Mass and Madaus, 2014;Nipen et al, 2020), operational weather monitoring (e.g., de Vos et al, 2019), mesoscale model evaluation (e.g., Hammerberg et al, 2018), hydrometeorological analyses and modelling (e.g., Smiatek et al, 2017;de Vos et al, 2020), high-resolution mapping of air temperature (e.g., Venter et al, 2020;Vulova et al, 2020;Zumwald et al, 2021), thermal-comfort assessment (Nazarian et al, 2021), and urban climate investigations (e.g., Fenner et al, 2017Fenner et al, , 2019Droste et al, 2020;Feichtinger et al, 2020). The potential of CWS data is especially large for cities, where population density and thus also CWS network density is high and where traditional meteorological observations are sparse.…”