Extreme temperatures and heat waves are a significant risk for public health. Climate change and ongoing urbanization particularly expose urban dwellers, especially vulnerable people such as children and the elderly. In this context, the Urban Heat Island (UHI) effect describes the increased temperatures in urban areas compared to their rural surroundings. This study aims at the quantification of the UHI based on land surface temperatures measured by remote sensing, and air temperatures recorded by climate stations. Since air temperature measurements from official networks are scarce particularly in cities, measurements from Citizen Science offer a complementary source of data. First of all, this work investigates the robustness of crowdsourced air temperatures by comparison with measurements of official measurement networks. In general, we found both data sources featuring similar characteristics. Secondly, the relation of land surface and air temperature measurements is explored through statistical analysis, which showed a clear correlation that varied depending on the season. Finally, the UHI was quantified for different seasons. The strongest UHI based on land surface temperatures was measured in summer with urban-rural difference of 3.0 K. Regarding air temperatures, the highest average difference of 1.3 K was found in spring. In this manner, this study demonstrates that the incorporation of Citizen Science data enhances the knowledge on the urban climate and thus assists the development of appropriate and targeted mitigation strategies to protect the health of citizens.