The measurements of an urban-rural observation network in Hannover obtained over almost four years were used to analyse a canopy layer urban heat island (UHIUCL). Especially during the summer months, the UHIUCL was pronounced on numerous nights with maximum values of more than 6 K. We have demonstrated that the choice of rural reference station is important to describe a UHIUCL in detail. The time evolution of UHIUCL differed greatly from night to night and four main types could be identified with comparable frequencies of occurrence. The difference between these types was the appearance of the maximum urban heat island intensity during very different times of the night. The main drivers for the development of a specific type were the rural winds and rural temperature changes caused by turbulent mixing within the near-surface inversion.