The urban heat island effect is a phenomenon observed worldwide, i.e. evening and nocturnal temperatures in cities are usually several degrees higher than in the surrounding countryside. In contrast, cities are sometimes found to be cooler than their rural surroundings in the morning and early afternoon. Here, a general physical explanation for this so-called daytime urban cool island effect is presented and validated for the cloud-free days in the BUBBLE campaign in Basel, Switzerland. Simulations with a widely-evaluated conceptual atmospheric boundary-layer model coupled to a land surface model, reveal that the urban cool island can form due to differences in the early morning mixed-layer depth over the city (deeper) and over the countryside (shallower). The magnitude of the urban cool island is estimated for various types of urban morphology, categorised by their respective local climate zones.