Cool city mornings by urban heat

N.E. Pietersen-Theeuwes, G.J. Steeneveld, R.J. Ronda, M. Rotach, A.A.M. Holtslag

Research output: Contribution to journalArticleAcademicpeer-review

31 Citations (Scopus)


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 (UCI) 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 UCI can form due to differences between the early morning mixed-layer depth over the city (deeper) and over the countryside (shallower). The magnitude of the UCI is estimated for various types of urban morphology, categorized by their respective local climate zones.
Original languageEnglish
Article number114022
Number of pages10
JournalEnvironmental Research Letters
Issue number11
Publication statusPublished - 2015

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