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The spatio-temporal variability of the canopy-level urban heat island (UHI) of Muscat is examined on the basis of meteorological observations and mobile measurements during a span of 1 year. The results indicate that the peak UHI magnitude occurs from 6 to 7 hours after sunset and it is well developed in the summer season. The warm core of the UHI is located in the Highland zone of Muscat, along a narrow valley characterized by low ventilation, high business activities, multi-storied buildings and heavy road traffic. Topographically, this valley is surrounded by mountains formed of dark-colored rocks such Ophiolites that can absorb short wave radiation and contribute, herewith, to the emergence of this warm urban core. In addition, this mountainous terrain tends to isolate this location from the cooling effect of the land-sea breeze circulation during the day time. In this warm valley the hottest temperature is encountered in the compact districts of old Muscat. In comparison, the urban thermal pattern in Lowland zone of Muscat is fragmented and the urban-rural thermal difference is reduced because of the lower urban density of the residential quarters. In addition, the flat alluvial terrain on which these residential quarters are located is consistently exposed to the land breeze circulation. Also, the study illuminates and emphasizes the importance giving due consideration to the nature of the rural baseline when assessing the urban effect on an area’s climate. For Muscat City, irrespective of the rural baseline used, a significant difference in the value of the urban heat island is registered. (C) 2011 Elsevier B.V. All rights reserved.