Temporal and spatial variability of urban heat island by geographical location: A case study of Ulsan, Korea

This paper investigated the temporal and spatial variability of the urban heat island (UHI) effect by geographical location in the study area of Ulsan, Korea. It focuses on the relationship between geometric features and geographical locations. For this research, air temperature measurements were collected from 44 monitoring stations in 2016. We classified these measurements into five geographical conditions and conducted trend and a random effect GLS regression analyses. The results of the trend analysis showed that the nocturnal air temperature of the urban center roads was approximately 2.5 degrees C and 3.5 degrees C higher during the summer and winter periods, respectively, than the temperature of the rural roads. However, when we controlled for the geometric features of the roads in the regression model, the UHI intensities of the urban center roads in the winter and summer nights decreased to 1.4 degrees C and 1.1 degrees C, respectively. This decrease was due to the mediation effect of the geometric features of the roads. In addition, nocturnal air temperature increased when the building surface fraction and road width increased, while the sky view factor (SVF) was positively related with daytime air temperature. Compared with other geographical contexts, the thermal conditions of the roads near the coastal area was mild in the winter and cool in the summer when the geometric features and the meteorological conditions of the roads were controlled in the regression models.