This paper presents a literature review on urban design indicators addressing the impact of urban geometry and vegetation on the outdoor thermal environment at the pedestrian level, as urban geometry and vegetation have been regarded as the most influential urban design factors that affect outdoor microclimate. The thermal balance concept is first introduced to elaborate how each component of energy fluxes is affected by the urban built environment, which helps to explore the underlying thermophysical mechanisms of how urban design modifies the outdoor thermal environment. The literature on numerous urban design descriptors addressing urban geometric characteristics is categorized into five groups in this paper according to the design features that the parameters entail, including land use intensity, building form, canyon geometry, space enclosure and descriptive characteristics. The literature on urban vegetation descriptors is reviewed together, followed by the combined effect of urban geometry and vegetation. This paper identifies a series of important urban design parameters and shows that the impact of design parameters on thermal environment varies with time, season, local climate and urban contexts. Contradictory impacts often occur between daytime and nighttime, or different seasons, which requests trade-offs to be achieved when proposing design strategies.

This study investigates the relationships between urban morphology and summertime urban heat island in Hong Kong. A total of 33 urban design parameters describing complex high-rise high-density urban morphology are proposed and categorized into three groups, i.e., land-use intensity, built form, and space enclosure. A process combining the python script and the geoprocessing function is proposed to quickly calculate the morphological parameters for 10 sites and 160 points. Microclimate data were collected using onsite measurement equipment. Pearson correlation and multiple linear regression analysis are conducted to quantify the impact of each parameter on urban heat island (UHI) intensities at 3 pm (UHI_3pm) and 9 pm (UHI_9pm). 16 parameters are found to be statistically correlated with UHI_3pm and 9 parameters with UHI_9pm. Results show land-use intensity parameters have the highest correlation with UHI, followed by built form parameters and then space enclosure parameters. Furthermore, it is found that site-level parameters can better explain UHI variation compared to point-level parameters. This research paves the path for quickly extracting urban morphological parameters and enhances the understanding of the impact of complex high-rise high-density urban environment on summertime UHI. The results can inform policy makers with the guidelines to create more comfortable urban environment.