The Impact of Urban Design Descriptors on Outdoor Thermal Environment: A Literature Review

Abstract

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.

A Numerical Study on the Correlation between Sky View Factor and Summer Microclimate of Local Climate Zones

“…In urban areas microclimate and ventilation, and thus thermal comfort, are strictly interconnected with built form and building packing density, as clearly stated in the literature, see for example recent reviews by Lin et al [5], Toparlar et al [8], Di Sabatino et al [14], Buccolieri and Hang [15]. The built form is commonly represented through morphological parameters such as the building density, the volume ratio, the street height to width ratio (aspect ratio), the plane permeability, the sky view factor, and so on.…”

“…The process of urbanization is crucial for a country's social evolution and economic development. Over the past two decades, large-scale urban constructions and high-density building arrangements have changed the existing urban physical space form, leading to a series of urban issues mainly related to climate and environment, such as the urban heat island (UHI) effect, the urban microclimate and air quality deterioration, the building energy consumption increase, and the enhanced greenhouse effect [1][2][3][4][5][6]. The concept of urban planning and design is experiencing a transformation from the traditional urban demands of rational functional layout, sustainable industrial structure, and complete infrastructure to the new requirements of livable, ecological, and sustainable modern urban quality.…”

“…There are two ways to define SVF: Based on space geometry as the ratio of the visible sky area of a point in space to the total sky area, or based on radiation as the ratio of the radiation received (or emitted) by a planar surface to the radiation emitted (or received) by the entire hemispheric environment [16]. Being a high-order index describing urban spatial form, it is generally used to characterize the intensity of UHI [5]. Since the urban climate is closely related to spatial form, geographical environment, and climatic conditions, any conclusion arisen from a specific urban climate research is mostly of relevance only for similar climate and geographical regions.…”

Urban Warming and Cities’ Microclimates: Investigation Methods and Mitigation Strategies—A Review

“…One of the most important negative outcomes of urbanization involves urban warming and high ambient urban air temperatures, attributed to various uncontrollable and controllable parameters. On one hand, the uncontrollable factors refer to environmental and nature-related parameters, whereas the controllable ones involve urban planning and design parameters [1,2] (see Figure 1). To address the urban warming issue and the degraded urban microclimatic conditions, the term "urban heat island phenomenon" (UHI) is also used, describing the divergence of atmospheric conditions and the higher ambient air temperatures (T air ) values occurring in an urban district, compared to those of the surrounding rural areas [3][4][5].…”

The Cooling Effect of Large-Scale Urban Parks on Surrounding Area Thermal Comfort

“…Despite the cooling effect of urban parks on their surroundings, such an outcome is not merely dependent on the traits of the green space [38]. The morphology of the surrounding area of parks, the sky view factor (SVF), the spatial configuration of the location, and the covered area, also affect the perception of the cooling effect of urban parks and thermal comfort [37,39].…”