Actual sky visibility under shading by trees with different crown densities

Abstract

Introduction. The development of green infrastructure is an important task in urban planning strategies to reduce urban heat island effect and improve urban ecosystem. The aim of this study is to determine the actual visibility of the skyline when shaded by trees of different crown densities. The scientific novelty is that for the first time a study of the actual sky visibility with trees shading as complex heterogeneous systems has been conducted to assess the effect of mitigating of urban heat island. The practical significance lies in the establishment of the actual visibility of the firmament under the shading of trees with different crown density, which enables the correction of simulation models to analyze the parameters of the microclimate of the urban environment under given climatic conditions.
 
 Materials and methods. Various approaches to determining the sky visibility are considered. Direct measurements were made by photographing with a special fisheye lens and a smartphone. The research was carried out in a temperate continental climate (according to the international climate classification Köppen — type Dfa). Several tree species most commonly found in landscaping under the climate conditions in question and having different crown densities were selected: elm, oak, maple and poplar. For each, several samples were selected as required for data averaging. A total of 27 samples were studied.
 
 Results. A diagram of measured averaged values of the sky visibility factor was plotted, taking into account shading by trees with different crown densities in different periods of the year. To protect trees with dense spreading crowns from the direct solar radiation in the summer daytime, it is better to use trees with dense spreading crowns to intensify the radiant heat exchange of the urban surface with the sky at night — trees with less dense crowns. In case of the need to soften the urban heat island during the day, hybrid solutions are the most effective.
 
 Conclusions. The actual assessment of the sky visibility, made by the authors taking into account shading by trees, allows us to make corrections in simulation models to analyze the parameters of the microclimate of the urban environment under the given climatic conditions. The results of the study make it possible to select the type of tree providing the required shading and urban heat island’s mitigation effect.