The sky visibility at shading by trees
The development of green infrastructure is an urgent task in developing an urban planning strategy to reduce the effect of an urban heat island and improve the urban ecosystem. The purpose of this study is to assess the visibility of the sky at shading by trees when solving the problems of predicting the formation of an urban heat island and developing recommendations for its mitigation. The object of research. Several species of trees were selected, the most common in greening in the hot climatic conditions: elm (Ulmus L.), oak (Quercus L., 1753), maple (Acer L., 1753) and poplar (Populus L., 1753). For each of these tree species, several samples required for data averaging were selected. A total of 27 samples were examined. Field studies were conducted in various months of the year (10.05.2021, 16.07.2021 and 03.10.2021). Method. A characteristic of sky visibility is sky visibility factor (SVF). Various approaches to determining the visibility of the sky are considered. A common limitation of computational methods is the ability to define SVF for relatively simple object configurations. Therefore, to find SVF complex heterogeneous systems, such as trees, we made direct measurements by photographing using a special fisheye lens and a smartphone. Data verification confirms the possibility of using this equipment for field studies. The method of black, white and gray pixels was used to process photo images, which allows you to increase the accuracy of the results. Results. Analysis of the results shows that the minimum SVF values for all tree species studied are observed in July. At this point in time, the tree crown is densest, limiting the visibility of most of the sky. The maximum effect of shading in summer is provided by oak (SVF = 22.9%), minimal – by elm (SVF = 30.4%). It is shown that trees with denser krone block long-wave radiation streams day at night that leads to growth of the urban heat island. Comparing the received results, it is possible to note that the minimum mitigation (8.0 K) gives an elm; maximum mitigation (8.8 K) gives an oak. Therefore, for protection against solar radiation in the afternoon it is better to use an oak, and for mitigation of the urban heat island it is better to use an elm at night. Thus, the method allows choosing a species of the tree providing the required shading and effect of mitigation of the urban heat island. Further researches will be connected with specification of mathematical model of the urban heat island for assessment of a microclimate of the urbanized territories during the hot period.