Review of methods used to estimate the sky view factor in urban street canyons

The sky view factor (SVF) is the ratio of the visible sky area of a point in space to the total sky area. It provides the relationship between the visible sky area and covered surroundings, such as by buildings or street trees. The SVF has been widely used as a key parameter in urban climate research and urban planning practices. Significant research has taken place in the past decades on methods of calculating/estimating SVFs to improve their accuracy and efficiency. This review lists the methods used to calculate/estimate SVFs including geometric methods, fish-eye photographical method, Global Positioning System methods, simulation methods based on 3D city models or digital surface models, and big data approaches using street view images. We stress the principles, input data, application, accuracy and efficiency of each method. This review is meaningful for climatologists in solar radiation modeling and energy balance modeling fields, as well as for urban planners in the development of design guidelines to improve outdoor thermal comfort in the urban environment.