## 1 Introduction
Because the decrease of the solar radiation is significant to energy saving for cooling, the impact of design factors of urban features on mitigating solar radiation was investigated by [PERSON] et al. [13]. They found that varying the height-to-width ratio in linear types of urban open-spaces has the greatest influence in decreasing total solar radiation gained by surrounding buildings facades. The implications of this study are that street height-to-width ratio can be determined by controlling solar radiation, since Qilou street is also a linear type of urban open-space. By now, few studies have been performed on urban open-spaces' thermal comfort in southern China [14]. Nanning is located in a hot summer and warm winter zone. As the urban commercial block, the commercial value and prosperity of Qilou street largely depends on outdoor thermal comfort in this region. In summer, commercial streets with arcades (Qilou) are crowded and hot. Excessive solar radiation will deteriorate the thermal environment of the streets, directly affecting people's shopping experience and reducing commercial profits. In order to enhance the commercial and tourism value of urban commercial pedestrian street, [PERSON] et al. [14] studied the thermal comfort conditions in the microclimate of the street; then, some strategies were proposed to improve thermal comfort: increasing the building height, adding tree canyon coverage, and changing paving material. The relationship between urban geometry and microclimate has significantly influences the thermal comfort of individuals. [PERSON] et al. [15] presents a long term numerical analysis of the effect of height/width proportions on the thermal comfort of courtyard typology. The results of the study reveal that high height/width proportions appear to have a stabilizing effect on thermal comfort. [PERSON] et al. [16] deals with the dependence of outdoor thermal comfort on the design of an urban street, including complex street geometry, orientation, greening coverage, etc. The results of the study indicated that if these factors are appropriately combined, it can effectively reduce heat stress in the summer and enhance thermal comfort. Among all the factors, sunshine-hours, morphology, pavements, greening and trees are the evident factors affecting the thermal environment of Qilou streets. Therefore, based on setting the height-to-width ratio of the streets reasonably, the sunshine hours of the streets on hot days can be minimized to create a pleasant street environment, improving residents' thermal comfort and shopping experience. Hence, the relationship between the planning characteristics of a block at a location and sunshine hours must be determined; in other words, a sunshine-hour model should be built up based on the planning characteristics of the block. According to the definition of sunshine hours by the World Meteorological Organization (WMO) [17], if the sunshine-hours are regulated in urban environment design, then the amount of solar radiation received by a location during that duration will be controlled to a certain extent [18]. This is critical to the optimization of the outdoor thermal environment through reasonable planning and design. In this regard, [PERSON] [19] proposed a sunshine-hours model; subsequently, [PERSON] [20] proposed a functional relationship between the coefficients in [PERSON]'s model and the percentage of sunshine. Based on the characteristics of the local climate, the sunshine-hours model was developed into a nonlinear model: [PERSON] [21] introduced a logarithmic function into the model. [PERSON] [22] proposed an exponential model for sunshine hours. All these models performed well as predictors of solar radiation from sunshine hours but the non-linear models were better than the basic linear model [23]. In order to obtain a more accurate sunshine-hours model, [PERSON] [24] presented a quadratic nonlinear sunshine-hours model. Then, [PERSON] et al. [25] further developed a cubic nonlinear model based on [PERSON]'s model.