Study On The Impact Of Urban Street Greening On Outdoor Thermal Comfort

In the context of global warming and rapid urbanization,the thermal comfort of cities has become particularly important.Severe heat has a negative impact on public health and also greatly reduces the use of outdoor spaces and activities that can be undertaken.Good thermal comfort helps to improve the vitality of a street and has become an important indicator of its walking potential.Street greening,as an important part of street space planning and design,has been proved by a large number of studies to be an important way to improve the thermal comfort of street space.Therefore,it is of great significance to explore the influence mechanism of street greening on outdoor thermal comfort in order to better use street planting design to improve pedestrian thermal comfort.The study selected living streets with different greening characteristics in Wuhan as the research object.In the summer,through the combination of microclimate measurement and questionnaire survey,the impact of different tree coverage and tree canopy characteristic factors on thermal comfort was revealed.The evaluation standard of thermal comfort in summer in Wuhan was established,and the thermal sensation prediction formula was proposed.The research results can provide a theoretical basis for the future climate-adaptive street planting design in Wuhan.The work and main research conclusions are as follows:（1）On August 18,2019 and August 19,2019 from 7:00-22:00,Wuhan Yejin 1st Street（the street is divided into three parts of high,medium and low tree coverage with 150m as a section）)Conducted a microclimate measurement（air temperature,relative humidity,black bulb temperature,wind speed）and a longitudinal experiment questionnaire survey,received 476 valid thermal comfort questionnaires.The influence of tree coverage in different streets on thermal comfort in summer was analyzed and compared.The results showed that high-percentage tree canopy cover over a street can reduce midday（12:00–14:00）air temperature and mean radiant temperature by up to 3.3°C and 13.9°C,respectively,compared with a similar street lacking tree shade.The thermal sensation is related to the degree of street greening.The higher the street tree coverage,the easier it is for people to be satisfied with the current thermal environment.The neutral and moderate PET thermal ranges that represented the comfort of people on the streets in Wuhan in summer were found to be 28.8°C and 26.35–31.29°C,respectively.The prediction formula of thermal sensation was proposed,and the relationship between human subjective thermal sensation and microclimate factors（air temperature,wind speed,globe temperature and relative humidity）was established.This formula provides a method for predicting the physical environment parameters required to obtain a neutral thermal sensation in the summer streets of Wuhan.In the summer of Wuhan,improving solar thermal comfort by blocking solar radiation and increasing wind speed is more feasible than controlling air temperature and humidity.（2）The canopy characteristics measurement and microclimate measurement were conducted on August 20,2019 and August 21,2019 from 7:00 to 19:00 on the municipal side street of Wuhan City（there are two species of camphor and plane tree）,comparative analysis The effects of different canopy characteristics of trees on street microclimate and outdoor thermal comfort are discussed.The results show that when the tree species are the same,trees with high leaf area index（3.03）and high crown width（13.55m）can better improve street microclimate and outdoor thermal comfort than trees with low leaf area index（1.83）and low crown width（7.65m）.At 15:00,the difference between the PET and T_mrt of the two reached the maximum,respectively 5.4℃and 12.09℃.When the leaf area index is the same between trees,even if the tree species are different,the difference in leaf morphology and crown width cannot determine the benefits of microclimate and PET.On regular streets in summer,the leaf area index is significantly correlated with globe temperature and PET at noon.