The influence of small green space type and structure at the street level on urban heat island mitigation

- Small green spaces can reduce air temperature of an urban block where they belong.
- Polygonal type of small green spaces has cooling effect more than other types have.
- Multi-layered small green spaces contribute to cool down the block where they belong.
- Polygonal as well as mixed green space of over 2000 m3 can reduce 1 °C of a block.

The purpose of this study was to determine the types and structures of small green spaces (SGs) that effectively reduce air temperature in urban blocks. Six highly developed blocks in Seoul, South Korea served as the research sites for this study. Air temperature was measured at the street level with mobile loggers on clear summer days from August to September in 2012. The measurements were repeated three times a day for three days. By analyzing the spatial characteristics, SGs within the six blocks were categorized into the four major types: polygonal, linear, single, and mixed. The result revealed that the polygonal and mixed types of SGs showed simple linear regression at a significant level (p < 0.01). It indicated that the blocks' urban heat island (UHI) mitigation (ΔTRmn) increased in a linear fashion when the area and volume of these two types of green spaces increased. The area and volume of a polygonal SG with mixed vegetation, over 300 m2 and 2300 m3, respectively, lowered the ΔTRmn by 1 °C; SG with an area and volume of larger than 650 m2 and 5000 m3, respectively, lowered the ΔTRmn by 2 °C. The results of this study will be useful to urban planners and designers for determine the types and structures of urban green spaces to optimize the cooling effect, as well as how such green spaces should be designed and distributed.