Effects of urban morphology and greenery on intra-urban outdoor thermal comfort using mobile transect measurements in Plymouth, UK

Global warming, urbanisation, and anthropogenic activities increase urban populations’ exposure to heat, affecting walkability and outdoor activity patterns. Urban greenery has been seen as an effective strategy to reduce heat burdens in cities. Yet, cities often do not provide sufficient tree shade for heat relief. Recent research has highlighted the heat vulnerability factors at the city scale. However, urban morphology and greenery synergistically affect outdoor thermal comfort at the neighbourhood level. Therefore, this study examines the effects of built environment factors and urban vegetation on outdoor thermal comfort at the neighbourhood scale.

The team developed a novel trolley system with GPS for mobile measurements of air temperature, relative humidity, wind speed, and global temperature. FLIR thermal cameras were attached to the trolley to measure surface temperatures. Moreover, fisheye photos were taken to calculate the sky view factor (SVF) at points of interest on our walking routes. Mobile monitoring campaigns occurred during May and June 2024. The mobile transect spanned built and natural environments, including high-density university buildings, a museum precinct, medium-density dwellings, highways and parks. Additional mapping data was introduced to develop a geospatial model. The study shows large variations in pedestrian-level air temperature, with unshaded, mid-density built-up areas reaching up to 10 °C higher than shaded parks around 3 pm. The Physiological Equivalent Temperature (PET) shows intra-city differences of up to 20 °C, which is consistent with higher PET values in streets with higher SVF. This intra-urban PET difference could also be due to surface temperature variations captured in thermal images, with the lowest ground surface temperature recorded under dense tree cover. In summary, the novel trolley system demonstrates the extent to which urban greenery improves thermal comfort depends on tree canopy density, SVF and the time of the day, highlighting key neighbourhoods that require urban planning interventions.