Effect of blue and green features on thermal exposure and thermal sensation in Czech cities

Climate change becomes more evident even in the generally mild climate of Central Europe. Analyses based on long-term temperature observations have confirmed growing numbers of hot days and nights and an increasing frequency of heat waves. In response to this risk, municipalities seek and invest into climate adaptation measures. However, the implemented measures are often inadequate or inefficient. In this study, we focus on the analyses of the effect of blue and green features to reduce heat stress in open public spaces in city centres; our research is conducted in collaboration with local representatives of four Czech cities of Brno, Olomouc, Ostrava and Pilsen. We selected representative locations in each of the four city centres as field measurement sites for air temperature, humidity, wind velocity and globe temperature during hot days. Based on our measurements the UTCI values in five-minute steps were calculated. At the same time, we conducted extensive questionnaire surveys of thermal comfort perceived by passers-by in each of the investigated locations. Our results show that in studied cities, trees within open areas of the city centre lead to a decrease in UTCI by 5–8°C during daytime in tree shade compared to sunlit paved areas. Irrigated and regularly cut lawns in open areas of the city centres cause a decrease of 0–1°C compared to sunlit paved areas. Small features of blue infrastructure (fountains, misting systems etc.) in open areas of the city centre lead to changes in UTCI in the range of −2.5 to +1.0°C, depending on the time of day and on the character of the water feature. Sprinkling the open areas (city squares) with water from tank trucks during heat waves lowers the UTCI by 1–3°C, yet this effect lasts only around 20 minutes (depending on air temperature and air flow). Influence of small water sprinklers and misting systems on the microclimate in distances exceeding 0.5 m from the given water feature was inconclusive. Finally, the results suggest a highly complex relationship between biometeorological indices and thermal sensation vote (TSV) in urban environments − open grassy areas exhibit a lower probability of higher TSV, on the other hand, the probability of higher TSV is higher under trees and near sprayed water-mist.