The impact of public space's temporary interventions in outdoor thermal comfort and microclimate : analysis of Lahti's market square during spring

Abstract

Temporary interventions in the public spaces are common in the urban landscape (e.g. street markets, parklets or sun sails); however, few studies have analysed their effects on microclimate and outdoor thermal comfort, especially in the Nordic countries. Outdoor thermal comfort has a physiological and psychological aspect; therefore, it has a site-specific understanding. Moreover, Climate Sensitive Urban Design (CSUD) searches for pleasant spaces by mitigating urbanization effects and weather. The study's relevance is to define effective and immediate low-cost solutions that cope with the rapid effects of climate change and users’ desires and create more liveable places using temporary interventions.

This study reviews and compares the thermal comfort and microclimatic implications of existing temporary elements in Lahti’s Market Square (Finland) during two critical spring days (warm and cold) in recent years to develop a list of strategies that bring an overall improvement of the site. Three scenarios were analysed on each day with ENVImet to analyse Potential air temperature, Mean Radiant Temperature (MRT) and Physiological Equivalent Temperature (PET). The scenarios considered: no interventions in the plaza as the base case (S1), the usual Daily Market (S2) and the Monthly Market (S3) based on field surveys made in May 2021. Field measurement validated the performance of ENVI-met. In parallel, an online survey revealed users’ behaviour, preferences and desires for the plaza, and two interviews presented the logistics and plans of the site.

Results exposed that existing temporary elements effects had a major impact on MRT and PET than T air. Additionally, they created more comfortable environments at specific hours (warming effect in the cold day and cooling effect in the warm day), especially for creating various microclimates. However, effects were more substantial on the warm day. Also, the higher the percentage of occupancy of the plaza was, the more significant effects they had. When comparing the existing vegetation with the temporary elements, the first ones had better results on the warm day because of evapotranspiration. Due to cultural preferences, users' time availability, and the lack of evening activities, the most “comfortable” hours were not voted as the preferred visiting hours (18:00 on warm day and 13:00 on cold day).

Finally, a practical list of recommendations for both critical dates was complemented with a set of simulations to validate their performances. The explorative proposals considered CSUD, placemaking and the recovered users’ desire for the area. The implementation of new activities involves using more temporary interventions, which simultaneously create more “comfortable” spaces and attract more people. The study is an essential reference for decision-makers and designers to develop more comfortable and attractive public spaces.