Capturing street-level heat: Citizen-based high-frequency observations of urban microclimates

Urban heat stress varies strongly at local scales, shaping everyday exposure to high temperatures and humidity across streets, neighbourhoods, and public spaces. However, official monitoring networks often lack the spatial and temporal detail needed to capture these fine-scale conditions. Citizen science and low-cost sensors offer a promising pathway to complement existing systems with localized, high-frequency observations that reflect how heat is experienced in cities. 

In this study we collaborated with residents and city partners in four European cities (Athens, Cascais, Riga, and Utrecht) to collect geolocated temperature and relative humidity data using more than 300 low-cost sensors. Participants contributed around 160,000 observations, capturing fine-scale variation in urban microclimates and illustrating how Urban ReLeaf, a Horizon Europe initiative, strengthens citizen-powered data ecosystems for urban climate resilience. 

Data collection followed three complementary approaches. Most participants carried sensors during their daily activities and collected data where and when they chose. A second approach equipped municipal street cleaners with sensors during their regular work routes, providing more systematic coverage of public spaces and their working conditions. A third approach deployed sensors for short periods at predefined locations to support targeted comparison and calibration. 

We demonstrate how these citizen-powered observations can be transformed into usable climate information, from filtering reliable spatial records to addressing uneven sampling in time and space. We also explore modelling approaches that leverage the richness of high-frequency, mobile measurements despite their inherent heterogeneity. The results reveal microclimate patterns that remain largely unseen by fixed monitoring networks, particularly at the spatial scales that matter for everyday heat exposure and urban design decisions. We share practical pathways for incorporating citizen science data into urban monitoring efforts and highlight their potential relevance for heat adaptation, greenspace planning, and public health.