Do Trees Cool the Same Every Summer's Day? Temporal Non-Stationarity in Urban Environments from Bike-Based Air Temperature Monitoring in New Haven, Ct

Abstract

Mitigating urban heat is a vital ecosystem services of urban trees, especially within a warming climate. Land surface temperature measures captured at a single time of day (in the morning) dominate the urban heat island literature. Less is known about how local tree canopy and impervious surface regulate air temperature throughout the day, and/or across many days with varied weather conditions, including cloud cover. We use bike-mounted air temperature sensors in New Haven, CT, from 2019 – 2021 and generalized additive mixed models across 156 rides to estimate the daily variation in cooling benefits associated with tree canopy cover, as well as warming from impervious surface cover. The cooling benefits from tree canopy cover were strongest in the mid-day (11:00-14:00, -1.62°C), afternoon (14:00-17:00, -1.19°C), and morning (8:00-11:00, -1.15°C) on clear days. The cooling effect was comparatively smaller on cloudy mornings -0.92°C and afternoons -0.51°C. Under cloudy conditions the role of trees’ shading is diminished, yet there was significant cooling, highlighting the unique contribution of evapotranspiration. The warming disservice of impervious surfaces were most pronounced in the evening (17:00-20:00, 1.11°C) on clear and cloudy rides, and during cloudy nights (20:00-23:00) and cloudy mornings 1.03C [1.03, 1.04]. Among the hottest observed days, tree canopy was associated with lower temperatures on clear afternoons -1.78C [-1.78, -1.78], cloudy mid-day -1.17C [-1.19, -1.15], clear mid-day -1.12C [-1.12, -1.11]. Future mobile sampling campaigns may broaden the spatial extent and include more sources of environmental variation, and represent an exciting opportunity for pubic science and engagement.