Influence of Vegetation Transitions on Air Temperature

Overall health of urban residents is influenced by local heat. Therefore, urban planners are considering on enlarging areas of vegetation primarily for microclimate cooling. However, the magnitude of cooling through vegetation transitions on urban climate is uncertain. Here we present the analytical results of air temperature variations from a network of 28 air temperature (T_a) sensors (iButtons) in six different plots on the main campus at UC Riverside in July 2017. T_a iButtons Thermocron (DS1922L, Maxim Integrated Products, Inc., San Jose, California, USA) used as network sensors offer high quality data (accuracy of ±0.5 °C from -10 to 65 °C) that help maximize the understanding on patterns and relationships between vegetation coverage and T_a. All T_a sensors were evenly distributed on all plots and placed at a height of 1.5 m within different types of scenarios: transect 1 had a highly vegetated tall canopy, transect 2 transitioned from tall canopy to turf, transect 3 transitioned from a dense greenery core to asphalt, transect 4 had tall canopy with grass to asphalt, transect 5 displayed a transition of turf to asphalt, and transect 6 had only asphalt as the surface. All T_a measurements were collected hourly in a 15-day time period (July 3^rd - 18^th) coinciding with the warmest period of the year. Alongside T_a sensors on transect 3, a tower mounted LST thermal camera (SC660, FLIR, Inc.) was used to record thermal images every 10 min for a 24-hr period (July 17^th - 18^th) at the edge of a tall vegetation and asphalt. Our analysis yielded a greater microclimate regulation observed with high vegetation with a mean T_a of 26.24 °C, more efficient cooling variations when transitioning from tall-dense vegetation to grass (transect 2), and an increasing thermal gradient when moving away from the core of the vegetation outwards. Less microclimate regulation was observed on impermeable surfaces (asphalt) with a mean T_a of 30.94 °C. Thermal images showed an increasing thermal gradient from vegetation to asphalt (transect 3) with T_a starting at 28.3 °C and reaching 66.5 °C. Thus, vegetation density increases cooling rates because of shading effect, albedo effect, and by possible air mixing in taller canopies. These findings further highlight the concept that an increase in urban vegetation can provide positive feedback for mesoclimate cooling.