Characteristics of the Park Cool Island in Golden Gate Park, San Francisco.

Mobile bicycle transects were conducted in Golden Gate Park and surrounding residential neighborhoods of San Francisco, CA, during the period July–October 2012. Measurements included air temperature at six heights from 0.15 to 2.5 m, as well as relative humidity, solar radiation, and surface radiometric temperature. Park temperatures were lower than surrounding neighborhoods at all measurement levels, and mean Park Cool Island (PCI) values ranged from 0.9 °C for grass playing fields to 1.9 °C for tree groves during typical cool (~ 16 °C) summer conditions. These increased to 2.6 °C and 3.7 °C, respectively, during periodic warm events (~ 30 °C). Grass fields produced strong temperature lapse profiles, similar to asphalt surfaces, while dense trees produced a near-isothermal profile. Shade enhanced cooling by 1–8 °C throughout the 2.5-m layer, peaking near the surface, and frequently producing inversions below 0.5 m. Complexity in the lower surface layer temperature profile suggests measurements of air temperature at one height may be insufficient to characterize the PCI. The park also produced a consistent humidity enhancement, with differences in vapor pressure between urban and park surfaces of up to 10 hPa. The humidity difference was positively correlated with the PCI and also doubled between cool and warm periods. This, along with assessment of the role of shade, and the daytime peak, suggests evaporative cooling and shade cooling are the main drivers of the PCI in Golden Gate Park. The advection of cool air from the park into leeward neighborhoods was limited below 2.5 m, and was only observable within the first few hundred meters. Implications for urban planning suggest urban green spaces that provide abundant vegetation, including dense tree canopies, and are widely accessible to inhabitants can be most effective for in situ thermal comfort, particularly temperature relief during heat events. [ABSTRACT FROM AUTHOR]