Differences between the temperature of vegetated urban parks and that of their surrounding built environment are reported. The study is based on observations of surface and air temperature in Vancouver, BC and Sacramento, CA. during summer conditions. A combination of remotely sensed surface temperature and air temperature from fixed station and mobile (car and bicycle) traverses is used to characterize the magnitude of park-induced coolness the 'park cool island' (PCI) effect. Relatively large surface PCI are present by day and at night, although for different reasons. Air temperature effects are smaller. In Vancouver, parks are typically 1-2 C, but in ideal conditions can be almost 5 C cooler than their surroundings. Larger PCI are possible in Sacramento where irrigated greenspace can be 5-7 C cooler. Park type, especially the extent of irrigation and the presence of trees, is important in PCI development. During the day, trees may play an important role in establishing a cool park effect, perhaps through a combination of shade and evaporative cooling. At night, it appears that the surface geometry and moisture status of the park are important controls on surface cooling. Open parks (with higher sky view factors) that have dry soils (and hence lower thermal admittance) cool the most. Nocturnal cooling in open grass parks is often similar to that at rural sites. The influence of parks on air temperatures appears to be restricted to a distance of about one park width.
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