Spatial and temporal variations of cool island (low-temperature area) phenomena in urban green parks were analyzed based on several field meteorological observations in and around the Meiji shrine/Yoyogi park (124ha) which is one of the largest parks in Tokyo. In order to clarify diurnal surface temperature variations in the green park and its peripheral area, an infrared thermography was settled on the top of a neighboring building. Vertical temperature variations were observed using a kite baloon (Meisei-electric Inc.) and a tower sonde (AIR Inc.). Cool island intensity (CII), which is defined as the temperature difference between green park area and its surrounding built-up area, shows diurnal variations. In a grass area, CII is weak in the daytime and strong at night. On the other hand, CII is strong for both day and night in forest areas. The reason for this is that radiative cooling from the surface of grass and forest at night might be more effective in the green park than in the peripheral built-up. It is also pointed out that CII shows little variability in either fine or cloudy weather.
Surface air temperature observations along a horizontal straight line in and around the green park reveal cooling in the peripheral built-up area caused by the advection of cooled air from a green park area. Vertical temperature profiles in the green park show ground inversion which reaches 60 meters in height during the night. No clear ground inversion could be found in urban built-up areas. These results suggests that nocturnal heat island in an urban area, particularly in the periphery of a large green park, would be partly mitigated by the cool island effect in urban green parks.