Seasonal variations in the cooling effect of urban green areas on surrounding urban areas

We measured air temperature in an urban green area that includes forest and grassland and in the surrounding urban area for a full year in Nagoya, central Japan, to elucidate seasonal variations of the difference in air temperature between urban and green areas. We determined the range of the “cool-island” effect as well as the relationship between vegetation cover and air temperature throughout the year. The temperature difference between urban and green areas was large in summer and small in winter. The maximum air temperature difference was 1.9 °C in July 2007, and the minimum was −0.3 °C in March 2004. The difference was larger during the day than during the night in summer, whereas in winter the opposite relationship was true. However, winter diurnal variation was not particularly noticeable, a behaviour thought to be related to reduced shading by deciduous trees in the green area. During the night, the cooling effect of the green area reached 200–300 m into the urban area. During the day, the cooling effect between August and October 2006 exceeded 300 m and varied widely, although there was no correlation beyond 500 m. The correlation between air temperature and forest-cover ratio within a radius of 200 m from each measurement site was significant from 16:00 to 19:00. There was also a correlation during the night; this correlation was weakest in the early morning. The effect of the forest-cover ratio on air temperature was most pronounced in August 2006 and June 2007.