Contemporary open space planning is required both approach of science and function. As one of the functions of the green, there is a temperature mitigation function of green spaces to solve the heat island problems. So in the new development, planners should consider to effective greening in there. In the case of planning the open space with temperature mitigation, it is significant to analyze based on the output index as green space in quantitative and the outcome index as temperature mitigation effect of them. This study aims, from the view point of easy adapting for open space planning in future development, to estimate how much effect of urban thermal environment mitigation in subtropical climatic conditions by ratio of ground coverage.

 We studied in Naha Shintoshin area in Naha City, Okinawa Prefecture. Naha Shintoshin area was returned land from USA base and had been developed by land readjustment project finished in 2003. The LR project area was 214 ha. But because of the limitation of lack of measurement tools and man power, we had to choose a part of area, 5508 m2, as our case study site in there.

 The case study site is almost flat in topographical condition and planned in modern urban planning theory which has block parks, neighborhood park, comprehensive park, and greening residential district. Because of this site is an example of entirely new development in Okinawa and it is planned in neighborhood planning theory, this site was suitable as case study site.

 Temperature measurements by hour was done at 39 spots where are in different environment such as in bank of a river, in park, on greenery street, and in residential parcels. The amount of measurements was 2846 hours. We set the finite difference between measurement temperature and temperature published Okinawa meteorological observatory as an explained variable and set land coverage ratio of the site as explanatory variables. The land coverage ratio was estimated by google aerial photos and was divided to tree, grass, water, bare, building and pavement.

 Analysis methods are followings. At first, we examined the ratio of ground coverage which is superior effect of the temperature mitigation for each temperature range, time zone, mesh grid size in subtropical climatic conditions. (Sec. 3) Next, we analyzed the influence of the temperature distribution from the relation with the ground covering condition in quantitatively. (Sec. 4) And then this study considers the findings of temperature mitigation of open space as a green space planning theory. (Sec. 6)

 And the consequences are followings. 1, It was confirmed that factors such as season, time zone, mesh grid size, affect the relationship between temperature mitigation and ground coverage. 2, It was confirmed from the single regression analysis result that the adjusted R2 becomes high in mesh grid size of 10m and 90m. This result means the natural land coverage has both effect of temperature mitigation in spot and wide area. 3, By principal component analysis, it was possible to obtain a multivariate regression analysis result avoiding Multiple correlations. 4, From the estimated temperature distribution of the target area, it was confirmed that the site that ration of green coverage is 20% or more, forms a low temperature zone.

 Considering of these result with real green planning, 10m mesh effect should be able to use in planning of house scale, and 90m mesh effect should be able to adapt as a greening regulation in special district plan.

 This study remains future subjects such as the multiple elements analysis and the continuity consideration to next meshes in wide-spread greenery land use.