Greening of cities is one of the mitigation strategies for urban hot environments in summer, because green parks in cities have a lower temperature than the surrounding built environment in summer, called cool islands phenomena. This study took the micro-meteorological measurements in an urban park (Shinjyuku Gyoen, Tokyo Japan) and analyzes the vertical structure of a cool island at night. Vertical profiles of air temperature were measured simultaneously in the park and the nearby town area by the tethered balloons. In the park area, a cold air pool was present in the first 50 m from the ground, as opposed to neutral stratification in the town area. The cold air pool grew vertically at night. Meanwhile, surface air temperature difference between the park and the town increased up to 2 °C.

The ground-based instruments recorded the park breeze which is cold airflow to town area for 21 nights over two summer months. In most cases, the park breeze had at least 16 m depth and traveled for more than 122 m from the park edge. It is hypothesized that the park breeze is the gravity current. The observations made in our study can aid evaluating worth of urban green.

城市绿化是夏季城市高温环境的缓解策略之一，因为城市绿色公园的温度低于夏季周围建筑环境的温度，这称为凉岛现象。这项研究在城市公园（新宿御苑，日本东京）进行了微气象测量，并分析了夜间凉爽岛屿的垂直结构。在公园和附近城镇地区，通过系留的气球同时测量了垂直空气温度分布。在公园区域，离地面的前50 m存在一个冷空气池，与城镇区域的中性分层相反。冷空气池在晚上垂直生长。同时，公园和城镇之间的地面空气温差增加到2°C。

地面仪器记录了公园的微风，在两个夏季的月份中，有微风向镇区流动了21个晚上。在大多数情况下，公园的微风至少有16 m的深度，并从公园边缘行进超过122 m。假设公园的微风是重力流。我们的研究结果可以帮助评估城市绿色价值。