Mitigating urban heat island (UHI) effects, especially under climate change, is necessary for the promotion of urban sustainability. Shade is one of the most important functions provided by urban trees for mitigating UHI. However, the cooling effect of tree shade has not been adequately investigated. In this study, we used a simple and straightforward method to quantify the spatial and temporal variation of tree shade and examined its effect on land surface temperature (LST). We used the hillshade function in a geographic information system to quantify the spatiotemporal patterns of tree shade by integrating sun location and tree height. Relationships between shade and LST were then compared in two cities, Tampa, Florida and New York City (NYC), New York. We found that: (1) Hillshade function combining the sun location and tree height can accurately capture the spatial and temporal variation of tree shade; (2) Tree shade, particularly at 07:30, has significant cooling effect on LST in Tampa and NYC; and (3) Shade has a stronger cooling effect in Tampa than in NYC, which is most likely due to the differences in the ratio of tree canopy to impervious surface cover, the spatial arrangements of trees and buildings, and their relative heights. Comparing the cooling effects of tree shade in two cities, this study provides important insights for urban planners for UHI mitigation in different cities.

缓解城市热岛效应（UHI），特别是在气候变化下，是促进城市可持续发展的必要条件。遮阳是城市树木缓解UHI的最重要功能之一。但是，尚未充分研究树荫的冷却效果。在这项研究中，我们使用一种简单明了的方法来量化树荫的时空变化，并研究了其对地表温度（LST）的影响。我们使用了地理信息系统中的hillshade函数，通过整合太阳位置和树高来量化树荫的时空模式。然后比较了佛罗里达州坦帕市和纽约州纽约市这两个城市的阴影与LST之间的关系。我们发现：（1）结合太阳位置和树高的山体阴影功能，可以准确捕捉树荫的时空变化；（2）树荫，特别是在07:30，对​​坦帕和纽约的LST具有明显的降温作用；（3）树荫在坦帕的降温效果比纽约州强，这很可能是由于树冠与不透水表面覆盖率之比，树木和建筑物的空间布置及其相对高度的差异。比较两个城市中树荫的冷却效果，本研究为城市规划者在不同城市缓解UHI提供了重要的见识。这很可能是由于树冠与不透水表面覆盖率的比率，树木和建筑物的空间布置及其相对高度的差异所致。比较两个城市中树荫的冷却效果，本研究为城市规划者在不同城市缓解UHI提供了重要的见识。这很可能是由于树冠与不透水表面覆盖率的比率，树木和建筑物的空间布置及其相对高度的差异所致。比较两个城市中树荫的冷却效果，本研究为城市规划者在不同城市缓解UHI提供了重要的见识。