Urban trees play a crucial role in mitigating the urban heat island (UHI) effects. However, the cooling efficiency (CE), defined as the reduction in land surface temperature (LST) per unit increase in fraction tree cover (FTC), and its response to changes in FTC remain poorly understood across climate zones, presenting challenges for optimizing urban tree planting strategies to maximize cooling benefits. Additionally, the interactions among key meteorological factors, such as air temperature (AT), soil moisture (SM) and vapor pressure deficit (VPD), complicate a comprehensive understanding of how these factors influence CE. In this study, we analyzed CE of 229 cities across the globe by 2100 m×2100 m grid cells. Using piecewise regression, we determined the FTC threshold values associated with CE variations across different climate zones. We further applied Partial Least Squares Path Modeling (PLS-PM) to explore the direct and indirect effects of these meteorological variables on CE. The results indicate that: (1) In arid zones during the day and semi-arid zones at night, no significant FTC threshold is observed as FTC increases. However, in semi-arid zones during the day and arid zones at night, the FTC threshold is 19%. In sub-humid zones, the FTC thresholds are 45% during the day and 30% at night, while in humid zones, the thresholds are 55% during the day and 23% at night. These thresholds underscore the importance of considering local climatic conditions when planning urban tree coverage to achieve optimal cooling benefits. (2) AT and SM generally enhance CE, while VPD directly suppresses CE during the day but promotes it at night. In (semi-)arid zones, AT and SM primarily influence CE indirectly by controlling VPD, whereas in (semi-)humid zones, AT and SM have a direct impact on CE. (3) In semi-arid regions, the direct effect direction of AT and VPD on daytime CE reverse around the FTC threshold, with the dominant factor shifting from AT to VPD. At night in humid regions, the influence of AT on CE transitions from negative to positive around the FTC threshold, while in other climate zones, the dominant factors remain consistent across the threshold. The FTC thresholds identified in this study can guide urban greening planning in different climate zones. By revealing the different impacts of meteorological variables on CE before and after these thresholds, our results provide crucial insights for urban planners aiming to enhance cooling benefits in cities, informing the development of climate-resilient urban environments.