This study investigates the cooling effect of urban trees across different background climates to understand how climatic conditions influence their performance as heat-mitigation strategies. Using the open-source urbanMicroclimateFoam solver, a total of 54 high-resolution CFD simulations are performed for an identical urban layout representing a mid-scale residential lot. Two tree-planting configurations are tested: single and double row along the main street, each including 112 trees. Simulations are conducted under three reference wind speeds (0.5, 3, and 6 m s-1) and four wind directions (0 degrees, 22.5 degrees, 45 degrees, 90 degrees) for meteorological conditions representative of continental, tropical, and arid climates. Thermal comfort and heat exposure are evaluated with the Universal Thermal Climate Index (UTCI) and related indicators. Results show that the cooling behavior of trees is nonlinear and highly dependent on both background climate and boundary conditions. Average UTCI reductions reach up to -3 degrees C in the arid climate, -1.5 to -2 degrees C in the continental climate, and about -1 degrees C in the tropical climate. In terms of cumulative heatexposure reduction, the continental climate shows the strongest benefit, followed by the arid and tropical ones. The lowest wind speed consistently provides the largest potential for UTCI reduction, while wind direction has only a secondary influence on overall heat-stress reduction. Tree transpiration in the arid climate enhances cooling efficiency with limited penalties from humidity or wind reduction, whereas in the tropical case, increased humidity offsets the reduction in air-temperature. The study concludes with a comparison to realistic neighborhoods, confirming the context-specific nature of tree cooling under different background climates.