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Accurately capturing the impact of urban trees on temperature can help optimize urban heat mitigation strategies. Recently, there has been widespread use of remotely sensed land surface temperature ( T _s ) to quantify the cooling efficiency (CE) of urban trees. However, remotely sensed T _s reflects emitted radiation from the surface of an object seen from the point of view of the thermal sensor, which is not a good proxy for the air temperature ( T _a ) perceived by humans. The extent to which the CEs derived from T _s reflect the true experiences of urban residents is debatable. Therefore, this study systematically compared the T _s -based CE (CE _T _s ) with the T _a -based CE (CE _T _a ) in 392 European urban clusters. CE _T _s and CE _T _a were defined as the reductions in T _s and T _a , respectively, for every 1% increase in fractional tree cover (FTC). The results show that the increase in FTC has a substantial impact on reducing T _s and T _a in most cities during daytime. However, at night, the response of T _s and T _a to increased FTC appears to be much weaker and ambiguous. On average, for European cities, daytime CE _T _s reaches 0.075 °C % ^−1 , which is significantly higher (by an order of magnitude) than the corresponding CE _T _a of 0.006 °C % ^−1 . In contrast, the average nighttime CE _T _s and CE _T _a for European cities are similar, both approximating zero. Overall, urban trees can lower daytime temperatures, but the magnitude of their cooling effect is notably amplified when using remotely sensed T _s estimates compared to in situ T _a measurements, which is important to consider for accurately constraining public health benefits. Our findings provide critical insights into the realistic efficiencies of alleviating urban heat through tree planting.