Urban heat island (UHI) countermeasures are of growing interest for cities. Field studies of their micro-climatic effects are scarce, yet are essential to properly evaluate their effectiveness and that of anti-UHI policies. The standard approach to determining their micro-climatic effects is to study the difference in measurements made at case and control stations. However, measurements conducted during a pavement-watering experiment in Paris, France reveal that this assumption is unfounded in the general case, making the approach ill-suited to its task. An alternative approach based on a two-sample t-test was developed. The Parisian pavement-watering field trial was used as a case study to test it and determine its feasibility. In the case of full-street-width watering, statistically significant daily effects were found. Watering was found to reduce maximum daily heat stress, while having smaller statistically significant UH-reducing effects. The greatest effects were reached during the day for all parameters with reductions of 0.79°C, 1.76°C and 1.03°C for air, mean radiant and UTCI-equivalent temperatures and a 4.1% increase in relative humidity. UHI-mitigation reached up to -0.22°C. The methodology developed is not specific to pavement-watering and recommendations for its improvement and its application to the field-evaluation of other UHI countermeasures are made.