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Human activities and urban infrastructure elevate city temperatures significantly above those of surrounding rural areas. This process, known as the urban heat island effect, exposes city residents to dangerously high temperatures in summer. We take a dual approach to treating urban heat, partnering with the Philadelphia Office of Sustainability (OOS) to pair fundamental materials research with an applied pilot project. Dark-colored surface coatings that absorb a high fraction of solar radiation are a major cause of extreme heat, and materials that prevent solar heat from entering the environment can passively cool buildings and cities. However, existing cool surface coatings are limited by the atmospheric IR transparency window, which can be blocked on humid or cloudy days. A promising alternative is evaporative cooling, which circumvents atmospheric transparency issues by absorbing moisture overnight and releasing it when heated by sunlight. For this study, we formulated a biocompatible hydrogel consisting of sodium carboxymethylcellulose (NaCMC), polyethylene glycol (PEG), and citric acid (CA). The cooling power of the coating was quantified in laboratory and outdoor environments, showing surface temperature reductions exceeding 10 ºC. Beyond the laboratory, a partnership with the Philadelphia OOS to evaluate a new cool pavement coating is described. This work uses materials science to guide public policy, bridging the gap between academic research and climate solutions.