Urban heat mitigation through misting, and its role in broader blue infrastructure portfolios

Abstract

Evaporative misters have long been used in urban spaces for heat mitigation, yet their thermal stress impacts and optimal operating conditions have not been fully explored. To fill this gap, we develop a misting model and embed it into an urban canopy model for the first time. Our tests confirm that misters can considerably reduce maximum urban canyon air temperature (up to 17.5 °C) and human skin temperature (up to 0.48 °C) in a hot and dry city (Phoenix, AZ). They continue to effectively reduce thermal stress, albeit with half of the cooling benefits, in a hot and humid city (Houston, TX). These thermal stress impacts are contingent upon wind speeds: the optimal wind speeds generally fall within an intermediate range—from light air (with low mist flow rates) to a moderate breeze (with higher mist flow rates). We then incorporate misting into a broader comparison of blue cooling strategies, including irrigation (on vegetation) and sprinkling (on pavements). With abundant water resources, sprinkling on asphalt and misting are the most effective cooling solutions, particularly suitable for middays and late afternoons, respectively. To balance cooling benefits with limited water resources, we propose a thermostatic control scheme that can save at least 10.5 m³/day of water compared to continuous misting for a 100-m stretch of street, equivalent to the water demand of about 20 Phoenix residents. Notably, misting and sprinkling generate rapid cooling in under 10 min with sufficient flow rates, demonstrating their potential as fast activation measures during extreme heat emergencies.