A cooling system spraying micro water droplets could prove useful in mitigating temperature increases in urban areas by using heat from water evaporation, a process that consumes only small amounts of water and energy. If water mist is sprayed in a semi-outdoor area, for example under a canopy, it could potentially improve conditions on hot days. However, there is little reference data concerning the design or control of such systems. In order to propose a method for designing and predicting the performance of a water mist system, we discuss differences in cooling effects in the context of particle size distribution of water mist. From the results of numerical fluid analysis, it was observed there is no significant difference in temperature reduction for distribution of different particle sizes. However, the water particles remained in a lower position when the particle size distribution increased.