Urban parks as effective airborne cooling sources: planning suggestions based on an empirical study in Xi'an, China

Urban parks are attempted as “cooling sources” to deliver cool air for mitigating the urban heat island phenomenon. However, the planning philosophy of urban parks that could guarantee the air-cooling benefits remains vague. The present study addressed this issue based on a high-resolution meteorological observation in an urban park of Xi'an—a temperate-climate city in China. Sensitivity and regression analyses were conducted to quantify the relationships between microclimate parameters and Normalized Difference Vegetation Index (NDVI) data at different scales. The results show that the air temperature between 15:00 and 17:00 (18:00 and 6:00) was most significantly correlated to the average NDVI value within the sampling area of 330–390 m (30 m) in diameter. While when the average NDVI value within the 390 m diametric sampling area was in the range of 0.29–0.33, the air enthalpy increased within 24 h. It meant the air-cooling benefit of the green lands could be offset by the humidification effect. To ensure the parks as “cooling sources” of urban air, it is recommended that the land-cover area of urban parks should exceed 0.12 km², and the average NDVI within the area should be maintained above 0.33.