Investigating the Relationship Between Urban Morphology and the Formation of Urban Heat Islands Through the Influence of Wind Flow: A Case Study of Sari, Iran

Abstract

Urban growth and the rapid migration of rural populations to cities have directly contributed to climatic changes in urban areas and have raised concerns regarding global warming. Accordingly, studies of urban climate can contribute to improving outdoor thermal comfort. The urban heat island (UHI) is a phenomenon in which the air temperature within a city becomes higher than that of its surrounding areas, primarily because of human activities. Urban heat island intensity (UHII) is used to quantify the magnitude of the UHI effect and the temperature difference between urban and rural areas. Over the past four decades, Sari, the largest city in Mazandaran Province, has experienced substantial changes in its urban structure. These transformations have affected both the older parts of the city and newly developed neighborhoods, many of which initially emerged as informal peripheral settlements. Changes in building regulations during the implementation phase, along with the construction of buildings without obtaining the required permits in many peripheral neighborhoods, have transformed the urban morphology of this historic Iranian city. The morphological characteristics of these areas include low-rise buildings with high building density, narrow streets and alleys, limited vegetation, a lack of urban open spaces, and high building coverage ratios. Extensive calculations conducted using Landsat 8 satellite imagery acquired on August 8, 2021, the hottest day of that year in Sari, were used to investigate the formation of urban heat islands. The findings showed that the Normalized Difference Vegetation Index (NDVI) was distributed relatively uniformly across Sari, and its effect on reducing ambient temperature was therefore approximately similar throughout the city. Regarding the moisture index, neighborhoods with higher moisture levels also exhibited higher temperatures, while wind strength was unable to produce a meaningful reduction in moisture. However, in locations with similar wind conditions, urban morphology was the determining factor influencing ambient temperature. Unless urban morphology facilitates airflow, wind strength alone cannot reduce ambient temperatures in areas affected by urban heat islands. Wind strength by itself cannot prevent the formation of urban heat islands and functions only as a contributing factor in reducing ambient temperature under favorable urban morphological conditions. Consequently, modifying urban morphology, as the most fundamental determinant of urban heat island formation, can help mitigate and control the urban heat island effect.