Quantifying the effects of settlement size on urban heat islands in fairly uniform geographic areas

City size is closely related to urban heat island intensity (UHII). To examine the relationship more accurately, it is necessary to eliminate the effects of landforms and climatic differences on urban heat islands (UHIs), through selecting settlement clusters in a large plain within a similar biome as the study area. This study selected 1124 land use clusters (cities, towns, and big rural settlements) and demarcated surrounding buffer areas; each buffer width equaled the radius of the clusters. The results showed that UHI increased with growth in cluster size, and the relationship could be described using a logarithmic function. For clusters with an area >2 km2, the city size accounted for about 60% of the variance in UHII during the night and only about 30% during the day. For clusters with areas of <2 km2, the uncertainty in the relationship increased significantly and the relationship became very weak. In addition, our study showed that daytime UHII was higher than nighttime UHII, particularly for large cities with a size >10 km2.