“Urban heat island,” a term first coined in the 1940s, refers to the elevated air temperature in a city compared to its rural surroundings. Heat islands occur in almost all urban areas, large or small, in warm climates or cold, due to the change in land cover and materials when natural landscapes are urbanized.
To fully understand the impact of cities on the thermal environment, researchers worldwide have measured the magnitude of the urban heat island effect for many decades through simple comparisons of “urban” and “rural” air temperatures. The problem with this approach is that the term “urban” does not have a single, objective meaning. All cities are different when it comes to the physical structure and surface properties. What is described as urban in one city or region differs from that of another city. For example, Phoenix Arizona has mostly 1-2 story single family homes with backyards, while Manhattan, New York has high-rise buildings. How can we compare the urban heat island in Phoenix to the heat island in Manhattan?
To provide a research framework for urban heat island studies and standardizes the worldwide exchange of urban temperature observations, a team of urban climatologists developed the local climate zone classification system. They defined 17 local climate zones, or LCZs, that are local in scale—meaning the size of a neighborhood that spans hundreds of meters to several kilometers—and unique in their combination of surface structure, cover, materials, and human activity. The 17 zones are divided into 10 “built types” and seven “natural types”. Built types are a combination of lowrise to highrise buildings that are arranged in an open or compact manner and include lightweight structures, such as mobile home parks, and large lowrise structures, such as shopping malls. The natural types include land cover such as rock, soil, scattered trees, and forests.
Each local climate zone has a characteristic air temperature regime that persists year-round. For example, the forest is cooler on average than a compact highrise downtown area, such as Manhattan.
Those differences in local climate are important when it comes to mitigating heat in urban areas. To determine hotspots in the city, it is less important how hot the urban area is compared to its rural surroundings. We can’t eliminate the city because we need the infrastructure for our daily lives. What matters more is the “intra-urban heat variability” and how it affects people in the most heat-stressed neighborhoods.
Local climate zones are a first step to defining this variability within cities based on physical and climatological characteristics.
