Time Evolution of the Surface Urban Heat Island

Abstract

A proper description of the climate of a place includes the expected sequence of air temperatures in a daily cycle. In cities, the observed sequence helps researchers to explain climatic phenomena such as the urban heat island (UHI). The UHI is a local-scale atmospheric-warming effect associated with urban areas, whose near-surface air temperatures are higher, on average, than those of the surrounding countryside. The diurnal cycles of heat islands were first investigated in major European cities by Renou (1868) and Hann (1885), who observed 3-hourly and daily maximum/minimum temperatures. Their work evolved with the first hourly observations made in urban and rural areas by Alfred Angot (1896). He reported the magnitude of the heat island in Paris (France) to vary greatly in the course of a day, reaching maximum values in the latter half of the night, and minimum values (sometimes negative-a "cool island") at noon or several hours before noon, depending on the season (Figures 1a and 1c). These early insights into the fine-scale sequencing of urban-rural air temperature differences commenced a generation of work on the time evolution of atmospheric heat islands in cities worldwide.A century later, Oke (1982) conceptualized the diurnal cycle of an atmospheric heat island based on decades of work preceding his own. His classic portrayal describes a 24-h screen-level (2 m) heat island for a large, temperate-climate city during calm and clear summer weather (Figure 1f). It exhibits the time-dependent features of the heat island that are observed-to some extent-in all cities (Figures 1d and 1e). Oke defined the heat island much like his predecessors had done: as a nocturnal phenomenon whose existence is mainly

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Integrated Assessment of Urban Overheating Impacts on Human Life

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