The Heat Island of the Urban Boundary Layer: Characteristics, Causes and Effects

Urban heat islands (UHI) are defined. The importance of distinguishing between different types is stressed and a simple classification scheme is forwarded. Emphasis in this paper is upon the heat island in the urban boundary layer (UBL) above roof-level (the UHI_UBL).The observed characteristics of the daytime and nocturnal UHI_UBL are illustrated including the evolution of its thermal structure and the transition between the day and night régimes. Effects of weather controls on the UBL are mentioned.The essential physics underlying the genesis of the daytime and nocturnal UHI_UBL is outlined including radiative flux divergence, heating from below due to the altered surface energy balance, and heating from above due to entrainment.UHI_UBL effects on urban airflow and air pollution dispersion are numerous. Examples given include effects on thermal turbulence, atmospheric stability, convective structures, nocturnal inversions, mixed layer depth, local circulation systems, plume trajectories, intra- and inter-urban fumigation, humidity and rates of chemical reactions and biogenic emission.