The Urban Heat Island's Effect on the diurnal temperature range

Abstract

(cont.) representing Phoenix and its rural equivalent is presented through a comparison with estimated energy fluxes for Houston, TX.

This study has found significant evidence of the Urban Heat Island Effect (UHIE) in Phoenix, AZ using historic and recent (up to 2002) temperature records. First, an explicit indicator of UHIE was identified by the -0.148⁰F per year decrease of the diurnal temperature range (DTR). This was explained by an increasing trend of 0.190⁰F per year in mean minimum temperature while the mean maximum increased at a much slower pace of 0.042⁰F per year. This has manifested itself in a significant nighttime heat island. Second, the UHIE for Phoenix was estimated to contribute about 15 [plus-minus] 5% of total climate variability for this city. That is, only about, 85% of the DTR for Phoenix can be accounted for by natural variability. This was established by comparing the average historical climate record for Phoenix Sky Harbor International Airport and several nearby rural sites. In this analysis, Casa Grande National Monument, a national park 40 miles southeast of Phoenix, was chosen as the most appropriate rural site for application of a method to assess the Phoenix UHIE by trading space-for-time. Finally, the basic concepts for the development of an urban heat island model were outlined. Land use changes and the materials used to build infrastructure were determined to be the most significant factors in determining the magnitude of the Phoenix (and similar cities) UHIE. These materials effectively reduce surface albedo and emissivity, decrease latent heat release by lowering the water permeability of surfaces, and increase the storage heat flux due to greater surface area density and much larger thermal mass offsetting the lower specific heat capacity. Anthropogenic heat release is also a factor, but mainly in dense urban cities. The energy balance