Temperature Cooling and Warming Rates in Three Different Built Environments within Nairobi City, Kenya

Urban canyon, urban park, and suburban surface air temperature data for hot-wet, hot-dry, cool-dry, and warm-wet periods in Nairobi city were analyzed to detect differences in the cooling and warming rates. Measurement of temperature for thirty continuous days was done at each of the three sites for each of the above periods. The cooling and warming rates were computed on an hourly basis beginning at 6.00 P.M., the approximate time of sunset. The results of the study showed that the largest cooling and warming rates were generally experienced during the hot-dry period while the lowest during the cool-dry period. Cooling and warming rates were also found to be the highest at the suburban site and the lowest at the urban canyon site. The differences in the conditions of the built environment at the three sites could explain the cause of the differential cooling and warming rates. The study recommends proper planning of the built environment to ameliorate the problem of excessive nocturnal heat loads within the built environment. 1. Introduction Urban geometry is one of the major factors leading to the modification of urban climate. Specifically, urban geometry that relates to the urban canopy layer (UCL) influences aspects such as increased substrate heat storage due to greater thermal admittance of surface materials and decreased latent heat fluxes arising from the replacement of soil and vegetated surfaces with impervious material [1–5]. It also leads to increase in solar radiation absorption due to lower albedo of urban materials and reduced wind speeds caused by the aerodynamically rougher urban fabric [6]. There is also the release of anthropogenic heat from domestic, commercial, industrial and transport energy sources and increased atmospheric radiation absorption from green house gases [4, 6, 7]. This paper examines the thermal behaviour of both the urban and suburban landscapes as a possible cause of the differences in the urban heat island within Nairobi city. The study proposes that the structure and composition of the urban canopy as well as the density of the built-up environment are significant factors in explaining the intensity of the modification of urban climate. 2. Study Area The study was carried out in the larger Nairobi area including the city centre, extending from longitude 1°15’S to 1°25’S and latitude 36°40’E and 37°05’E. The area extends from the Kikuyu highlands in the west to the Athi-Kapiti plains in east, covering approximately 690 square kilometers (Figure 1). Nairobi has a diversified physical environment, with altitude