Comparison of transects and fixed stations, Phoenix, Arizona

Various observations of an urban heat island (UHI) have been conducted by urban climate researchers and ecologists using mobile surveys (airplanes, autos, bikes, and walks), fixed weather stations, and remote sensing platforms. Each method has advantages, but each has its distinct disadvantages which may account for differing estimates of heat island magnitudes. We use fixed stations versus automobile transects to investigate the degree to which these two methods produce different estimates of the UHI at night for a region stretching from urban/xeric/mesic residential to agricultural/desert in the Phoenix, Arizona, USA region. The advantage of the transect method is that a researcher is able to more fully characterize climate conditions along the “landscape spatial gradient”, whereas fixed sites are typically in a station network that is suspect as to accurately representing land cover spatially, especially if the stations were not put in as part of an urban climate study. The disadvantages lay in the fact that a mobile transect is very difficult to do simultaneously (time sampling problem over large distances) and with great repetition over time (hours and days on end). Fixed sites record continuously. We use an urban, residential, and rural weather site for fixed stations, and over a 27 day period (spanning July through November, 2001) ran post-sundown mobile transects for comparitive purposes to determine the dimension of the UHI observed along the entire transect. An urban (U) and residential (RE) comparison to rural (RU) values was determined from the fixed sites by calculating DT(U-RU) and DT(RE-RU) for the period ca. 9-10 pm at night – the same time as each transect was completed. Similarly, we calculated the urban to rural and residential to rural DT as determined from more complete information along the transect route. On average, transects revealed DT(U-RU) of 7.3°C and DT(RE-RU) of 3.0°C, whereas the fixed site result was DT(U-RU) of 4.8°C and DT(RE-RU) of 2.3°C.