Summer and winter patterns of air temperature and humidity under calm conditions in relation to urban land use

Urban areas are composed of a heterogeneous patchwork of discrete land use types. In desert cities, the relative proportions of landscape plant density to anthropogenic surfaces and associated differences in hydrology and water inputs via irrigation might be expected to influence microclimate under the urban canopy layer especially under calm conditions. During June and December 1999, air temperature (T_a) and relative humidity (RH) were measured along two transects in Phoenix, AZ, USA metropolitan area around 0500 and 1500 HR on days of clear, calm, anticyclonic conditions. Measurements were made on surface streets over asphalt from a vehicle traveling at an average speed of 14.7 m/s. T_a was measured with shielded copper constantan thermocouple wire at 0.5, 2.0, and 5.0 m above ground. RH was measured with a shielded Vaisala RH probe at 2.0 m above ground. From these data, dew point temperature (T_d) was calculated. Transects were chosen to include combinations of the following land use types in order of increasing relative plant cover; industrial, commercial, residential, greenbelt, and agricultural. Generally, T_a was highest in industrial and commercial zones. Furthermore, RH and T_d in residential and greenbelt zones were higher than in industrial and commercial zones. In winter, RH and T_d was lowest in agricultural zones reflecting fallow conditions. Effects of land use on T_a were most evident during June early mornings, least evident during December afternoons, and were the same regardless of the height of measurement. Effects of land use on RH and T_d were least evident during June afternoons. These data underscore the importance of understanding local variations in microclimate due to heterogeneous land use types when modeling larger scale ecosystem processes in and around an urban environment.