Between the dates of 28 June and 31 July 2003, the Joint Urban 2003 (JU2003) field project was conducted in Oklahoma City and was the largest urban dispersion experiment ever in North America. Because the focus of JU2003 was on atmospheric processes within the urban environment, an extremely dense network of instrumentation was deployed in and around the central business district (CBD) both prior to and during the field experiment. Among the variables collected were high-resolution observations of air temperature from various instrument sources. Additional observations of air temperature were also collected at Oklahoma Mesonet stations in the rural areas surrounding Oklahoma City. Using an index value, the diurnal cycle of the urban heat island (UHI) for Oklahoma City, with respect to the surrounding rural terrain, was quantified. The results revealed a consistent mean nocturnal UHI greater than 1.5°C at both 2 and 9 m. However, observations at 2 m during JU2003 revealed a significant urban “cool” island during the convective portion of the day. The mean variability of temperature within the urban core of Oklahoma City increased significantly after sunrise, increased to a maximum near solar noon, and decreased following sunset. These results were inconsistent with the rural observations wherein the variability among sites was maximized during the nocturnal period. Finally, the vertical temperature gradient between 2 and 9 m demonstrated a clear and strong diurnal trend at the rural locations, whereas observations from the urban environment were nearly isothermal and consistent with near-neutral conditions throughout JU2003.