In order to further evaluate the factors that can influence thermal environmental stresses, a characterization of how urban areas affect local microclimates, air temperature in particular, is necessary. To assist the CalEPA in evaluating the thermal impacts of urban areas in California, this study characterizes the urban heat island (UHI) and quantifies an urban heat island index (UHII) for each census tract in a number of urban areas. For this purpose, detailed multi-scale meteorological modeling was carried out and a UHII definition was developed.