A homogeneous, high-density, daily maximum and minimum air temperature dataset was assembled for the Twin Cities Metropolitan Area (TCMA), Minnesota, USA, to conduct basic urban climatological investigations on the spatial structure and temporal-scale dependence of the urban heat island, and to quantify the urban heat island effect upon several derived environmental indices. By combining data from National Weather Service cooperative stations, the University of Minnesota-St. Paul field station, and the previously unused KSTP-TV cooperative weather station network, a merged dataset of 26 stations was assembled for the TCMA for the year l989. Extensive quality control was conducted to identify suspect data values, estimate missing data, and adjust for time-of-observation bias. Eight environmental indices were examined to overview the impact of the TCMA urban heat island upon a range of physical and biological activities. These included 2 growing degree-day indices, melting degree-days, cooling and heating degree-days, freezing degree-days, number of frost change-days, and the freeze-free season length. Results illustrate the magnitude and spatial pattern of the urban heat island and derived thermal indices which might be typical of a large midlatitude midcontinental metropolitan area. The mean annual air temperature urban heat island is approximately 2.1*C, and resembles the classic spatial pattern consisting of a peripheral zone of rapid temperature increase, a large plateau of elevated air temperatures, and a small central core of peak temperatures. The impact of the urban heat island upon the magnitude and spatial pattern of the 8 environmental indices was highly index-specific. In many cases, the urban heat island effects are so profound that the response and adaptation of selected environmental systems to the historical urban warming should be evident. Urban environments would appear to offer a suitable laboratory for selected climate change impact studies
