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Abstract
Scenarios of future climates must be made so that the potential impacts of climatic change can be assessed. A simple method is presented for developing regional scenarios of the number of days per month above a selected temperature. The method uses the observational record of daily maximum temperatures to develop linear regressions between monthly average temperature and the number of days above the threshold. The relationships are then extrapolated to the average temperature conditions estimated by General Circulation Models (GCMs) and the new number of days determined. The method is demonstrated by developing a set of scenarios for the United States for a 90° F threshold. The validity of the approach, including consideration of the linearity of the relationships and the amount of extrapolation needed, is discussed. The regional variability of the results can be explained in climatological terms while local variations are associated with topographic and hydrologic effects. The results are in general agreement with estimates obtained directly from GCM results.