Spatial climate data sets of 1971-2000 mean monthly precipitation and minimum and maximum temperature were developed for the conterminous United States. These 30-arcsec (∼800-m) grids are the official spatial climate data sets of the U.S. Department of Agriculture. The PRISM (Parameter-elevation Relationships on Independent Slopes Model) interpolation method was used to develop data sets that reflected, as closely as possible, the current state of knowledge of spatial climate patterns in the United States. PRISM calculates a climate-elevation regression for each digital elevation model (DEM) grid cell, and stations entering the regression are assigned weights based primarily on the physiographic similarity of the station to the grid cell. Factors considered are location, elevation, coastal proximity, topographic facet orientation, vertical atmospheric layer, topographic position, and orographic effectiveness of the terrain. Surface stations used in the analysis numbered nearly 13 000 for precipitation and 10 000 for temperature. Station data were spatially quality controlled, and short-period-of-record averages adjusted to better reflect the 1971-2000 period.PRISM interpolation uncertainties were estimated with cross-validation (C-V) mean absolute error (MAE) and the 70% prediction interval of the climate-elevation regression function. The two measures were not well correlated at the point level, but were similar when averaged over large regions. The PRISM data set was compared with the WorldClim and Daymet spatial climate data sets. The comparison demonstrated that using a relatively dense station data set and the physiographically sensitive PRISM interpolation process resulted in substantially improved climate grids over those of WorldClim and Daymet. The improvement varied, however, depending on the complexity of the region. Mountainous and coastal areas of the western United States, characterized by sparse data coverage, large elevation gradients, rain shadows, inversions, cold air drainage, and coastal effects, showed the greatest improvement. The PRISM data set benefited from a peer review procedure that incorporated local knowledge and data into the development process.

Temperature and precipitation records from 1949 to 1998 were examined for 25 stations throughout the State of Alaska. Mean, maxima, and minima temperatures, diurnal temperature range, and total precipitation were analyzed for linear trends using least squares regressions. Annual and seasonal mean temperature increases were found throughout the entire state, and the majority were found to be statistically signi®cant at the 95% level or better. The highest increases were found in winter in the Interior region (2.2 C) for the 50 year period of record. Decreases in annual and seasonal mean diurnal temperature range were also found, of which only about half were statistically signi®cant. A statewide decrease in annual mean diurnal temperature range was found to be 0.3 C, with substantially higher decreases in the South/Southeastern region in winter. Increases were found in total precipitation for 3 of the 4 seasons throughout most of Alaska, while summer precipitation showed decreases at many stations. Few of the precipitation trends were found to be statistically signi®cant, due to high interannual variability. Barrow, our only station in the Arctic region, shows statistically signi®cant decreases in annual and winter total precipitation. These ®ndings are largely in agreement with existing literature, although they do contradict some of the precipitation trends predicted by the CO 2-doubling GCM's.

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Over the Arctic during the last few decades a decrease in annual precipitation and snow depths have been observed; this decrease is especially pronounced during the winter months. This decrease was not only found over northern Alaska but also over the high latitude Canadian stations and Russian drift stations. Further, satellite monitoring of North America snow cover has revealed a significant decreasing trend in mid‐spring cover since 1972. The temperature increased during the last few decades in the Arctic, hence the simplest explanation—normally increased temperature leads to high precipitation—is not valid. A causal explanation for these trends had been related to the shift of the Aleutian low and Arctic high. This study, with special emphasis on the surface observation data from Barrow and Barter Island, indicates: (i) not only the frequency, but the mean intensity of precipitation has decreased; (ii) the amount of total cloud cover, and in particular, low cloudiness, has decreased with time; (iii) sea‐level pressure did not show any significant trends. Variability in atmospheric pressure, however, decreased with time, suggesting that either the intensity and/or frequency of cyclones has decreased; (iv) a shift in seasonal resultant winds at Barrow has been observed. © 1998 Royal Meteorological Society

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