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Hendry F. Diaz
Robert G. Quayle


Time series of temperature and precipitation weighted by area and grouped by season for each of the 48 contiguous United States were analyzed. Within an 83-year period of record (1895–1977) three subperiods or climatic regimes are identified and the differences in their means and standard deviations plotted and analyzed. The statistical significance of the changes in the mean was calculated by using a two-tailed t test; for changes in the standard deviation, the F-ratio test was used.

The variation patterns suggest that an east-west mode for changes in both temperature and precipitation is dominant over the continental United States. Over the past 25 years the average temperature of the United States has decreased∼1°F (0.6°C) from the relatively warm interval of the 1920's to the middle 1950's. However, most of this cooling has occurred in the eastern United States. In winter, for example, the southeastern United States cooled ∼3°F (1.7°C), whereas the Far West actually recorded warmer mean temperatures amounting to ∼0.5°F (0.3°C). Increases in precipitation in the past 25 years have favored the eastern United States, as many areas of the western United States experienced diminished precipitation. No systematic relationship could be found between changes in mean temperature and precipitation and corresponding changes in their variance.

Among the potential effects of similar climatic fluctuations in the future are increased energy costs for heating and possible water shortages in the western states.

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