Policy Documents

Trends in Twentieth-Century Temperature Extremes across the United States

Arthur T. DeGaeton and Robert J. Allen –
June 4, 2001

ABSTRACT

A long-term, homogeneous set of daily maximum and minimum temperature data representing a subset of daily U.S. Historical Climatology Network stations is used to analyze trends in extreme temperature occurrence across the contiguous United States. Time series of various lengths are analyzed, with the longest spanning the period 1900–96. Trends in the annual occurrence of extreme maximum and minimum temperatures (e.g., values greater than the 90th, 95th, or 99th percentile) are strongly influenced by high exceedence counts during drought periods in the 1930s and 1950s. Peaks in exceedences during these years result in predominantly decreasing warm exceedence trends across the country during the 1930–96 period. This is uncharacteristic of recent years (1960–96) in which a large majority of stations show increases in warm extreme temperature exceedences. Significant increases in warm minimum temperature exceedences are found at nearly one-third of the stations during this period. Multiday warm temperature exceedence runs also show strong increases during this more recent period. The most rapid increases in high maximum and minimum temperature extremes occur at stations classified as urban, by satellite land use information. Trends in the annual occurrence of extremely cold maximum and minimum temperatures display an analogous decrease during the 1960–96 period. Here again, there is a distinct shift in the number of decreasing trends between the 1950–96 and 1960–96 periods. Based on starting decades prior to 1960, there is not a strong tendency for either increasing or decreasing trends. The period 1910–96 is an exception, with almost all stations exhibiting decreasing cold extreme occurrence trends. The extreme cold exceedence trends during the 1960–96 period are also influenced by urbanization, but to a lesser degree than the warm extremes.