end of the time series is significantly affected by the drought. Furthermore, variance of the time series is significantly affected when such large regions drop out of the analysis, which is why the series ends prematurely. Nonetheless, Figure 3 reflects a gradual decrease of the DTR through much of the past several decades.
Unfortunately, the availability of maximum and minimum temperature data covering the globe is currently limited prior to 1951. In the United States a network of approximately 500 high-quality stations has remained intact since the turn of the century, and in the former Soviet Union a fixed network of 224 stations (165 if only rural stations are used) is available back to the 1930s. The time series from these two countries shown in Figure 4 reflect significant decadal variations in the DTR, which are particularly evident during the dry 1930s and the early 1950s in the United States. The general decrease of the DTR did not begin in the United States until the late 1950s, and the DTR decreased rather dramatically in the mid-to-late 1970s over the former Soviet Union as part of substantial increases in the minimum temperature. The decrease of the DTR in these two countries is a phenomenon of recent decades. Data are also available further back in time for smaller areas and countries, notably Japan, eastern Australia, and South Africa. Figure 5 indicates that the decrease in the DTR in eastern Australia has occurred rather gradually since the decline in the late 1940s. In South Africa the decrease is predominantly due to the sharp decline in the early 1950s.
A very long record of maximum and minimum temperatures was available from the Klementinum Observatory in Prague, Czech Republic, and another from a benchmark station from northern Finland. Figure 6 portrays a remarkable increase of the DTR at the Klementinum Observatory from the early twentieth to the mid-twentieth century, with a substantial decrease since about 1950. The increase coincides with the increase of global mean temperature since the turn of the century, and the decrease occurs when the