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82 assessment of climate change in the southwest united statesFigure 5.3 Occurrence ofcold waves and heat waves,1901–2010. Cold waves (top) aredefined as four-day periods that arecolder than the threshold of a onein-five-yearoccurrence; heat waves(bottom) are four-day periodswarmer than the same threshold.The thresholds are computedfor the entire 1901–2010 period.Source: NOAA National ClimaticData Center for the CooperativeObserver Network (http://www.ncdc.noaa.gov/land-based-stationdata/cooperative-observer-networkcoop).significant. This means that in recent years there were fewer events exceeding high precipitationthresholds for multi-day episodes than during earlier in the period of analysis.The Southwest is thus unlike some other areas of the United States, such as the GreatLakes and Ohio Valley regions, where significant upward trends in very heavy dailyprecipitation (during 1908–2000) have been noted (Groisman et al. 2004).5.5 Summertime Drought During 1901–2010To quantify variability in drought across the region, the Palmer Drought Severity Index(PDSI) was used, based on data from the NOAA National Climate Data Center (NCDC).The PDSI is a widely used indicator of dryness, representing the balance of water inputs