Similarly, but at the opposite extreme, are regions which have conditions at or near the NWS excessive heat criteria for much of the summer, and where lifestyles may already be adapted to heat stress. A period of at least 48 hours during which both the overnight low and daytime high apparent temperature have values exceeding those observed 1 percent of the time at the station, but where conditions fail to meet the criteria for a heat wave. Three-hourly data, based on the standard synoptic observing hours, were used. Stations with virtually complete records for 1951-1990 were selected. Observations suggest in the former area rather few heat waves per decade in comparison to adjacent areas, but the results must be treated with caution.
Heat waves are, largely by definition, unusual events.
Consequently changes in the decadal totals at individual stations were used to characterize temporal trends. The southern Great Plains showed a decrease, while no clear pattern emerged from the southeast . It has been proposed that it is possible to quantitatively define heat waves on a national basis. The threshold criteria are either those already established by the National Weather Service for the promulgation of excess heat advisories and warnings, or the values which exceed a fixed percentage of a stations climatological record of apparent temperature values. The appropriate selection is that method which gives the minimum number of events per decade.