On the definition of a heat wave

Heat waves are a major cause of weather-related deaths. With the current co ncern for global warming it is reasonable to suppose that they may increase in frequency, severity, duration, or areal extent in the future. However, in the absence of an adequate definition of a heat wave, it is impossible t o assess either changes in the past or possible consequences for the future . A set of definitions is proposed here, based on the criteria for heat str ess forecasts developed by the National Weather Service (NWS). Watches or w arnings are issued when thresholds of daytime high and nighttime low heat i ndex (H-i) values are exceeded for at least two consecutive days. The heat index is a combination of ambient temperature and humidity that approximate s the environmental aspect of the thermal regime of a human body, with the NWS thresholds representing a generalized estimate of the onset of physiolo gical stress. These thresholds cannot be applied directly nationwide. In ho t and humid regions, physical, social, and cultural adaptations will requir e that the thresholds be set higher to ensure that only those events percei ved as stressful are identified. In other, cooler, areas the NWS criteria m ay never be reached even though unusually hot events may be perceived as he at waves. Thus, it is likely that a similar number of perceived heat events will occur in all regions, with the thresholds varying regionally. Hourly H-i for 178 stations in the coterminous United States was analyzed for the 1951-90 period to determine appropriate threshold criteria. Use of the NWS criteria alone indicated that much of the nation had less than three heat w aves per decade, and this value was adopted as the baseline against which t o establish suitable thresholds. For all areas, a percentile threshold appr oach was tested. Using all available data, daytime high and nighttime low t hresholds were established separately for each specific percentile. Heat wa ves were treated as occurring when conditions exceeded both the daytime hig h and the nighttime low thresholds of the same percentile for two consecuti ve days. Several thresholds were tested. For much of the South, 1% threshol ds produced appropriate values. Consequently, a heat wave was defined as a period of at least 48 h during which neither the overnight low nor the dayt ime high H-i falls below the NWS heat stress thresholds (80 degrees and 105 degreesF, respectively), except at stations for which more than 1% of both the annual high and low H-i observations exceed these thresholds, in which case the 1% values are used as the heat wave thresholds. As an extension, "hot spells'' were similarly defined, but for events falling between the 1% values and NWS thresholds, with "warm spells'' occurring between the 2% an d 1% values. Again, stations for which the 1% or 2% H-i values exceed the N WS thresholds were given modified definitions. The preliminary investigatio n of the timing and location of heat waves resulting from these definitions indicated that they correctly identified major epidemiological events. A t entative climatic comparison also suggests that heat waves are becoming les s frequent in the southern and more frequent in the midwestern and eastern parts of the nation.