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A heat wave is a prolonged period of unusually warm and humid weather, lasting for a period of days to weeks; a minimum of three consecutive hot days is a common rule of thumb. Similarly, the temperature threshold that indicates a heat wave depends on what is normal for the region. In assessing the impact of a heat wave, the most useful measure is the apparent temperature, which is an index that combines air temperature and humidity to assess the total stress that individuals will experience.
Heat waves create uncomfortable conditions, and are often associated with increases in human mortality. The hot, humid conditions characteristic of heat waves raise the body temperature while simultaneously limiting evaporative cooling, producing discomfort and increased stress on the body. Heat wave deaths are higher among elderly populations, and are often attributed to cardiovascular and respiratory problems, exacerbated by the heat stress. Increases in death rates during heat waves are more common in northern cities, where air conditioning is less common and the population is not acclimated to high temperature and humidity. In southern cities, where high temperatures are typical during summer, unusually warm events tend to have less of an impact. In addition, there are socioeconomic patterns in heat-related deaths, as air conditioning and effective medical care are often less available to the poor. Overall, heat-related deaths tend to be more frequent than any other form of weather-related mortality, although the direct cause of death is often attributed to an underlying medical problem that increase a person’s vulnerability to heat stress.
In addition to mortality, heat waves result in vast increases in energy consumption. Failures of the energy infrastructure due to increased load can contribute to discomfort and mortality. Economic impacts can include a decrease in shopping and worker efficiency. Agriculture is also affected: livestock mortality rises during heat waves, and the production of milk and eggs is reduced.
Heat waves tend to be a larger problem in urban areas than rural areas. Due to the urban heat island effect, cities can be several degrees warmer than the surrounding countryside. More significantly, cities do not cool off as much during the night due to the slow release of heat stored in concrete and other surfaces, as well as waste heat produced by transportation and industry. Unusually warm nights can be particularly important during heat waves. Nighttime normally provides an opportunity for the body to relax and recover. When nights are unusually warm as well, this recovery is limited and mortality can be increased. During a heat wave, the majority of excessive deaths tend to occur in the early days of the event, as the more vulnerable members of the population succumb. As the heat wave continues, the death rate tends to drop, due to the gradual acclimatization of the populace.
Heat Waves in History
In recent years, several significant heat waves have produced excessive mortality in the United States and Europe. In July 1995, a period of high temperature and humidity was responsible for over 1,000 deaths in the U.S. Midwest, including over 500 in Chicago, which was ill-prepared for the severity of the event. Many of these deaths were attributable to extremely high nighttime temperatures, and disproportionately occurred among the elderly and poor residents in urban areas. Four years later, a similar heat wave affected nearly the same area with far fewer deaths. The reduced death toll in 1999 can be at least partly attributed to the improved responses of state and city governments, including better public notification, the opening of cooling centers for residents without air conditioning, and careful attention to electrical infrastructure.
In the summer of 2003, a record heat wave struck much of Europe and resulted in over 35,000 deaths, many of them elderly. This effect was exaggerated due to the demographic structure of many European countries, which have relatively high proportions of elderly people. As in Chicago, national and regional governments were not prepared for such extreme heat in a region that does not typically experience hot summers. Because heat waves are unusual, air conditioning is uncommon and people are not wellacclimated to high temperatures. In addition, the heat wave struck in August, a month when many people, including physicians and other health workers, traditionally vacation.
Of significant concern is the question of whether global warming will result in increased frequencies of heat waves and the associated mortality. Over the past century, the global average temperature has risen by approximately 0.6 degrees C (1.1 degrees F), and continued warming is anticipated. As much of this warming is expected to occur at higher latitudes, it is reasonable to expect that heat waves should become more common in northern cities. However, heat-related mortality has in fact decreased in the United States since the 1960s, despite the fact that stressful weather conditions have become more common. There are many possible explanations for this trend, including improvements in health care, the wider availability of air conditioning, better governmental responses to extreme weather, and a general acclimatization to heat in the population. Still, heat remains the largest weather-related killer and will continue to be a serious concern.
- Frank C. Curriero, Karlyn S. Heiner, Jonathan M. Samet, Scott L. Zeger, Lisa Strug, and Jonathan A. Patz, “Temperature and Mortality in 11 Cities of the Eastern United States” (American Journal of Epidemiology, 155(1):80-87, 2002);
- Robert E. Davis, Paul C. Knappenberger, Patrick J. Michaels, and Wendy M. Novicoff, “Changing Heat-Related Mortality in the United States” (Environmental Health Perspectives, 111(14):1712-1718, 2003);
- Thomas R. Karl and Richard W. Knight, “The 1995 Chicago Heat Wave: How Likely is a Recurrence?” (Bulletin of the American Meteorological Society, 78(6):1107-1119, 1997);
- Frederick K. Lutgens and Edward J. Tarbuck, The Atmosphere, 8th ed. (Prentice-Hall, 2001);
- Michael A. Palecki, Stanley A. Changnon, and Kenneth E. Kunkel, “The Nature and Impacts of the July 1999 Heat Wave in the Midwestern United States: Learning from the Lessons of 1995” (Bulletin of the American Meteorological Society, 82(7):1353-1367, 2001).