Heat Wave

Aheat 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 [Page 859]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.