A heat wave is a prolonged period of excessive heat, often combined with excessive humidity.
Excessive heat is defined as temperatures that hover 10 degrees or more above the average high temperature for the region during summer months, last for an extended period, and often are accompanied by high humidity.
North American summers are hot; most summers see heat waves in one section or another of the United States. East of the Rockies, they tend to combine both high temperature and high humidity although some of the worst have been catastrophically dry.
Despite the common perception that hurricanes and tornadoes are the most dangerous weather event, heat waves kill more Americans than any other type of natural disaster.
The American Meteorological Society reports that on average heat kills more than 1,000 people each year.
In the 40-year period from 1936 through 1975, nearly 20,000 people were killed in the United States by the effects of heat and solar radiation.
Heat kills by pushing the human body beyond its limits. Under normal conditions, the body's internal thermostat produces perspiration that evaporates and cools the body. However, in extreme heat and high humidity, evaporation is slowed and the body must work extra hard to maintain a normal temperature.
Most heat disorders occur because the victim has been overexposed to heat or has over-exercised for his or her age and physical condition. Other conditions that can induce heat-related illnesses include stagnant atmospheric conditions and poor air quality.
Because men sweat more than women, men are more susceptible to heat illness because they become more quickly dehydrated.
People living in urban areas are at greater risk from the effects of a prolonged heat wave than people living in rural regions. Stagnant atmospheric conditions – including heat and moisture - trap pollutants, adding unhealthy air to excessively hot temperatures, which may then in aggravate health problems like asthma, especially for those with respiratory difficulties like asthma.
Asphalt and concrete store heat longer and gradually releases heat at night, which produces significantly higher nighttime temperatures in urban areas known as the “urban heat island effect.”