Climate Fact: Heat Waves
In Brief: Nights are warmer than they were in the early 1970s, which exacerbates the urban heat island effect and heat related health problems.
Heat waves, which in the United States kill up to 1,000 people per year, are defined as prolonged periods of abnormally hot weather. They can occur at any time of the year. What “abnormally hot” weather is will vary from place to place and from season to season, making a standard range of dangerous temperatures difficult to determine. What’s more, people become acclimated to seasonal temperatures. This means that sudden onsets of summer weather early in the year are generally much more dangerous than equivalent temperatures later in the summer. Some basic human physiological limits, however, can provide set temperatures that are dangerous. Heat index temperatures of 130 degrees Fahrenheit or higher are considered extremely dangerous, with heat stroke or sunstroke likely. Temperatures between 105 and 129 degrees are in the danger zone, when sunstroke, muscle cramps, and/or heat exhaustion likely. Heatstroke is possible with prolonged exposure and/or physical activity. When the heat index is between 90 and 105 degrees, extreme caution with the heat is recommended.
Because they are built with heat trapping materials like concrete and asphalt, cities tend to be hotter than surrounding rural areas. This is particularly true at night, when cool temperatures are important for giving the body a break from the heat. Urbanization, in addition to a general warming trend, means that extreme heat events are becoming more common. In North America over the last 50 years, average nighttime low temperatures have risen faster than average daytime high temperatures. There has been a 50 percent increase in the number of unusually warm nights, and nights with temperatures that would have fallen into the top tenth percentile during the 1950s now fall into the top fifteenth percentile. Almost all of this increase has happened since 1975.
Seasons: Spring, Summer, Fall
Sources: Meisner, BN. “Heat Wave.” National Weather Service Southern Region Homepage. NOAA. 15 May 2000. Accessed Online 16 August 2010
