Skip directly to search Skip directly to A to Z list Skip directly to navigation Skip directly to page options Skip directly to site content

Heat Waves

Heat waves, or extreme heat events, are characterized by several days of temperatures greater than 90° F; warm, stagnant air masses; and consecutive nights with higher–than-usual minimum temperatures. As a result of climate change, heat waves are expected to increase in severity and frequency, particularly in the northern latitudes. The nature of the temperature shifts brought by climate change, which include both a shift in the average temperature overall as well as an increase in temperature variability, are depicted in the graphs below. The images show that there is likely to be much more hot weather with climate change, and more record hot weather, as well. The graphs are copied with permission from the Intergovernmental Panel on Climate Change.

Shifts in the distribution of cold and hot weather

Shifts in the distribution of cold and hot weather

Heat waves are already the most deadly weather-related exposure in the U.S., and account for more deaths annually than hurricanes, tornadoes, floods, and earthquakes combined. Children and older adults are at greater risk from heat. Other risk factors for heat-related death include living alone, lack of air-conditioning, and use of certain medications. Additionally, there is an element of adjustment to heat as body processes change to compensate for increased temperatures; this can be protective during heat waves.

Climate change will bring more heat waves to the U.S. Increases in the number of people living in cities, as well as population aging, will further increase heat-related health risks. Studies suggest that, if current emissions hold steady, excess heat-related deaths in the U.S. could climb from an average of about 700 each year currently, to between 3,000 and 5,000 per year by 2050.

Effective strategies have been identified for preventing heat-related illness and death among individuals and populations. Individually, gaining access to cooler temperatures, usually achieved through air conditioning, is key to preparing for heat waves. Checking elderly and homebound neighbors, relatives, and acquaintances during heat waves can be life-saving. Avoiding or rescheduling strenuous activities during heat waves, drinking lots of water, and dressing in light-colored, loose fitting clothing are all important behavior changes. On a community level, municipal heat response plans can be very effective, and vulnerability mapping, a process that highlights the locations of vulnerable populations, can target efforts even more effectively. Urban planning that increases green space, reduces heat build-up in buildings and other structures, and decreases the size and intensity of the urban heat island is also an effective strategy.

Although all heat-related deaths are preventable, climate change will bring increased heat waves and heat stress to the U.S., and is likely to cause increased illness and death, particularly in cities that are unprepared. CDC and its partners have resources to facilitate planning for heat waves, and stand ready to assist when needed.

Additional Information about Heat Waves

CDC Extreme Heat Website

Recognizing, Preventing and Treating Heat-Related Illness: An e-learning course. Available at:

Additional Readings about Climate Change and Heat Waves

Bernard S, McGeehin M. Municipal heat wave response plans. Amer J Publ Hlth. 94(9): 1520-1522, 2004.

Golden J, Hartz D, Brazel A, Luber G, and Phelan P. A biometeorology study of climate and heat-related morbidity in Phoenix from 2001 to 2006. Intl J of Biometeorology. In press.

McGeehin M, Mirabelli M. The potential impacts of climate variability and change on temperature-related morbidity and mortality in the United States. Envr Health Pers. 109: 191-198, 2001.

Semenza J, McCullough J, Flanders W, McGeehin M. Excess hospital admissions during the July 1995 heat wave in Chicago. Am J Prev Med. 16 (4): 269-277, 1999.

 Top of Page