Geography Extreme Heat
Cathleen McAnneny
  • LAST MODIFIED: 22 September 2021
  • DOI: 10.1093/obo/9780199874002-0234


Extreme heat events (EHEs) are periods of high temperatures and humidity that are considered to be unusual for a specific geographic location. For example, in 1995 an extended heat wave in Chicago, Illinois, in the United States was blamed for the deaths of 550 citizens. Most of the dead were elderly, poor individuals who may not have realized that heat could kill, or who had no means of mitigating the rising temperatures in their homes or any way to escape to a cooler environment. In 2003, Europe was subjected to an EHE that is estimated to have resulted in the deaths of 70,000, with 15,000 of those deaths in Paris, France. “Extreme heat” is a relative term. Individuals adapt to their local climate, so it is difficult to use an absolute number to describe the conditions meteorologists consider a relative change from past conditions. The Centers for Disease Control and Prevention (CDC) defines extreme heat as “summertime temperatures that are substantially hotter and/or more humid than average for location at that time of year.” According to the Public Health Institute’s Center for Climate Change, the state of California defines extreme heat days as those days above the 98th percentile of maximum temperatures based on 1961–1990 data for a specific location. Crucial to understanding extreme heat events is the collection of data about temperature and humidity. The US Global Change Research Program provides heat wave data spanning 1961 to 2018. The site links to a variety of programs related to global climate modeling. The National Resources Defense Council is a nongovernmental organization that has excellent maps which show change over time in the frequency of extreme heat events that overlay the human impact of these events. The National Centers for Environmental Information provides graphic data of current weather conditions along with lists of significant climate anomalies. The site has links to weather records and tools. All of these sites rely on the National Oceanic and Atmospheric Administration for their data. There are equivalent agencies all over the world. The World Meteorological Organization, part of the United Nations, is also a valuable resource for data.

Measurement Technology, Statistics

Extreme heat events are defined as periods of unusually high temperatures and humidity. Perkins and Alexander 2013 uses historical data to predict intensity and duration of EHEs. This is measured using the daily maximum temperature and the daily apparent temperature, also known as the heat index. Some authors use the diurnal temperature differences as part a metric to define an extreme heat event. The duration of such events can be two to four consecutive days. Peterson, et al. 2013 details the importance of this pattern at a variety of scales. Data quality issues are discussed in Alexander, et al. 2006, which describes the creation of indices that are used to better understand warming trends. The threshold for declaring an EHE can be based on the daily absolute temperatures or on a relative heat based on a set standard for a location. How intense such an event may be is based on deviations above a certain percentile, such as above the 95th percentile, or above a certain absolute temperature threshold of 95 degrees Fahrenheit. More detail about the methods currently in use to model and predict heat events can be found in Vose 2005 and Caesar, et al. 2006. A multiregional approach is used by Dittus, et al. 2016 to assess the role of humans in areas affected by increased temperatures.

  • Alexander, L. V., X. Zhang, T. C. Peterson, et al. “Global Observed Changes in Daily Climate Extremes of Temperature and Precipitation.” Journal of Geophysical Research 111 (2006): D05109.

    DOI: 10.1029/2005JD006290

    Using available meteorological data, this paper addresses issues of data quality and reliability, and creates indices that can be used to measure global warming and precipitation trends

  • Caesar, John, Lisa Alexander, and Russell Vose. “Large-Scale Changes in Observed Daily Maximum and Minimum Temperatures: Creation and Analysis of a New Gridded Data Set.” Journal of Geophysical Research 111 (2006): D05101.

    DOI: 10.1029/2005JD006280

    Uses land-only station data to discuss the gridding of the data, evaluates the data set in terms of interpolation errors, and presents an assessment of observed maximum and minimum temperatures from 1946 to 2000.

  • Dittus, Andrea J., David J. Karoly, Sophie C. Lewis, Lisa V. Alexander, and Markus G. Donat. “A Multiregional Model Evaluation and Attribution Study of Historical Changes in the Area Affected by Temperature and Precipitation Extremes.” Journal of Climate 29.23 (December 2016): 8285–8299.

    DOI: 10.1175/JCLI-D-16-0164.1

    Data from eight coupled climate models are used to assess the roles of natural and anthropocentric forcings driving changes in areas affected by temperature and precipitation extremes across four continents and one hemispheric region

  • Fischer, E. M., and C. Schär. “Consistent Geographical Patterns of Changes in High-Impact European Heatwaves.” Nature Geoscience 3 (2010): 398–403.

    DOI: 10.1038/ngeo866

    Uses simulations to predict where the warmest weather will occur in Europe to analyze change and risk

  • Perkins, S. E., and L. V. Alexander. “On the Measurement of Heat Waves.” Journal of Climate 26 (2013): 4500–4517.

    DOI: 10.1175/JCLI-D-12-00383.1

    Data rom two climate periods along with data from other environmental indicators are used to better define extreme heat events.

  • Peterson, Thomas C., Richard R. Heim Jr., Robert Hirsch, et al. “Monitoring and Understanding Changes in Heat Waves, Cold Waves, Floods, and Droughts in the United States: State of Knowledge.” Bulletin of the American Meteorological Society 94 (2013): 821–834.

    DOI: 10.1175/BAMS-D-12-00066.1

    Examines on multiple scales the patterns of changing weather and the resulting impacts linking the reduction in cold waves and increasing warming events to an overall warming climate.

  • Vose, Russell S., David R. Easterling, and Byron Gleason. “Maximum and Minimum Temperature Trends for the Globe: An Update through 2004.” Geophysical Research Letters 32 (2005): L23822.

    DOI: 10.1029/2005GL024379

    This study combines data from twenty data sets to develop a global time series model to compare temperature trends from several time periods using increasing temperature minima rather than maxima to explain the increase in extreme heat events.

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