Severe and extreme heatwaves have claimed more lives than any other natural hazard in Australia1. Heatwaves can be dangerous because they pose health risks to the most vulnerable, such as elderly people and very young children. Heatwaves can also affect the transport, agriculture and energy sectors and associated infrastructure. So, when is it a heatwave, rather than just hot, and how will you know if one is on the way? Show A heatwave occurs when the maximum and the minimum temperatures are unusually hot over a three-day period at a location. This is considered in relation to the local climate and past weather at the location. Your
browser does not support the video tag. It takes more than just a high daily maximum temperature to define a heatwave. It's also about how much it cools down overnight. Hot days without hot nights allow some recovery from each day's heat, but if the temperature stays high overnight, the maximum will be reached earlier the following day and will last longer. When unusually high night and daytime temperatures persist, heat stress becomes a critical factor in human health and whether
infrastructure functions properly. In heatwaves, hot nights make it harder to recover from the heat of the day and this puts more stress on the body. For each part of the country, we compare the forecast maximum and minimum temperatures for each three-day period in the coming week (e.g. Monday-Wednesday, Tuesday-Thursday) to what would be considered hot for that location, and also to observed temperatures over the last 30 days. Heatwaves
are classified into three types, based on intensity. This indicator describes trends in multi-day extreme heat events across the United States.
Key Points
BackgroundA persistent period of unusually hot days is referred to as an extreme heat event or a heat wave. Heat waves are more than just uncomfortable: they can lead to illness and death, particularly among older adults, the very young, and other vulnerable populations (see the Heat-Related Deaths and Heat-Related Illnesses indicators).1 Prolonged exposure to excessive heat can lead to other impacts as well—for example, damaging crops, injuring or killing livestock, and increasing the risk of wildfires. Prolonged periods of extreme heat can lead to power outages as heavy demands for air conditioning strain the power grid. Unusually hot days and heat wave events are a natural part of day-to-day variation in weather. As the Earth’s climate warms, however, hotter-than-usual days and nights are becoming more common (see the High and Low Temperatures indicator) and heat waves are expected to become more frequent and intense.2 Increases in these extreme heat events can lead to more heat-related illnesses and deaths, especially if people and communities do not take steps to adapt.3 Even small increases in extreme heat can result in increased deaths and illnesses.4 About the IndicatorThis indicator examines trends over time in four key characteristics of heat waves in the United States:
Heat waves can be defined in many different ways. For consistency across the country, Figures 1 and 2 define a heat wave as a period of two or more consecutive days when the daily minimum apparent temperature (the actual temperature adjusted for humidity) in a particular city exceeds the 85th percentile of historical July and August temperatures (1981–2010) for that city. EPA chose this definition for several reasons:
The National Oceanic and Atmospheric Administration (NOAA) calculated apparent temperature for this indicator based on temperature and humidity measurements from long-term weather stations, which are generally located at airports. Figures 1 and 2 focus on the 50 most populous U.S. metropolitan areas that have recorded weather data from a consistent location without many missing days over the time period examined. The year 1961 was chosen as the starting point because most major cities have collected consistent data since at least that time. Figure 3 provides another perspective to gauge the size and frequency of prolonged heat wave events. It shows the U.S. Annual Heat Wave Index, which tracks the occurrence of heat wave conditions across the contiguous 48 states from 1895 to 2021. This index defines a heat wave as a period lasting at least four days with an average temperature that would only be expected to persist over four days once every 10 years, based on the historical record. The index value for a given year depends on how often such severe heat waves occur and how widespread they are. About the DataIndicator NotesAs cities develop, vegetation is often lost, and more surfaces are paved or covered with buildings. This type of development can lead to higher temperatures—part of an effect called an “urban heat island.” Compared with surrounding rural areas, built-up areas have higher temperatures, especially at night.10 Urban growth since 1961 may have contributed to part of the increase in heat waves that Figures 1 and 2 show for certain cities. This indicator does not attempt to adjust for the effects of development in metropolitan areas, because it focuses on the temperatures to which people are actually exposed, regardless of whether the trends reflect a combination of climate change and other factors. Figures 1 and 2 focus on the 50 most populous metropolitan areas that had complete data for the entire period from 1961 to 2021. Several large metropolitan areas did not have enough data, such as New York City, Houston, Minneapolis–St. Paul, and Denver. In some of these cases, the best available long-term weather station was relocated sometime between 1961 and 2021—for example, when a new airport opened. As noted above, Figures 1 and 2 focus on daily minimum temperatures because of the connection between nighttime cooling (or lack thereof) and human health. For reference, EPA’s technical documentation for this indicator shows the results of a similar analysis based on daily maximum temperatures. Temperature data are less certain for the early part of the 20th century because fewer stations were operating at that time. In addition, measuring devices and methods have changed over time, and some stations have moved. The data in Figure 3 have been adjusted to the extent possible to account for some of these influences and biases, however, and these uncertainties are not sufficient to change the fundamental nature of the trends. Data SourcesFigures 1 and 2 are adapted from an analysis by Habeeb et al. (2015).11 They are based on temperature and humidity measurements from weather stations managed by NOAA’s National Weather Service. NOAA’s National Centers for Environmental Information compiled and provided the data. Figure 3 is based on measurements from weather stations in the National Weather Service Cooperative Observer Network. The data are available online at: www.ncdc.noaa.gov. Components of this indicator can also be found at: www.globalchange.gov/indicators. Technical Documentation
References1 Sarofim, M.C., S. Saha, M.D. Hawkins, D.M. Mills, J. Hess, R. Horton, P. Kinney, J. Schwartz, and A. St. Juliana. 2016. Chapter 2: Temperature-related death and illness. In: The impacts of climate change on human health in the United States: A scientific assessment. U.S. Global Change Research Program. https://health2016.globalchange.gov. 2 USGCRP (U.S. Global Change Research Program). 2017. Climate science special report: Fourth National Climate Assessment, volume I. Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.). https://science2017.globalchange.gov. doi:10.7930/J0J964J6.. 3 U.S. EPA (U.S. Environmental Protection Agency). 2015. Climate change in the United States: Benefits of global action. EPA 430-R-15-001. www.epa.gov/cira. 4 Sarofim, M.C., S. Saha, M.D. Hawkins, D.M. Mills, J. Hess, R. Horton, P. Kinney, J. Schwartz, and A. St. Juliana. 2016. Chapter 2: Temperature-related death and illness. In: The impacts of climate change on human health in the United States: A scientific assessment. U.S. Global Change Research Program. https://health2016.globalchange.gov. 5 Sarofim, M.C., S. Saha, M.D. Hawkins, D.M. Mills, J. Hess, R. Horton, P. Kinney, J. Schwartz, and A. St. Juliana. 2016. Chapter 2: Temperature-related death and illness. In: The impacts of climate change on human health in the United States: A scientific assessment. U.S. Global Change Research Program. https://health2016.globalchange.gov. 6 CCSP (U.S. Climate Change Science Program). 2008. Synthesis and Assessment Product 3.3: Weather and climate extremes in a changing climate. www.globalchange.gov/browse/reports/sap-33-weather-and-climate-extremes-changing-climate. 7 NOAA (National Oceanic and Atmospheric Administration). 2022. Heat stress datasets and documentation. Provided to EPA by NOAA in February 2022. . 8 NOAA (National Oceanic and Atmospheric Administration). 2022. Heat stress datasets and documentation. Provided to EPA by NOAA in February 2022. 9Kunkel, K. 2022. Updated version of Figure 2.3 in: CCSP (U.S. Climate Change Science Program). 2008. Synthesis and Assessment Product 3.3: Weather and climate extremes in a changing climate. www.globalchange.gov/browse/reports/sap-33-weather-and-climate-extremes-changing-climate. 10 U.S. EPA (U.S. Environmental Protection Agency). 2016. Heat island effect. www.epa.gov/heatislands. 11 Habeeb, D., J. Vargo, and B. Stone, Jr. 2015. Rising heat wave trends in large U.S. cities. Nat. Hazards 76(3):1651–1665. doi:10.1007/s11069-014-1563-z. Learn about other indicators in this sectionHow long does a heatwave usually last?In recent years, the average heat wave in major U.S. urban areas has been about four days long. This is about a day longer than the average heat wave in the 1960s (see Figure 1). The average heat wave season across the 50 cities in this indicator is about 49 days longer now than it was in the 1960s (see Figure 1).
How do you get rid of heat waves?Keep animals in shade and give them plenty of water to drink. Keep your home cool, use curtains, shutters or sunshade and open windows at night. Use fans, damp clothing and take bath in cold water frequently.
How often do heat waves occur?Heat waves are occurring more often than they used to in major cities across the United States. The frequency of heat waves has increased steadily over time, from an average of two heat waves per year during the 1960s to more than six per year during the 2010s.
Are heat waves serious?Extreme heat events can be dangerous to health – even fatal. These events result in increased hospital admissions for heat- related illness, as well as cardiovascular and respiratory disorders. Extreme heat events can trigger a variety of heat stress conditions, such as heat stroke.
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