Heat Waves

2023-03-22

Table of contents

Heat Waves
Climate Change Indicators: Heat Waves
Key Points
Background
About the Indicator
About the Data
- Indicator Notes
- Data Sources

Heat Waves

A heat wave, or heat wave, is a period of scorching weather that can be accompanied by high humidity, especially in countries with an oceanic climate. While definitions vary, a heat wave is usually measured relative to the familiar environment of the area and close to the average temperatures of the season.

Temperatures that people from warmer climates consider normal can be called heat waves in a colder area if they are outside the typical climate pattern for that area.

Climate Change Indicators: Heat Waves

This indicator describes trends in extreme heat events over several days in the United States.

Figure 1. Heat Wave Characteristics in the United States by Decade, 1961–2021

Heat Wave Characteristics in the United States by Decade, 1961–2021

This figure shows the evolution of the number of heat waves per year; the average duration of heat waves in days; the number of days between the first and the last heat wave of the year; and the heat of the heat waves compared to the local temperature threshold to define a heat wave.

These data were analyzed from 1961 to 2021 for 50 large metropolitan areas. The graphs show the averages of the 50 urban areas by decade.

Decade	Frequency (average number of heat waves per year)	Duration (average number of individual heat waves)	Season (average length of annual heat wave season)	Intensity (average temperature above local threshold during heat waves)
1960s	2.172	3.003418	23.75	1.949738
1970s	2.842	3.206548	33.97	2.063236
1980s	3.216	3.29976	38.81	2.15336
1990s	3.956	3.466631	47.512	2.25061
2000s	4.668	3.654336	53.498	2.297838
2010s	5.964	4.044645	68.518	2.36048
2020s	6.14	4.034601	72.68	2.322827

Figure 1. Heat Wave Characteristics in the United States by Decade, 1961–2021
Data source: NOAA, 2022
Web update: July 2022
Units: number of heat waves; length of heat waves (days); length of heat wave season (days); °F

Figure 2. Heat Wave Characteristics in 50 Large U.S. Cities, 1961–2021

Heat Wave Characteristics in 50 Large U.S2

These maps show the evolution of the number of heat waves per year; the average duration of heat waves in days; the number of days between the first and the last heat wave of the year; and the heat of the heat waves compared to the local temperature threshold to define a heat wave.

These data were analyzed from 1961 to 2021 for 50 large metropolitan areas. The size of each circle indicates the total variation over the measured period. Solid-colored circles represent cities where the trend was statistically significant.

Station	Latitude	Longitude	Frequency Change	Duration Change	Season Change	Intensity Change
ALBANY_NY	42.75	-73.8	3.182443	0.922206	37.7578	0.498143
ALBUQUERQUE_NM	35.05	-106.62	4.080381	1.156348	40.02961	0.23716
ATLANTA_GA	33.65	-84.43	6.212586	2.241648	85.37388	0.409455
AUSTIN_TX	30.3	-97.7	6.942359	1.361569	62.35748	0.418575
BALTIMORE_MD	39.18	-76.67	3.915389	1.030942	46.51507	0.709783
BATON_ROUGE_LA	30.53	-91.15	3.861449	1.202755	53.78741	0.16557
BIRMINGHAM_AL	33.57	-86.75	5.530407	2.500855	67.46272	0.492552
BOSTON_MA	42.37	-71.03	1.830777	0.667505	28.53094	1.254814
BUFFALO_NY	42.93	-78.73	2.525648	0.338473	19.71338	0.370466
CHARLOTTE_NC	35.22	-80.93	3.575886	1.360237	44.12586	0.362877
CHICAGO_IL	41.78	-87.75	3.350608	0.128501	46.38498	-0.4263
CLEVELAND_OH	41.4	-81.85	5.355896	0.683032	48.15547	0.846733
COLUMBUS_OH	40	-82.88	3.594923	0.872303	47.9175	0.74362
COVINGTON_KY	39.07	-84.67	2.236912	0.462915	23.86991	0.174406
DETROIT_MI	42.42	-83.02	5.758858	1.142514	54.77419	0.83671
FORT_WORTH_TX	32.83	-97.05	4.902168	3.707752	55.19302	0.483303
FRESNO_CA	36.77	-119.72	4.362771	2.213152	66.97409	0.778161
HARTFORD_CT	41.93	-72.68	1.634056	0.13007	27.13802	0.40141
HONOLULU_HI	21.33	-157.92	5.622422	1.775997	47.81914	0.315821
INDIANAPOLIS_IN	39.73	-86.28	1.824432	0.932072	24.11105	-0.18958
JACKSONVILLE_FL	30.5	-81.7	0.996298	0.342432	20.90005	-0.03682
KNOXVILLE_TN	35.82	-83.98	3.556848	0.569831	47.64463	0.297246
LAS_VEGAS_NV	36.08	-115.17	5.746166	2.981493	61.15177	1.174505
LOS_ANGELES_CA	33.93	-118.4	0.999471	2.543069	29.28926	0.501711
LOUISVILLE_KY	38.18	-85.73	4.892649	0.897163	50.96351	0.145496
MEMPHIS_TN	35.05	-89.98	2.522475	-0.84148	20.4146	-0.35357
MIAMI_FL	25.8	-80.27	8.852459	1.433866	82.25172	0.338878
MILWAUKEE_WI	42.95	-87.9	4.080381	0.408456	50.60497	0.948615
NASHVILLE_TN	36.12	-86.68	3.172924	0.670081	37.76415	0.244342
NEW_ORLEANS_LA	29.98	-90.25	9.055526	4.129092	102.5806	0.632466
NORFOLK_VA	36.9	-76.2	5.920677	1.710693	75.31888	0.428747
OKLAHOMA_CITY_OK	35.4	-97.6	2.144897	0.135713	22.1597	0.698864
PHILADELPHIA_PA	39.88	-75.25	4.521417	1.692348	52.25806	1.509821
PHOENIX_AZ	33.43	-112.02	5.352723	1.749148	58.18191	1.069157
PITTSBURGH_PA	40.5	-80.22	4.115283	1.117904	58.19778	1.433639
PORTLAND_OR	45.6	-122.6	5.279746	1.101745	57.47118	0.646406
PROVIDENCE_RI	41.73	-71.43	2.852459	0.382778	31.31359	0.410295
RALEIGH_NC	35.87	-78.78	5.346378	1.715923	64.17874	0.72915
RICHMOND_VA	37.5	-77.33	4.045479	0.921881	58.36594	0.875282
ROCHESTER_NY	43.12	-77.67	1.154944	-0.3986	9.636171	-0.22777
SALT_LAKE_CITY_UT	40.77	-111.97	4.753041	3.390715	48.90428	1.341896
SAN_ANTONIO_TX	29.53	-98.47	4.956108	1.421069	49.77366	0.091336
SAN_DIEGO_CA	32.73	-117.17	1.180328	0.300123	13.60867	0.283486
SAN_FRANCISCO_CA	37.62	-122.38	6.793231	2.897614	111.4268	0.205518
SAN_JUAN_PR	18.43	-66	14.25278	1.369659	134.6843	0.303653
SEATTLE_WA	47.45	-122.3	4.508726	1.961303	56.89371	0.36543
ST_LOUIS_MO	38.75	-90.38	3.937599	-0.60269	52.77525	0.48518
TAMPA_FL	27.97	-82.53	9.639344	2.039686	92.34479	0.516167
TUCSON_AZ	32.12	-110.93	5.359069	1.706698	49.30407	0.488453
TULSA_OK	36.2	-95.9	3.398202	0.96528	50.62401	0.588124

Figure 2. Heat Wave Characteristics in 50 Large U.S. Cities, 1961–2021
Data source: NOAA, 2022
Web update: July 2022
Units: degrees; degrees; number of heat waves; length of heat waves (days); length of heat wave season (days); °F

Figure 3. U.S. Annual Heat Wave Index, 1895–2021

This figure shows the annual values of the U.S. Heat Wave Index from 1895 to 2021. This data covers the 48 contiguous states. An index value of 0.2 could mean that 20% of the country experienced one heat wave, 10% experienced two heat waves, or some other combination of frequency and area gave this value.

Year	Heatwave index
1890
1891
1892
1893
1894
1895	0.034
1896	0.089
1897	0.166
1898	0.054
1899	0.043
1900	0.124
1901	0.328
1902	0.052
1903	0.001
1904	0.006
1905	0.037
1906	0.027
1907	0.019
1908	0.023
1909	0.043
1910	0.051
1911	0.204
1912	0.033
1913	0.097
1914	0.105
1915	0.03
1916	0.145
1917	0.182
1918	0.238
1919	0.026
1920	0.004
1921	0.139
1922	0.004
1923	0.024
1924	0.032
1925	0.159
1926	0.058
1927	0.008
1928	0.046
1929	0.02
1930	0.524
1931	0.572
1932	0.118
1933	0.199
1934	0.898
1935	0.166
1936	1.338
1937	0.063
1938	0.035
1939	0.09
1940	0.221
1941	0.185
1942	0.067
1943	0.066
1944	0.062
1945	0.003
1946	0.008
1947	0.168
1948	0.095
1949	0.058
1950	0.014
1951	0.061
1952	0.191
1953	0.076
1954	0.284
1955	0.145
1956	0.027
1957	0.004
1958	0.016
1959	0.015
1960	0.035
1961	0.042
1962	0.021
1963	0.021
1964	0.032
1965	0.004
1966	0.042
1967	0.003
1968	0.016
1969	0.027
1970	0.014
1971	0.037
1972	0.032
1973	0.017
1974	0.015
1975	0.064
1976	0.053
1977	0.078
1978	0.033
1979	0.022
1980	0.271
1981	0.067
1982	0.007
1983	0.148
1984	0.011
1985	0.023
1986	0.079
1987	0.052
1988	0.349
1989	0.03
1990	0.063
1991	0.028
1992	0.027
1993	0.035
1994	0.063
1995	0.078
1996	0.018
1997	0.008
1998	0.124
1999	0.088
2000	0.081
2001	0.099
2002	0.111
2003	0.114
2004	0.013
2005	0.056
2006	0.217
2007	0.178
2008	0.032
2009	0.053
2010	0.15
2011	0.3
2012	0.252
2013	0.086
2014	0.027
2015	0.112
2016	0.053
2017	0.079
2018	0.107
2019	0.051
2020	0.123
2021	0.226

Figure 3. U.S. Annual Heat Wave Index, 1895–2021
Data source: Kunkel, 2022
Web update: July 2022
Units: heat wave index

Key Points

Heat waves happen more often than before in major cities across the United States. Their frequency has continued to increase, going from an average of two heat waves per year during the 1960s to six per year during the years 2010 and 2020 (see Figure 1).
The average heat wave in major US urban areas has lasted about four days in recent years. That’s about a day longer than the intermediate heat wave of the 1960s (see Figure 1).
The average heat wave season in the 50 cities in this indicator is about 49 days longer today than in the 1960s (see Figure 1). Timing can matter, as heat waves that occur earlier in the spring or later in the fall can catch people off guard and increase exposure to health risks associated with heat waves.
The heat waves have become more intense over time. During the 1960s, the average heat wave in the 50 cities in Figures 1 and 2 was 2.0°F above the local 85th percentile threshold. During the 2020s, the intermediate heat wave was 2.3°F above the local threshold (see Figure 1).
Of the 50 metropolitan areas in this indicator, 46 experienced a statistically significant increase in the frequency of heat waves between the 1960s and the 2020s. The duration of heat waves increased significantly in 29 locations, during the heat wave season in 44, and intensity in 17 (see Figure 2).
Longer-term records show that the heat waves of the 1930s remain the most severe in U.S. history (see Figure 3). The peak in Figure 3 reflects extreme and persistent heat waves in the Great Plains region during a period known as the “Dust Bowl.”

Poor land use practices and many years of intense drought have contributed to these heat waves by depleting soil moisture and reducing the moderating effects of evaporation.

Background

A continuous period of scorching days is an extreme heat event or heat wave. Heat waves are more than uncomfortable: they can lead to illness and death, especially among the elderly, the very young, and other vulnerable populations.

Prolonged exposure to excessive heat can also have different effects, for example, damaging crops, injuring or killing livestock, and increasing the risk of forest fires. Extended periods of extreme heat can lead to power outages, as high demands for air conditioning strain the power grid.

Sweltering days and heat waves are a natural part of daily weather variations. However, as the Earth’s climate warms, warmer-than-usual days and nights are becoming more frequent, and heat waves are expected to become more frequent and intense.

The increase in these extreme heat events can lead to more heat-related illnesses and deaths, especially if people and communities do not take action to adapt. Even small increases in extreme heat can lead to increased deaths and illnesses.

About the Indicator

This indicator examines the trends of four critical characteristics of heat waves in the United States over time.

Frequency: the number of heat waves that occur each year.
Duration: The duration of each heat wave in days.
Duration of the season: number of days between the first heat wave of the year and the last.
Intensity: how hot it is during the heat wave.

Heat waves can be defined in different ways. For consistency across the country, Figures 1 and 2 illustrate a heat wave as a period of two or more consecutive days during which the daily minimum apparent temperature in a given city exceeds the 85th percentile of historical temperatures of July and August for this city. The EPA chose this definition for several reasons:

A heat wave’s most severe health effects are often associated with high temperatures at night, which is usually the daily minimum. The human body must cool down at night, especially after a hot day.

If the air remains too warm at night, the body faces additional strain as the heart pumps more complicated to regulate body temperature.
Humidity adjustment is necessary because when the humidity is high, water does not evaporate as quickly, so it is more difficult for the human body to cool down by sweating. This is why extreme heat health warnings are often based on the “heat index,” which combines temperature and humidity.
The 85th percentile of July and August temperatures are the nine hottest days of the two hottest months of the year. A temperature typically only recorded nine times during the hottest part of the year is rare enough for most people to consider it unusually warm.
Using the 85th percentile for each city, Figures 1 and 2 define “unusual” in terms of local conditions. After all, a specific temperature like 95°F may be boiling in one town but perfectly normal in another. Also, people in relatively warm regions may be better acclimatized and adapted to hot weather.

The National Oceanic and Atmospheric Administration calculated the apparent temperature for this indicator based on temperature and humidity measurements from long-term weather stations, usually located at airports.

Figures 1 and 2 focus on the 50 most populous U.S. metropolitan areas that recorded weather data from a consistent location without many missing days during the time examined. The year 1961 was chosen as the starting point because most major cities have collected consistent data since that time.

Figure 3 provides another perspective for assessing the magnitude and frequency of prolonged heat waves. It shows the annual United States Heat Wave Index, which tracks the occurrence of heat wave conditions in the 48 contiguous states from 1895 to 2021.

This index defines a heat wave as a period of at least four days with an average temperature that would only be expected to persist for four days once every ten years, based on historical records. The value of the index for a given year depends on the frequency with which such violent heat waves occur and their magnitude.

About the Data

Indicator Notes

As cities grow, vegetation is often lost, and more and more surfaces are paved or covered with buildings. This type of development can lead to higher temperatures, which is part of an effect called the “urban heat island.”

Compared to surrounding rural areas, built-up areas have higher temperatures, especially at night. Urban growth since 1961 may have contributed partly to the increase in heat waves that Figures 1 and 2 show for some cities show. T

his indicator does not attempt to adjust for the effects of development in metropolitan areas, as it focuses on the temperatures to which people are exposed and whether or not 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 major urban areas, such as New York, Houston, Minneapolis– St. Paul, and Denver, did not have enough data. In some of these cases, the best available long-term weather station had moved between 1961 and 2021, for example, when a new airport opened.

As noted above, Figures 1 and 2 focus on daily minimum temperatures due to the link between nighttime cooling and human health. For reference, the EPA technical documentation for this indicator shows the results of a similar analysis based on daily maximum temperatures.

Temperature data are less specific for the first part of the 20th century because fewer stations were operating then. In addition, measuring devices and methods have changed over time, and some stations have moved.

However, the data in Figure 3 has been adjusted where possible to account for some of these influences and biases. These uncertainties are insufficient to change the fundamental nature of the trends.

Data Sources

Figures 1 and 2 are adapted from an analysis by Habeeb et al. They are based on temperature and humidity measurements from weather stations operated 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 National Weather Service Cooperative Observer Network weather stations.