High and Low Temperatures

Climate Change Indicators: High and Low Temperatures

This indicator describes trends in unusually high and low temperatures in the United States.
Figure 1. Area of the Contiguous 48 States with Unusually Hot Summer Temperatures, 1910–2020

This chart shows the percentage of the land area of ​​the contiguous 48 states with scorching daily high and low temperatures during June, July, and August. Thin lines represent individual years, while thick lines show the nine-year weighted average. Red lines represent daily highs, while orange lines represent daily lows. In this case, the term “unusual” is based on the long-term average conditions at each location.

Year	Hot daily highs	Hot daily highs (smoothed)	Hot daily lows	Hot daily lows (smoothed)
1910	0.074	0.071973	0.02	0.021004
1911	0.087	0.072895	0.031	0.023023
1912	0.004	0.080355	0	0.029832
1913	0.155	0.087617	0.033	0.039355
1914	0.139	0.08141	0.117	0.04232
1915	0.013	0.06507	0.004	0.035055
1916	0.005	0.060227	0.005	0.027449
1917	0.066	0.07627	0.013	0.029707
1918	0.153	0.095879	0.06	0.040785
1919	0.153	0.09991	0.063	0.056004
1920	0	0.089551	0.001	0.071945
1921	0.142	0.075813	0.202	0.079676
1922	0.052	0.065832	0.052	0.070625
1923	0.006	0.064969	0.031	0.051199
1924	0.084	0.072391	0.015	0.035711
1925	0.142	0.074578	0.044	0.02809
1926	0.06	0.061434	0.028	0.023027
1927	0.011	0.045438	0.002	0.018813
1928	0.004	0.051836	0.019	0.020203
1929	0.022	0.089527	0.009	0.032082
1930	0.219	0.139379	0.035	0.05391
1931	0.256	0.17943	0.129	0.081723
1932	0.062	0.209891	0.063	0.113754
1933	0.297	0.236883	0.119	0.147219
1934	0.411	0.253516	0.331	0.173672
1935	0.015	0.251668	0.017	0.184363
1936	0.484	0.22759	0.348	0.172543
1937	0.13	0.181828	0.133	0.135848
1938	0.063	0.131883	0.035	0.088617
1939	0.045	0.099523	0.014	0.056676
1940	0.193	0.084527	0.036	0.051488
1941	0.007	0.078223	0.096	0.06334
1942	0.009	0.081078	0.017	0.074965
1943	0.197	0.085699	0.204	0.071707
1944	0.101	0.074926	0.001	0.050797
1945	0.005	0.049363	0.002	0.025895
1946	0.007	0.029781	0.002	0.013879
1947	0.008	0.029586	0.007	0.019348
1948	0.038	0.043105	0.006	0.033758
1949	0.116	0.062316	0.133	0.045199
1950	0.001	0.092578	0	0.051402
1951	0.093	0.139941	0.023	0.061105
1952	0.322	0.188902	0.125	0.078547
1953	0.184	0.210813	0.067	0.094418
1954	0.303	0.192543	0.161	0.095555
1955	0.091	0.146445	0.09	0.079457
1956	0.101	0.098113	0.02	0.058621
1957	0.004	0.069664	0.008	0.047734
1958	0.06	0.068867	0.092	0.048785
1959	0.083	0.087121	0.046	0.055543
1960	0.105	0.103855	0.006	0.063352
1961	0.218	0.098922	0.177	0.06657
1962	0.015	0.070555	0	0.059121
1963	0.018	0.037387	0.081	0.042543
1964	0.004	0.017957	0.002	0.025242
1965	0	0.015137	0	0.014508
1966	0.02	0.020281	0.003	0.011598
1967	0.06	0.024746	0.035	0.012328
1968	0	0.027074	0	0.012648
1969	0.009	0.029383	0.009	0.012355
1970	0.089	0.029313	0.023	0.013051
1971	0.001	0.023438	0.005	0.015957
1972	0.003	0.015102	0.004	0.020207
1973	0.016	0.010828	0.069	0.02193
1974	0.002	0.013422	0	0.020125
1975	0.002	0.020824	0.004	0.020863
1976	0.068	0.028168	0.001	0.027918
1977	0.023	0.036043	0.107	0.036727
1978	0.019	0.055023	0.005	0.045422
1979	0.002	0.087156	0	0.058016
1980	0.316	0.111227	0.165	0.07168
1981	0.073	0.108824	0.075	0.078598
1982	0.002	0.089664	0.006	0.077785
1983	0.162	0.070848	0.176	0.070215
1984	0.018	0.056469	0.027	0.05634
1985	0.03	0.052422	0.004	0.046863
1986	0.027	0.070629	0.024	0.054813
1987	0.047	0.103102	0.083	0.073586
1988	0.356	0.11741	0.184	0.083223
1989	0.011	0.095988	0.014	0.078426
1990	0.021	0.060441	0.046	0.07193
1991	0.034	0.043559	0.116	0.073336
1992	0.015	0.056473	0.046	0.08309
1993	0.057	0.086336	0.081	0.099977
1994	0.236	0.109371	0.138	0.117453
1995	0.068	0.110887	0.183	0.124637
1996	0.127	0.099098	0.107	0.122629
1997	0.005	0.09216	0.01	0.122398
1998	0.17	0.097316	0.286	0.124242
1999	0.049	0.112582	0.057	0.124594
2000	0.16	0.138121	0.073	0.13575
2001	0.11	0.169617	0.137	0.166992
2002	0.293	0.187715	0.296	0.200004
2003	0.263	0.178695	0.279	0.215723
2004	0.02	0.163238	0.063	0.226195
2005	0.093	0.17134	0.286	0.245926
2006	0.311	0.190953	0.347	0.255652
2007	0.307	0.184551	0.33	0.237824
2008	0.045	0.155887	0.043	0.227566
2009	0.043	0.154387	0.076	0.273133
2010	0.153	0.204734	0.557	0.354852
2011	0.364	0.264707	0.568	0.401328
2012	0.489	0.272102	0.304	0.38357
2013	0.144	0.222043	0.364	0.344211
2014	0.039	0.172352	0.173	0.336945
2015	0.118	0.170012	0.386	0.371012
2016	0.271	0.201555	0.543	0.414891
2017	0.264	0.226715	0.338	0.436367
2018	0.263	0.234953	0.627	0.433582
2019	0.099	0.248465	0.191	0.431555
2020	0.347	0.280672	0.514	0.450316

Figure 1. Area of the Contiguous 48 States with Unusually Hot Summer Temperatures, 1910-2020
Data source: NOAA, 2021
Web update: April 2021
Units: Percent of land area

Figure 2. Area of the Contiguous 48 States with Unusually Cold Winter Temperatures, 1911–2020

This chart shows the percentage of the land area of ​​the contiguous 48 states with freezing daily high and low temperatures during December, January, and February.

Thin lines represent individual years, while thick lines show the nine-year weighted average. Blue lines represent daily highs, while purple lines represent daily lows. In this case, the term “unusual” is based on the long-term average conditions at each location.

Year	Cold Highs	9-pt High	Cold Lows	9-pt Low
1911	0.035	0.126672	0.04	0.145301
1912	0.374	0.169902	0.402	0.190211
1913	0.184	0.185695	0.286	0.184309
1914	0.038	0.18891	0.015	0.148504
1915	0.323	0.208172	0.037	0.150484
1916	0.108	0.247656	0.102	0.219836
1917	0.383	0.280922	0.479	0.302086
1918	0.434	0.27516	0.575	0.318707
1919	0.131	0.225828	0.106	0.257629
1920	0.194	0.163551	0.184	0.175063
1921	0.002	0.118738	0.01	0.119426
1922	0.183	0.093094	0.18	0.091336
1923	0.043	0.072402	0.044	0.072672
1924	0.049	0.05034	0.014	0.056512
1925	0.026	0.032305	0.098	0.044688
1926	0.008	0.029699	0.007	0.045633
1927	0.002	0.052902	0.002	0.074652
1928	0.019	0.093586	0.019	0.126039
1929	0.38	0.122879	0.514	0.159504
1930	0.004	0.129309	0.024	0.153152
1931	0.056	0.134301	0.031	0.139086
1932	0.195	0.148723	0.126	0.147207
1933	0.295	0.160012	0.359	0.165746
1934	0.031	0.178164	0.052	0.190156
1935	0.015	0.22559	0.022	0.23616
1936	0.608	0.27102	0.59	0.277863
1937	0.393	0.256207	0.423	0.260285
1938	0	0.188992	0	0.18857
1939	0.037	0.128203	0.03	0.121723
1940	0.254	0.09407	0.219	0.084949
1941	0	0.065992	0	0.059266
1942	0.015	0.038836	0.026	0.03466
1943	0.003	0.024688	0.003	0.019914
1944	0.034	0.023375	0.011	0.019281
1945	0.045	0.026516	0.017	0.029172
1946	0.004	0.040141	0.069	0.05102
1947	0.005	0.080367	0.006	0.092496
1948	0.09	0.138492	0.119	0.145758
1949	0.446	0.171566	0.386	0.175898
1950	0.117	0.151602	0.165	0.155871
1951	0.001	0.101492	0.036	0.102668
1952	0.108	0.060086	0.045	0.057066
1953	0	0.041238	0.001	0.042055
1954	0	0.038957	0.004	0.05143
1955	0.099	0.043531	0.177	0.064582
1956	0.04	0.048742	0.032	0.070137
1957	0.007	0.055641	0.026	0.074922
1958	0.104	0.06532	0.101	0.084164
1959	0.081	0.072949	0.167	0.089902
1960	0.074	0.07893	0.018	0.093539
1961	0.058	0.095363	0.103	0.11907
1962	0.066	0.127113	0.024	0.17909
1963	0.27	0.153867	0.424	0.237027
1964	0.226	0.149449	0.434	0.236797
1965	0.038	0.116379	0.066	0.17366
1966	0.097	0.08498	0.034	0.100953
1967	0.001	0.081414	0.036	0.065742
1968	0.081	0.103637	0.046	0.070082
1969	0.223	0.124039	0.086	0.087422
1970	0.175	0.120508	0.208	0.092121
1971	0.018	0.103211	0.014	0.079359
1972	0.027	0.09273	0.039	0.06241
1973	0.246	0.085773	0.086	0.051914
1974	0.007	0.075773	0.033	0.05584
1975	0.013	0.091891	0.02	0.09784
1976	0	0.175918	0.005	0.199332
1977	0.343	0.313629	0.496	0.329117
1978	0.585	0.414547	0.529	0.403469
1979	0.816	0.392965	0.665	0.366383
1980	0	0.268578	0	0.253539
1981	0.005	0.145371	0.1	0.150031
1982	0.119	0.093359	0.109	0.103453
1983	0.01	0.096633	0	0.101066
1984	0.244	0.103703	0.229	0.104914
1985	0.098	0.085695	0.124	0.088453
1986	0.001	0.051082	0.001	0.05591
1987	0.002	0.023641	0	0.031422
1988	0.004	0.013	0.004	0.026695
1989	0.033	0.012039	0.08	0.030668
1990	0	0.016492	0.026	0.030941
1991	0.002	0.032039	0.012	0.029953
1992	0.016	0.057594	0.001	0.034293
1993	0.202	0.074215	0.095	0.039164
1994	0.069	0.065203	0.048	0.034602
1995	0	0.038977	0	0.021188
1996	0	0.016766	0.001	0.008496
1997	0.01	0.007461	0	0.00216
1998	0.006	0.007852	0	0.000863
1999	0	0.015793	0	0.002039
2000	0	0.029176	0	0.004117
2001	0.12	0.039754	0.018	0.005391
2002	0.001	0.039656	0.001	0.004758
2003	0.059	0.03009	0.002	0.002969
2004	0.001	0.018262	0.001	0.001512
2005	0.003	0.010133	0	0.001012
2006	0	0.008891	0	0.001426
2007	0.017	0.019895	0.004	0.00348
2008	0.009	0.051105	0.001	0.009098
2009	0	0.094637	0.001	0.018215
2010	0.403	0.117566	0.06	0.026539
2011	0.024	0.099402	0.037	0.032973
2012	0	0.064129	0.002	0.045207
2013	0.004	0.044805	0.013	0.064535
2014	0.123	0.040332	0.246	0.073563
2015	0.014	0.032801	0.007	0.058449
2016	0	0.019734	0	0.031039
2017	0.014	0.009328	0.01	0.011238
2018	0	0.004199	0	0.003367
2019	0.001	0.001859	0	0.001121
2020	0	0.000656	0	0.000313

Figure 2. Area of the Contiguous 48 States with Unusually Cold Winter Temperatures, 1911-2020
Data source: NOAA, 2021
Web update: April 2021
Units: Percent of land area

Figure 3. Change in Unusually Hot Temperatures in the Contiguous 48 States, 1948–2020

This map shows trends in unusually high temperatures at individual weather stations that have operated consistently since 1948. In this case, “unusually hot” refers to a daily maximum temperature more remarkable than the 95th percentile from 1948 to 2020.

Thus, the maximum temperature on a particular day at a specific station would be considered “unusually hot” if it falls within the warmest 5 percent of readings from 1948–2020. The map shows changes in the total number of days per year that were warmer than the 95th percentile.

The red upward-pointing symbols show where these scorching days are becoming more common. Blue downward-pointing symbols show where scorching days are becoming less common.

Figure 4. Change in Unusually Cold Temperatures in the Contiguous 48 States, 1948–2020

This map shows the trends of frigid temperatures at individual weather stations operating consistently since 1948. In this case, the term “unusually cold” refers to a daily minimum temperature below the 5th percentile for the period 1948–2020.

Thus, the minimum temperature on a particular day in a specific station would be considered “unusually cold” if it falls within the coldest 5 percent of readings from 1948 to 2020. The map shows changes in the total number of days per year harder than the 5th percentile.

Blue upward-pointing symbols show where these freezing days are more common. Red downward-pointing signs show where snowy days are becoming less common.

Figure 5. Record Daily High and Low Temperatures in the Contiguous 48 States, 1950–2009

This figure shows the percentage of daily temperature records set at weather stations in the contiguous 48 states over a decade. Record highs are compared to record lows.

Decade	High %	Low %
1950s	52.07%	-47.93%
1960s	43.55%	-56.45%
1970s	43.92%	-56.08%
1980s	53.17%	-46.83%
1990s	57.55%	-42.45%
2000s	67.16%	-32.84%

Figure 5. Record Daily High and Low Temperatures in the Contiguous 48 States, 1950-2009
Data source: Meehl et al., 2009
Web update: April 2021
Units: Percent of daily records

Key Points

• Sweltering summer days (highs) have become more common across the country in the past few decades (see Figure 1). The incidence of muggy summer nights (lows) increased even faster. This trend suggests less “cooling” at night.
• The 20th century had many winters with widespread patterns of shallow temperatures, including a considerable increase in the late 1970s (see Figure 2). However, since the 1980s, freezing winter temperatures have become less common – especially very cold nights (lows).
• These two maps show where there have been changes in the number of days with unusually warm and cold days since 1948. Based on this way of looking at hot days, extraordinarily high temperatures increased in the western United States.
  
  They decreased in a few areas along the Gulf and Atlantic coasts in much of the central part of the country (see Figure 3). The number of freezing days generally decreased across the country, especially in the western United States (see Figure 4).
• If the climate were utterly stable, highs and lows would be expected to account for about 50 percent of recorded records. However, since the 1970s, record daily high temperatures have been more common in the United States than record lows (see Figure 5). The decade from 2000 to 2009 had twice as many record highs as record lows.

Background

• Unusually high or low temperatures can result in prolonged extreme weather events such as summer heat waves or winter cold spells. Heat waves can lead to illness and death, especially among older adults, the very young, and other vulnerable populations.
  
  People can also die from exposure to extreme cold. Additionally, prolonged exposure to excessive heat and cold can damage crops and injure or kill livestock. Extreme heat can lead to power outages as heavy demands for air conditioning strain the power grid, while extremely cold weather increases the need for heating fuel.
  
  
• Record daily temperatures, heat waves, and cold spells are a natural part of daily weather changes. But as the Earth’s climate warms overall, heat waves are expected to become more frequent and intense. Higher heat index values ​​are expected to increase discomfort and exacerbate health problems.
  
  On the contrary, the cold season is expected to decrease. In most places, scientists expect daily minimum temperatures — typically at night — to warm faster than daily maximum temperatures. This change will provide fewer opportunities to cool down and recover from the day’s heat.

About the Indicator

This indicator examines unusual temperature trends from several perspectives.

• The country has scorching summer and cold winter temperatures (Figures 1 and 2).
• Change in the number of days with unusually high and low temperatures at individual weather stations (Figures 3 and 4).
• Changes in record high and low temperatures (Figure 5).

1. The data comes from thousands of weather stations across the United States. National patterns can be determined by dividing the country into a grid and examining the data for one station in each grid cell. This method ensures that the results are not biased towards areas with many stations close together.
   
   
2. Figures 1 and 2 show trends in the percentage of the country’s area experiencing unusually high summer temperatures and shallow winter temperatures.
   
   These charts are based on daily maximum temperatures that typically occur during the day and daily minimum temperatures that usually occur at night. At each station, the recorded highs and lows are compared to the complete set of historical records.
   
   When averaged over a particular month or period of interest, the coldest 10 percent of years are considered “unusually cold” and the warmest 10 percent “unusually hot.” For example, if last year’s summer highs were the 10th most generous for a particular location with more than 100 years of data, this year’s summer highs would be considered unusually warm.
   
   
3. Figures 3 and 4 show how trends in unusually high and low daily temperatures vary by location throughout the year. These maps cover about 1,100 weather stations that have been in operation since 1948.
   
   Figure 3 was created by summarizing all daily maximum temperatures from 1948 to 2020 and identifying the 95th percentile temperature at each station. Additionally, the total number of days with maximum temperatures more excellent than the 95th percentile was determined for each year.
   
   The map shows how the number of sweltering days per year at individual stations has changed. Figure 4 is similar, except it looks at freezing days based on the 5th percentile of daily minimum temperatures.
   
   
4. Many people are familiar with record daily high and low temperatures that are often mentioned in weather reports. Figure 5 shows the trends in these records by comparing the number of record highs with the number of record lows for the decade. This data comes from a set of weather stations that have been collecting data consistently since 1950.

About the Data

Indicator Notes

Temperature data is less certain for the early 20th century because fewer stations were operating then. Additionally, measuring instruments and methods have changed over time, and some stations have moved. The data have been adjusted as much as possible to account for some of these effects and biases. Still, these uncertainties are insufficient to change the underlying trends shown in the figures.

Data Sources

The data for this indicator is based on measurements from weather stations managed by the National Oceanic and Atmospheric Administration. Figures 1 and 2 are from the US Index of Climate Extremes, which is based on a smaller group of long-term weather stations monitored by the National Center for Environmental Information and referred to as the US Historical Climatology Network.

Figures 3 and 4 use data from a more extensive set of stations monitored by the National Center for Environmental Information, known as the Global Historical Climatology Network. Figure 5 uses National Weather Service data processed by Meehl et al.