cdp test w03

Test Document W03: Climate Change Indicators

Test document compiled from US federal public-domain sources for software testing.

Compiled in October 2026 from U.S. federal government web pages, which are in the public domain. This is not an official government publication and is not endorsed by any agency.

Climate change indicators (EPA)#

Climate Change Indicators: Drought#

This indicator measures drought conditions of U.S. lands.

• This chart shows annual values of the Palmer Drought Severity Index, averaged over the entire area of the contiguous 48 states. Positive values represent wetter-than-average conditions, while negative values represent drier-than-average conditions. A value between -2 and -3 indicates moderate drought, -3 to -4 is severe drought, and -4 or below indicates extreme drought. The thicker line is a nine-year weighted average.

Data source: NOAA, 2016⁵
Web update: August 2016

Key Points#

• Average drought conditions across the nation have varied since records began in 1895\. The 1930s and 1950s saw the most widespread droughts, while the last 50 years have generally been wetter than average (see Figure 1).
• Over the period from 2000 through 2015, roughly 20 to 70 percent of the U.S. land area experienced conditions that were at least abnormally dry at any given time (see Figure 2). The years 2002–2003 and 2012–2013 had a relatively large area with at least abnormally dry conditions, while 2001, 2005, and 2009–2011 had substantially less area experiencing drought.
• During the latter half of 2012, more than half of the U.S. land area was covered by moderate or greater drought (see Figure 2). In several states, 2012 was among the driest years on record.⁴ See Temperature and Drought in the Southwest for a closer look at recent drought conditions in one of the hardest-hit regions.

Background#

There are many definitions and types of drought. Meteorologists generally define drought as a prolonged period of dry weather caused by a lack of precipitation that results in a serious water shortage for some activity, population, or ecological system. Drought can also be thought of as an extended imbalance between precipitation and evaporation.

As average temperatures have risen because of climate change, the Earth’s water cycle has sped up through an increase in the rate of evaporation. An increase in evaporation makes more water available in the air for precipitation, but contributes to drying over some land areas, leaving less moisture in the soil. Thus, as the climate continues to change, many areas are likely to experience increased precipitation (see the U.S. and Global Precipitation indicator) and increased risk of flooding (see the Heavy Precipitation indicator), while areas located far from storm tracks are likely to experience less precipitation and increased risk of drought.

As a result, since the 1950s, some regions of the world have experienced longer and more intense droughts, particularly in southern Europe and West Africa, while other regions have seen droughts become less frequent, less intense, or shorter (for example, in central North America).¹

Drought conditions can negatively affect agriculture, water supplies, energy production, and many other aspects of society. The impacts vary depending on the type, location, intensity, and duration of the drought. For example, effects on agriculture can range from slowed plant growth to severe crop losses, while water supply impacts can range from lowered reservoir levels and dried-up streams to major water shortages.

Prolonged droughts pose a particular threat to indigenous populations because of their economic and cultural dependence on land and water supplies. Warming and drought can threaten medicinal and culturally important plants and animals and can reduce water quality and availability, making tribal populations particularly vulnerable to waterborne illnesses.² Lower streamflow and groundwater levels can also harm plants and animals, and dried-out vegetation increases the risk of wildfires.

About the Indicator#

During the 20th century, many indices were created to measure drought severity by looking at precipitation, soil moisture, stream flow, vegetation health, and other variables.³ Figure 1 shows annual values of the most widely used index, the Palmer Drought Severity Index, which is calculated from precipitation and temperature measurements at weather stations.

An index value of zero represents the average moisture conditions observed between 1931 and 1990 at a given location. A positive value means conditions are wetter than average, while a negative value is drier than average. Index values from locations across the contiguous 48 states have been averaged together to produce the national values shown in Figure 1.

For a more detailed perspective on recent trends, Figure 2 shows a newer index called the Drought Monitor, which is based on several indices (including Palmer), along with additional factors such as snow water content, groundwater levels, reservoir storage, pasture/range conditions, and other impacts. The Drought Monitor uses codes from D0 to D4 (see table below Figure 2) to classify drought severity. This part of the indicator covers all 50 states and Puerto Rico.

About the Data#

Indicator Notes

Because this indicator focuses on national trends, it does not show how drought conditions vary by region. For example, even if half of the country suffered from severe drought, Figure 1 could show an average index value close to zero if the rest of the country was wetter than average. Thus, Figure 1 might understate the degree to which droughts are becoming more severe in some areas while other places receive more rain as a result of climate change.

The U.S. Drought Monitor (Figure 2) offers a closer look at the percentage of the country that is affected by drought. This index is relatively new, however, and thus too short-lived to be used for assessing long-term climate trends or exploring how recent observations compare with historical patterns. With several decades of data collection, future versions of this indicator should be able to paint a more complete picture of trends over time.

Overall, this indicator gives a broad overview of drought conditions in the United States. It is not intended to replace local or state information that might describe conditions more precisely for a particular region.

Data Sources

Data for Figure 1 were obtained from the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information, which maintains a large collection of climate data online at: www7.ncdc.noaa.gov/CDO/CDODivisionalSelect.jsp. Data for Figure 2 were provided by the National Drought Mitigation Center. Historical data in table form are available at: http://droughtmonitor.unl.edu.

Source: U.S. Environmental Protection Agency, "Climate Change Indicators: Drought", https://19january2021snapshot.epa.gov/climate-indicators/climate-change-indicators-drought_.html, Web update: August 2016.

Climate Change Indicators: U.S. and Global Precipitation#

This indicator describes trends in average precipitation for the United States and the world.

• This figure shows how the total annual amount of precipitation in the contiguous 48 states has changed since 1901\. This graph uses the 1901–2000 average as a baseline for depicting change. Choosing a different baseline period would not change the shape of the data over time.

Data source: NOAA, 2016¹
Web update: August 2016

• This figure shows the rate of change in total annual precipitation in different parts of the United States since the early 20th century (since 1901 for the contiguous 48 states and 1925 for Alaska). The data are shown for climate divisions, as defined by the National Oceanic and Atmospheric Administration.

Data source: NOAA, 2016³
Web update: August 2016

Key Points#

• On average, total annual precipitation has increased over land areas in the United States and worldwide (see Figures 1 and 2). Since 1901, global precipitation has increased at an average rate of 0.08 inches per decade, while precipitation in the contiguous 48 states has increased at a rate of 0.17 inches per decade.
• Some parts of the United States have experienced greater increases in precipitation than others. A few areas, such as the Southwest, have seen a decrease in precipitation (see Figure 3). Not all of these regional trends are statistically significant, however.

Background#

Precipitation can have wide-ranging effects on human well-being and ecosystems. Rainfall, snowfall, and the timing of snowmelt can all affect the amount of surface water and groundwater available for drinking, irrigation, and industry. They also influence river flooding and can determine what types of animals and plants (including crops) can survive in a particular place. Changes in precipitation can disrupt a wide range of natural processes, particularly if these changes occur more quickly than plant and animal species can adapt.

As average temperatures at the Earth’s surface rise (see the U.S. and Global Temperature indicator), more evaporation occurs, which, in turn, increases overall precipitation. Therefore, a warming climate is expected to increase precipitation in many areas. Just as precipitation patterns vary across the world, however, so do the precipitation effects of climate change.

By shifting the wind patterns and ocean currents that drive the world’s climate system, climate change will also cause some areas to experience decreased precipitation. In addition, higher temperatures lead to more evaporation, so increased precipitation will not necessarily increase the amount of water available for drinking, irrigation, and industry (see the Drought indicator).

About the Indicator#

This indicator examines U.S. and global precipitation patterns, based on rainfall and snowfall measurements from land-based weather stations worldwide. This indicator starts at 1901 except for the detailed map of Alaska, where reliable statewide records are available back to 1925\. The indicator extends through 2015.

This indicator shows annual anomalies, or differences, compared with the average precipitation from 1901 to 2000\. At each weather station, annual precipitation anomalies were calculated from total annual precipitation in inches. Anomalies for the contiguous 48 states and Alaska have been determined by calculating average precipitation anomalies for areas within each state based on station density and topography. These regional anomalies are then averaged together in proportion to their area to develop national results. Similarly, global anomalies have been determined by dividing the world into a grid, averaging the data for each cell of the grid, and then averaging the grid cells together.

About the Data#

Indicator Notes

Data from the early 20th century are somewhat less precise because there were fewer stations collecting measurements at the time. To ensure that overall trends are reliable, the data have been adjusted where possible to account for any biases that might be introduced by factors such as station relocation or changes in measurement instruments.

Hawaii and U.S. territories are not included, due to limitations in available data.

Data Sources

The data for this indicator were provided by the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information, which maintains a large collection of climate data online at: www.ncei.noaa.gov. The precipitation anomalies shown here were calculated based on monthly values from a network of long-term monitoring stations.

Source: U.S. Environmental Protection Agency, "Climate Change Indicators: U.S. and Global Precipitation", https://19january2021snapshot.epa.gov/climate-indicators/climate-change-indicators-us-and-global-precipitation_.html, Web update: August 2016.

Climate Change Indicators: Heavy Precipitation#

This indicator tracks the frequency of heavy precipitation events in the United States.

• This figure shows the percentage of the land area of the contiguous 48 states where a much greater than normal portion of total annual precipitation has come from extreme single-day precipitation events. The bars represent individual years, while the line is a nine-year weighted average.

Data source: NOAA, 2016⁶
Web update: August 2016

• This figure shows the percentage of the land area of the contiguous 48 states that experienced much greater than normal precipitation in any given year, which means it scored 2.0 or above on the annual Standardized Precipitation Index. The thicker line shows a nine-year weighted average that smooths out some of the year-to-year fluctuations.

Data source: NOAA, 2016⁷
Web update: August 2016

Key Points#

• In recent years, a larger percentage of precipitation has come in the form of intense single-day events. Nine of the top 10 years for extreme one-day precipitation events have occurred since 1990 (see Figure 1).
• The prevalence of extreme single-day precipitation events remained fairly steady between 1910 and the 1980s, but has risen substantially since then. Over the entire period from 1910 to 2015, the portion of the country experiencing extreme single-day precipitation events increased at a rate of about half a percentage point per decade (see Figure 1).
• The percentage of land area experiencing much greater than normal yearly precipitation totals increased between 1895 and 2015\. There has been much year-to-year variability, however. In some years there were no abnormally wet areas, while a few others had abnormally high precipitation totals over 10 percent or more of the contiguous 48 states’ land area (see Figure 2). For example, 1941 was extremely wet in the West, while 1982 was very wet nationwide.³
• Figures 1 and 2 are both consistent with other studies that have found an increase in heavy precipitation over timeframes ranging from single days to 90-day periods to whole years.⁴,⁵ For more information on trends in overall precipitation levels, see the U.S. and Global Precipitation indicator.

Background#

Heavy precipitation refers to instances during which the amount of rain or snow experienced in a location substantially exceeds what is normal. What constitutes a period of heavy precipitation varies according to location and season.

Climate change can affect the intensity and frequency of precipitation. Warmer oceans increase the amount of water that evaporates into the air. When more moisture-laden air moves over land or converges into a storm system, it can produce more intense precipitation—for example, heavier rain and snow storms.¹ The potential impacts of heavy precipitation include crop damage, soil erosion, and an increase in flood risk due to heavy rains (see the River Flooding indicator)—which in turn can lead to injuries, drownings, and other flooding-related effects on health.² In addition, runoff from precipitation can impair water quality as pollutants deposited on land wash into water bodies.

Heavy precipitation does not necessarily mean the total amount of precipitation at a location has increased—just that precipitation is occurring in more intense events. However, changes in the intensity of precipitation, when combined with changes in the interval between precipitation events, can also lead to changes in overall precipitation totals.

About the Indicator#

Heavy precipitation events can be measured by tracking their frequency, examining their return period (the chance that the event will be equaled or exceeded in a given year), or directly measuring the amount of precipitation in a certain period (for example, inches of rain falling in a 24-hour period).

One way to track heavy precipitation is by calculating what percentage of a particular location’s total precipitation in a given year has come in the form of extreme one-day events—or, in other words, what percentage of precipitation is arriving in short, intense bursts. Figure 1 of this indicator looks at the prevalence of extreme single-day precipitation events over time.

For added insight, this indicator also tracks the occurrence of unusually high total yearly precipitation. It does so by looking at the Standardized Precipitation Index (SPI), which compares actual yearly precipitation totals with the range of precipitation totals that one would typically expect at a specific location, based on historical data. If a location experiences less precipitation than normal during a particular period, it will receive a negative SPI score, while a period with more precipitation than normal will receive a positive score.

The more precipitation (compared with normal), the higher the SPI score. The SPI is a useful way to look at precipitation totals because it allows comparison of different locations and different seasons on a standard scale. Figure 2 shows what percentage of the total area of the contiguous 48 states had an annual SPI score of 2.0 or above (well above normal) in any given year.

About the Data#

Indicator Notes

Weather monitoring stations tend to be closer together in the eastern and central states than in the western states. In areas with fewer monitoring stations, heavy precipitation indicators are less likely to reflect local conditions accurately.

Data Sources

The data used for this indicator come from a large national network of weather stations and were provided by the National Oceanic and Atmospheric Administration’s (NOAA’s) National Centers for Environmental Information. Figure 1 is based on Step #4 of NOAA’s U.S. Climate Extremes Index; for data and a description of the index, see: www.ncdc.noaa.gov/extremes/cei.

Figure 2 is based on the U.S. SPI, which is shown in a variety of maps available online at: www.ncdc.noaa.gov/sotc/drought. The data used to construct these maps are available at: ftp://ftp.ncdc.noaa.gov/pub/data/cirs/climdiv/.

Source: U.S. Environmental Protection Agency, "Climate Change Indicators: Heavy Precipitation", https://19january2021snapshot.epa.gov/climate-indicators/climate-change-indicators-heavy-precipitation_.html, Web update: August 2016.

Climate Change Indicators: U.S. and Global Temperature#

This indicator describes trends in average surface temperature for the United States and the world.

• This figure shows how annual average air temperatures have changed in different parts of the United States since the early 20th century (since 1901 for the contiguous 48 states and 1925 for Alaska). The data are shown for climate divisions, as defined by the National Oceanic and Atmospheric Administration.

Data source: NOAA, 2016³
Web update: August 2016

Key Points#

• Since 1901, the average surface temperature across the contiguous 48 states has risen at an average rate of 0.14°F per decade (see Figure 1). Average temperatures have risen more quickly since the late 1970s (0.29 to 0.46°F per decade since 1979). Eight of the top 10 warmest years on record for the contiguous 48 states have occurred since 1998, and 2012 and 2015 were the two warmest years on record.
• Worldwide, 2015 was the warmest year on record and 2006–2015 was the warmest decade on record since thermometer-based observations began. Global average surface temperature has risen at an average rate of 0.15°F per decade since 1901 (see Figure 2), similar to the rate of warming within the contiguous 48 states. Since the late 1970s, however, the United States has warmed faster than the global rate.
• Some parts of the United States have experienced more warming than others (see Figure 3). The North, the West, and Alaska have seen temperatures increase the most, while some parts of the Southeast have experienced little change. Not all of these regional trends are statistically significant, however.

Background#

Temperature is a fundamental measurement for describing the climate, and the temperature in particular places can have wide-ranging effects on human life and ecosystems. For example, increases in air temperature can lead to more intense heat waves, which can cause illness and death, especially in vulnerable populations. Annual and seasonal temperature patterns also determine the types of animals and plants that can survive in particular locations. Changes in temperature can disrupt a wide range of natural processes, particularly if these changes occur more quickly than plant and animal species can adapt.

Concentrations of heat-trapping greenhouse gases are increasing in the Earth’s atmosphere (see the Atmospheric Concentrations of Greenhouse Gases indicator). In response, average temperatures at the Earth’s surface are increasing and are expected to continue rising. Because climate change can shift the wind patterns and ocean currents that drive the world’s climate system, some areas are warming more than others, and some have experienced cooling.

About the Indicator#

This indicator examines U.S. and global surface temperature patterns over time. U.S. surface measurements come from weather stations on land, while global surface measurements also incorporate observations from buoys and ships on the ocean, thereby providing data from sites spanning much of the surface of the Earth. This indicator starts at 1901 except for the detailed map of Alaska, where reliable statewide records are available back to 1925\. For comparison, this indicator also displays satellite measurements that can be used to estimate the temperature of the Earth’s lower atmosphere since 1979.

This indicator shows annual anomalies, or differences, compared with the average temperature from 1901 to 2000\. For example, an anomaly of +2.0 degrees means the average temperature was 2 degrees higher than the long-term average. Anomalies have been calculated for each weather station. Daily temperature measurements at each site were used to calculate monthly anomalies, which were then averaged to find an annual temperature anomaly for each year.

Anomalies for the contiguous 48 states and Alaska have been determined by calculating average anomalies for areas within each state based on station density, interpolation, and topography. These regional anomalies are then averaged together in proportion to their area to develop national results. Similarly, global anomalies have been determined by dividing the world into a grid, averaging the data for each cell of the grid, and then averaging the grid cells together.

About the Data#

Indicator Notes

Data from the early 20th century are somewhat less precise than more recent data because there were fewer stations collecting measurements at the time, especially in the Southern Hemisphere. The overall trends are still reliable, however. Where possible, the data have been adjusted to account for any biases that might be introduced by factors such as station moves, urbanization near the station, changes in measuring instruments, and changes in the exact times at which measurements are taken.

Hawaii and U.S. territories are not included, due to limitations in available data.

Data Sources

The data for this indicator were provided by the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information, which maintains a large collection of climate data online at: www.ncei.noaa.gov. The surface temperature anomalies shown here were calculated based on monthly values from a network of long-term monitoring stations. Satellite data were analyzed by two independent groups—the Global Hydrology and Climate Center at the University of Alabama in Huntsville (UAH) and Remote Sensing Systems (RSS)—resulting in slightly different trend lines.

Source: U.S. Environmental Protection Agency, "Climate Change Indicators: U.S. and Global Temperature", https://19january2021snapshot.epa.gov/climate-indicators/climate-change-indicators-us-and-global-temperature_.html, Web update: August 2016.

Climate Change Indicators: High and Low Temperatures#

This indicator describes trends in unusually hot and cold temperatures across the United States.

• This graph shows the percentage of the land area of the contiguous 48 states with unusually hot daily high and low temperatures during the months of June, July, and August. The thin lines represent individual years, while the thick lines show a nine-year weighted average. Red lines represent daily highs, while orange lines represent daily lows. The term “unusual” in this case is based on the long-term average conditions at each location.

Data source: NOAA, 2015⁷
Web update: August 2016

• This graph shows the percentage of the land area of the contiguous 48 states with unusually cold daily high and low temperatures during the months of December, January, and February. The thin lines represent individual years, while the thick lines show a nine-year weighted average. Blue lines represent daily highs, while purple lines represent daily lows. The term “unusual” in this case is based on the long-term average conditions at each location.

Data source: NOAA, 2016⁸
Web update: August 2016

• This map shows trends in unusually hot temperatures at individual weather stations that have operated consistently since 1948\. In this case, the term “unusually hot” refers to a daily maximum temperature that is hotter than the 95th percentile temperature during the 1948–2015 period. Thus, the maximum temperature on a particular day at a particular station would be considered “unusually hot” if it falls within the warmest 5 percent of measurements at that station during the 1948–2015 period.

The map shows changes in the total number of days per year that were hotter than the 95th percentile. Red upward-pointing symbols show where these unusually hot days are becoming more common. Blue downward-pointing symbols show where unusually hot days are becoming less common.

Data source: NOAA, 2016⁹
Web update: August 2016

• This map shows trends in unusually cold temperatures at individual weather stations that have operated consistently since 1948\. In this case, the term “unusually cold” refers to a daily minimum temperature that is colder than the 5th percentile temperature during the 1948–2015 period. Thus, the minimum temperature on a particular day at a particular station would be considered “unusually cold” if it falls within the coldest 5 percent of measurements at that station during the 1948–2015 period.

The map shows changes in the total number of days per year that were colder than the 5th percentile. Blue upward-pointing symbols show where these unusually cold days are becoming more common. Red downward-pointing symbols show where unusually cold days are becoming less common.

Data source: NOAA, 2016¹⁰
Web update: August 2016

Key Points#

• Heat waves in the 1930s remain the most severe heat waves in the U.S. historical record (see Figure 1). The spike in Figure 1 reflects extreme, 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 contributed to these heat waves by depleting soil moisture and reducing the moderating effects of evaporation.⁵
• Nationwide, unusually hot summer days (highs) have become more common over the last few decades (see Figure 2). The occurrence of unusually hot summer nights (lows) has increased at an even faster rate. This trend indicates less “cooling off” at night.
• The 20th century had many winters with widespread patterns of unusually low temperatures, including a particularly large spike in the late 1970s (see Figure 3). Since the 1980s, though, unusually cold winter temperatures have become less common—particularly very cold nights (lows).
• The two maps show where changes in the number of days with unusually hot (above the 95th percentile) and cold (below the 5th percentile) days have occurred since 1948\. Unusually high temperatures have increased in the western United States and in several areas along the Gulf and Atlantic coasts, but decreased in much of the middle of the country (see Figure 4). The number of unusually cold days has generally decreased throughout the country, particularly in the western United States (see Figure 5).
• If the climate were completely stable, one might expect to see highs and lows each accounting for about 50 percent of the records set. Since the 1970s, however, record-setting daily high temperatures have become more common than record lows across the United States (see Figure 6). The most recent decade had twice as many record highs as record lows.

Background#

Unusually hot or cold temperatures can result in prolonged extreme weather events like summer heat waves or winter cold spells. Heat waves 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 People can also die from exposure to extreme cold (hypothermia). In addition, 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-setting daily temperatures, heat waves, and cold spells are a natural part of day-to-day variation in weather. As the Earth’s climate warms overall, however, heat waves are expected to become more frequent, longer, and more intense.²,³ Higher heat index values (which combine temperature and humidity to describe perceived temperature) are expected to increase discomfort and aggravate health issues.

Conversely, cold spells are expected to decrease. In most locations, scientists expect daily minimum temperatures—which typically occur at night—to become warmer at a faster rate than daily maximum temperatures.⁴ This change will provide less opportunity to cool off and recover from daytime heat.

About the Indicator#

This indicator examines trends in unusual temperatures from several perspectives:

• The size and frequency of prolonged heat wave events (Figure 1).
• Unusually hot summer temperatures and cold winter temperatures nationwide (Figures 2 and 3).
• The change in the number of days with unusually hot and cold temperatures at individual weather stations (Figures 4 and 5).
• Changes in record high and low temperatures (Figure 6).

The data come 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 cell of the grid. This method ensures that the results are not biased toward regions that happen to have many stations close together.

Figure 1 shows the U.S. Annual Heat Wave Index, which tracks the occurrence of heat wave conditions across the contiguous 48 states from 1895 to 2015\. While there is no universal definition of a heat wave, this index defines a heat wave as a period lasting at least four days with an average temperature that would only be expected to occur once every 10 years, based on the historical record. The index value for a given year depends on how often heat waves occur and how widespread they are.

Figures 2 and 3 show trends in the percentage of the country’s area experiencing unusually hot temperatures in the summer and unusually cold temperatures in the winter. These graphs are based on daily maximum temperatures, which usually occur during the day, and daily minimum temperatures, which usually occur at night. At each station, the recorded highs and lows are compared with the full set of historical records.

After averaging over a particular month or season of interest, the coldest 10 percent of years are considered “unusually cold” and the warmest 10 percent are “unusually hot.” For example, if last year’s summer highs were the 10th warmest on record for a particular location with more than 100 years of data, that year’s summer highs would be considered unusually warm. Data are available from 1910 to 2015 for summer (June through August) and from 1911 to 2016 for winter (December of the previous year through February).

Figures 4 and 5 show how trends in unusually hot and cold daily temperatures throughout the year vary by location. These maps cover 1,100 weather stations that have operated since 1948\. Figure 4 was created by reviewing all daily maximum temperatures from 1948 to 2015 and identifying the 95th percentile temperature (a temperature that one would only expect to exceed in five days out of every 100) at each station.

Next, for each year, the total number of days with maximum temperatures higher than the 95th percentile (that is, unusually hot days) was determined. The map shows how the total number of unusually hot days per year at each station has changed over time. Figure 5 is similar except that it looks at unusually cold days, based on the 5th percentile of daily minimum temperatures.

Many people are familiar with record daily high and low temperatures, which are frequently mentioned in weather reports. Figure 6 depicts trends in these records by comparing the number of record-setting highs with the number of record-setting lows by decade. These data come from a set of weather stations that have collected data consistently since 1950.

About the Data#

Indicator Notes

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 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 trends shown in the figures.

Data Sources

The data for this indicator are based on measurements from weather stations managed by the National Oceanic and Atmospheric Administration. Figure 1 uses data from the National Weather Service Cooperative Observer Network. Figures 2 and 3 come from the U.S. Climate Extremes Index, which is based on a smaller group of long-term weather stations that are tracked by the National Centers for Environmental Information and referred to as the U.S. Historical Climatology Network. Figures 4 and 5 use data from a somewhat larger set of stations tracked by the National Centers for Environmental Information, known as the Global Historical Climatology Network. Figure 6 uses National Weather Service data processed by Meehl et al. (2009).¹² All of these weather station records are available online at: www.ncdc.noaa.gov, and information about the Climate Extremes Index can be found at: www.ncdc.noaa.gov/extremes/cei.

Source: U.S. Environmental Protection Agency, "Climate Change Indicators: High and Low Temperatures", https://19january2021snapshot.epa.gov/climate-indicators/climate-change-indicators-high-and-low-temperatures_.html, Web update: August 2016.

Sources#

1. U.S. Environmental Protection Agency, "Climate Change Indicators: Drought", https://19january2021snapshot.epa.gov/climate-indicators/climate-change-indicators-drought_.html, Web update: August 2016
2. U.S. Environmental Protection Agency, "Climate Change Indicators: U.S. and Global Precipitation", https://19january2021snapshot.epa.gov/climate-indicators/climate-change-indicators-us-and-global-precipitation_.html, Web update: August 2016
3. U.S. Environmental Protection Agency, "Climate Change Indicators: Heavy Precipitation", https://19january2021snapshot.epa.gov/climate-indicators/climate-change-indicators-heavy-precipitation_.html, Web update: August 2016
4. U.S. Environmental Protection Agency, "Climate Change Indicators: U.S. and Global Temperature", https://19january2021snapshot.epa.gov/climate-indicators/climate-change-indicators-us-and-global-temperature_.html, Web update: August 2016
5. U.S. Environmental Protection Agency, "Climate Change Indicators: High and Low Temperatures", https://19january2021snapshot.epa.gov/climate-indicators/climate-change-indicators-high-and-low-temperatures_.html, Web update: August 2016