Illinois's location in the interior of North America and the lack of mountains to the north and south expose the state to incursions of bitterly cold air masses from the Arctic in the winter and warm, humid air masses from the Gulf of America (formerly Gulf of Mexico) in the summer. Annual average temperature varies widely across the state, with a range of about 10°F from north to south. Typical summer maximum temperatures range from the low 80s (°F) in northern parts of the state to the high 80s in the south (1991–2020 averages). Average winter minimum temperatures range from less than 10°F in the north to around 25°F in the south. In northeastern Illinois, Lake Michigan moderates the temperature, causing cooler summers and warmer winters. Topography and urbanization also have local impacts on climate. Annual precipitation varies widely across the state, ranging from more than 50 inches in the south to less than 37 inches in the north. For annual snowfall, the pattern is reversed, with the northeastern part of the state averaging more than 35 inches compared to less than 10 inches in the southernmost section. Due to its proximity to Lake Michigan, the Chicago metropolitan area occasionally receives heavy winter precipitation from lake-effect snows (localized, enhanced snowfall caused by cold, dry air moving across warmer lake water).
Time series of observed and projected temperature change (in degrees Fahrenheit) for Illinois from 1900 to 2100 as described in the caption.
Y-axis values range from negative 4 to positive 20 degrees. Observed annual temperatures from 1900 to 2024 show variability and range from negative 3.3 to positive 4.1 degrees. By the end of the century, projected increases in temperature range from 2.2 to 8.6 degrees under the lower emissions pathway, from 4.8 to 11.1 degrees under the intermediate pathway, and from 8.8 to 18.3 degrees under the higher pathway.
Annual average temperatures in Illinois have increased more than 1.5°F since the beginning of the 20th century (Figure 1). Temperatures in the 2000s have generally been higher than in any other historical period, with the exception of comparable temperatures during the early 1930s Dust Bowl era. Warming has been concentrated in winter and spring, while summers have not warmed substantially, a feature characteristic of much of the Midwest (Figures 2a and 2b). The lack of summer warming is reflected in a mostly below-average number of very hot days since the 1960s (Figure 3a) and no overall trend in very warm nights since the beginning of the 20th century (Figure 3b). The winter warming trend is reflected in a generally below-average number of very cold nights since the 1990s (Figure 4).
Two graphs of the observed (a) winter and (b) summer average temperatures for Illinois from 1895–96 to 2023–24 and 1895 to 2024, respectively, as described in the caption.
Panel A: Winter Average Temperature
Y-axis values range from 16 to 40 degrees Fahrenheit. Annual values show year-to-year variability and range from 19.5 to 36.4 degrees. Five-year values also show variability and are all below the long-term average of 28.3 degrees between 1895 and 1919, all near or above average between 1920 and 1954, and all below average between 1955 and 1989. Since 1990, 5-year values are all above average except for the 2010 to 2014 period. The 1975 to 1979 period has the lowest 5-year value at 24.1 degrees, and the 2020 to 2024 period has the highest at 32.1 degrees.
Panel B: Summer Average TemperatureY-axis values range from 68 to 80 degrees Fahrenheit. Annual values show year-to-year variability and range from 69 to 78.2 degrees. Five-year values also show variability and are mostly near or below the long-term average of 73.6 degrees between 1895 and 1929, mostly near or above average between 1930 and 1959, and all below average between 1960 and 1979. Since 1980, 5-year values are mostly near or above average. The 1925 to 1929 period has the lowest 5-year value at 72.1 degrees, and the 1930 to 1934 period has the highest at 75.7 degrees.
Two graphs of the observed number of (a) very hot days and (b) very warm nights for Illinois from 1900 to 2024 as described in the caption.
Panel A: Very Hot Days
Y-axis values range from 0 to 50 days. Annual values show year-to-year variability and range from 0.3 to 45.3 days. Five-year values also show variability and are mostly near or above the long-term average of 9.2 days between 1900 and 1954 but are all near or below average since 1955. The 2015 to 2019 and 2020 to 2024 periods have the lowest 5-year values at 2.0 and 2.6 days, respectively, and the 1930 to 1934 period has the highest at 26.8 days.
Panel B: Very Warm NightsY-axis values range from 0 to 16 nights. Annual values show year-to-year variability and range from 0.1 to 13.3 nights. Five-year values also show variability and are all near or below the long-term average of 1.9 nights between 1900 and 1929, mostly above average between 1930 and 1954, all below average between 1955 and 1979, and mostly near or above average between 1980 and 2014. The 2015 to 2019 and 2020 to 2024 periods are below average. The 1925 to 1929 period has the lowest 5-year value at 0.6 nights, and the 1930 to 1934 and 1935 to 1939 periods have the highest at 5.3 and 4.7 nights, respectively.
Graph of the observed number of very cold nights for Illinois from 1900 to 2024 as described in the caption.
Y-axis values range from 0 to 32 nights. Annual values show year-to-year variability and range from 0.2 to 28.5 nights. Five-year values also show variability and are mostly near or above the long-term average of 8.3 nights between 1900 and 1919, all near or below average between 1920 and 1959, and mostly near or above average between 1960 and 1989. Since 1990, 5-year values are all near or below average. The 1950 to 1954 period has the lowest 5-year value at 5.1 nights, and the 1975 to 1979 period has the highest at 17.0 nights.
Graph of the observed total annual precipitation for Illinois from 1895 to 2024 as described in the caption.
Y-axis values range from 24 to 56 inches. Annual values show year-to-year variability and range from 25.5 to 51.2 inches. Five-year values also show variability and are mostly near or below the long-term average of 38.1 inches between 1895 and 1989 but are all above average since 1990. The 1910 to 1914 and 1930 to 1934 periods have the lowest 5-year values at 33.8 and 33.6 inches, respectively, and the 2015 to 2019 period has the highest at 44.4 inches.
Two graphs of the observed total (a) spring and (b) summer precipitation for Illinois from 1895 to 2024 as described in the caption.
Panel A: Total Spring Precipitation
Y-axis values range from 4 to 20 inches. Annual values show year-to-year variability and range from 5.1 to 18.4 inches. Five-year values also show variability and are mostly near or below the long-term average of 11.1 inches between 1895 and 1969 but are all near or above average since 1970 except for the 1985 to 1989 period. The 1930 to 1934 and 1985 to 1989 periods have the lowest 5-year values at 8.6 and 8.5 inches, respectively, and the 1995 to 1999 and 2015 to 2019 periods have the highest at 13.0 and 12.9 inches, respectively.
Panel B: Total Summer PrecipitationY-axis values range from 4 to 20 inches. Annual values show year-to-year variability and range from 5.6 to 18.5 inches. Five-year values also show variability and are mostly near or below the long-term average of 11.4 inches between 1895 and 1944 but are mostly near or above average since 1945. The 1910 to 1914 period has the lowest 5-year value at 8.8 inches, and the 2015 to 2019 period has the highest at 14.4 inches.
Illinois has experienced an increase in the number of 2-inch extreme precipitation events since the beginning of the 20th century (Figure 7). These heavy rainfall events have led to several impactful flooding episodes in recent years. In the summer of 1993, persistent heavy rainfall over the upper Midwest caused severe flooding along the Mississippi River. The 1993 flood was one of the greatest natural disasters in US history, causing billions of dollars in damages to homes, businesses, agriculture, and infrastructure. More recently, during April 16–19, 2013, heavy rainfall from a slow-moving storm system caused severe flooding across parts of northern and central Illinois, with some areas receiving up to 10 inches of rain. This event, which contributed to the second-wettest January–June on record in the state, caused planting delays and diminished revenue for many farmers. Illinois has also struggled with urban flooding caused by heavy rains falling on impervious surfaces (e.g., roads, sidewalks, and driveways) and inadequate infrastructure. In 2023, two extreme rainfall events on July 2 and September 17 led to significant flooding in the Chicago metropolitan area. Impacts from the July event included the flooding of many roads (including of Interstate 290) and at least 70,000 basements.
Graph of the observed annual number of 2-inch extreme precipitation events for Illinois from 1900 to 2024 as described in the caption.
Y-axis values range from 0.0 to 3.5 events. Annual values show year-to-year variability and range from 0.4 to 3.2 events. Five-year values also show variability and are mostly below the long-term average of 1.6 events between 1900 and 1964 but are mostly near or above average since 1965. The 1910 to 1914 and 1930 to 1934 periods have the lowest 5-year values at 1.1 events each, and the 2015 to 2019 period has the highest at 2.48 events.
Illinois experiences storms during all seasons. During February 1–3, 2011, one of the most powerful winter storms in history hit Illinois. Antioch, which normally averages only one snowfall greater than 6 inches per year, received the greatest accumulation at 27 inches. Chicago O’Hare International Airport recorded wind gusts of more than 60 mph and 21.2 inches of snow. More than 9.8 million Illinois residents were in areas that received 12 or more inches of snow. Severe thunderstorms occur frequently during late spring and early summer. These storms can occasionally cause tornadoes, which sometimes cause major damage and loss of life. Illinois averages 62 tornadoes each year (1995–2024). Tornadoes are also a threat in other seasons, for example, a tornado outbreak occurred on December 1, 2018, generating 29 tornadoes across the state (the largest December outbreak on record for Illinois). On August 10, 2020, a powerful derecho (a widespread, long-lived straight-line windstorm associated with a series of severe thunderstorms) traveled through Illinois, triggering 15 tornadoes across northeastern Illinois, with several affecting the Chicago metropolitan area. On July 15, 2024, a powerful derecho affected the northern half of Illinois and surrounding states. The derecho brought very strong winds and spawned many tornadoes, setting records for the most tornadoes from a single weather event in Illinois (48) and in the Chicago forecast area (32). The storm resulted in significant tree damage and flash flooding. Illinois saw a record 139 tornadoes in 2024, breaking the previous mark of 125 set in 2006.
Changes in seasonal and multiyear precipitation, evaporation, and temperature can affect water levels in the Great Lakes, causing serious environmental and socioeconomic impacts. Extreme high lake levels can damage beaches, erode shorelines, and flood and damage nearshore structures. Low lake levels can affect the supply and quality of water, restrict shipping, and result in the loss of wetlands. Reduced winter ice cover from rising temperatures also leaves shores vulnerable to erosion and flooding. Water levels in the Great Lakes have fluctuated widely since the early 20th century. For Lake Michigan–Huron (Figure 8), levels were generally high in the early 20th century, early 1950s, early 1970s, mid-1980s, and late 2010s. Levels were lowest in the mid-1920s, mid-1930s, mid-1960s, and, for an extended period, in the early 2000s to mid-2010s—averaging 577.5 feet over 2000–2013. Lake levels rose rapidly after that, reaching a record high of 581.7 feet in 2020 before dropping off to early-1990s levels by 2023.
Time series of annual average water levels for Lake Michigan–Huron from 1918 to 2023 as described in the caption.
Y-axis values range from 576 to 583 feet. Annual values show wide variability and range from a high of nearly 582 feet in 1986 and 2020 to a low of about 576 feet in 1964. Annual values are mostly between 577 and 579 feet from the beginning of the period to the late 1960s but are mostly between 579 and 581 feet from the early 1970s to late 1990s. The 2000s through the early 2010s show consistently low water levels between 577 and 578 feet. Levels rose sharply in the late 2010s to nearly 582 feet in 2020 but then dropped to nearly 579 feet by 2023.
Under the intermediate and higher emissions pathways, historically unprecedented warming is projected during this century (Figure 1). Even under the lower emissions pathway, most projections show annual average temperatures exceeding historical levels by the middle of the century. However, projections for all three pathways show a large range of potential temperature increases, and some projections for the lower pathway are comparable to recent observed values.
During July 12–16, 1995, Chicago experienced a severe heat wave—the worst weather-related disaster in the city’s history. Over a 5-day period, more than 700 people died. In addition to daytime highs of 90°F or higher (including two days of 100°F or higher), nighttime temperatures dropped only into the 80s. Furthermore, the heat index (a measure that combines air temperature and relative humidity) reached values of 105°F or more for 42 hours during the event. Values of 105°F or higher are considered dangerous by the National Weather Service. The number of hours above critical thresholds of the heat index were the highest on record (Figure 9). With rising temperatures, the intensity of cold waves is projected to decrease, while future heat waves are projected to be more intense. High temperatures combined with high humidity pose risks to human health, particularly for residents of Chicago and other urban areas. Heat absorbed and emitted by buildings and pavement, as well as other aspects of urban environments, makes cities even warmer than the surrounding countryside, a phenomenon known as the urban heat island effect.
Time series of the annual number of hours with heat index values at or above three thresholds for Chicago Midway International Airport during for the months June to September from 1930 to 2022 as described in the caption.
Y-axis values range from 0 to 140 hours. A legend identifies the three thresholds as 100 degrees Fahrenheit (yellow), 105 degrees (orange), and 110 degrees (red). Annual heat index values for the 100-degree threshold range from a low of 0 to a high of 135 hours, values for the 105-degree threshold range from 0 to 76 hours, and values for the 110-degree threshold range from 0 to 29 hours. Across the entire period, the number of hours with heat index values over 100 degrees are mostly below 40, the number of hours over 105 degrees are mostly below 20, and the number of hours over 110 degrees are mostly below 5. The highest number of heat index values above all three thresholds occurred in 1995 and the second highest in 1999.
Under the intermediate emissions pathway, projections indicate increases in total annual precipitation for Illinois by midcentury; however, the projected changes are not statistically significant (Figure 10). There is similar uncertainty in projections for seasonal precipitation, although models generally indicate statewide increases in spring, fall, and winter but decreases in summer. The frequency and intensity of extreme precipitation events are projected to increase, potentially increasing the frequency and intensity of floods. Heavier spring precipitation increases the risk of springtime flooding, posing a threat to Illinois’s important agricultural economy by delaying planting and reducing crop yields.
Map of the contiguous United States showing the projected changes in total annual precipitation by the middle of this century as described in the caption. The state of Illinois is outlined in black.
Legend values range from less than negative 10 to greater than positive 15 percent. Note: unless otherwise specified, projected values are not statistically significant.
Illinois
The majority of Illinois is projected to see increases in total annual precipitation of 0 to 5 percent, with a small region in the central and east-central portion expecting increases of 5 to 10 percent.
Contiguous US
Projected increases in total annual precipitation are as follows: up to 5 percent for most of the Southeast, as well as the central Midwest, central Great Plains, central West, and Northwest; and 5 to 10 percent for all of the Northeast, as well as portions of the Mid-Atlantic, Southeast, northern Midwest, northern Great Plains, and Pacific Northwest, with statistically significant increases in the 5 to 10 percent range for the Northeast, northeastern Midwest, and portions of the northern Great Plains and Pacific Northwest.
Projected decreases of up to 5 percent are expected across most of Texas, New Mexico, and Arizona, as well as southern California.
Droughts, a natural part of Illinois’s climate, are projected to increase in intensity due to higher temperatures and the associated increase in evaporation rates and soil moisture loss.
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