Illinois Climate – Sep 2026September 2026 Update

Novarupta (Katmai) eruption

The largest volcanic eruption anywhere in the world in the 20th century - 13 km3 of magma, about thirty times the 1980 Mount St. Helens eruption - at Novarupta in Alaska on 6-8 June 1912. Ash reached 100,000 ft and cooled the Northern Hemisphere by roughly 0.3°C over 1912-13. Unlike a tropical eruption such as Pinatubo, a high-latitude eruption's haze stays in its own hemisphere, so it did little to the Southern Hemisphere.

Smithsonian Global Volcanism Program, Alaska Volcano Observatory, National Park Service, Oman et al. 2005

Post-WWII industrial expansion begins

After the Second World War, coal and oil use grew rapidly across Europe, North America and the Soviet Union. Global sulphur dioxide emissions rose from about 30 to about 130 million tonnes a year by 1980, and the sulphate haze they formed masked part of the greenhouse warming underneath - the effect known as 'global dimming'.

Klimont et al. 2013

The 1950s drought

1953 was Illinois's driest year in this site's 1950-2025 record (697 mm, 29% below its 1961-90 average) and also its 2nd-highest for heatwave days at 28.7.

NOAA Drought.gov, NOAA Climate Program Office, Texas Water Development Board

Agung eruption

Agung's eruption in Bali put a veil of sulphate haze into the stratosphere that cooled global temperature by about 0.3°C over 1963-66.

Smithsonian Global Volcanism Program, NOAA Global Monitoring Laboratory

US Clean Air Act

The 1970 Clean Air Act set strict national standards for sulphur dioxide and particulates, and American skies got measurably cleaner over the decades that followed. There is a climate sting in that success: sulphate aerosols reflect sunlight, so removing them removed a cooling effect that had been partly masking greenhouse warming. Measured here, Illinois averaged +0.88°C warmer across 2006-2025 than across the two decades before the Act, 1951-1970, against a median of +1.09°C across the 49 states whose records reach back that far. Every one of them warmed. The range runs from +0.66°C in Oklahoma to +1.67°C in Alaska, so the question a state's own chart answers is not whether it warmed, but by how much.

US EPA

The 1973-74 La Niña

The strongest La Niña of the 1950-present record, part of a multi-year cool phase that also included a second, separate La Niña in 1975-76.

ONI -2.0°C · RONI -1.9°C

NOAA CPCENSO Tracker

The 1976-77 drought

1976 was Illinois's 3rd-driest year on record here - 725 mm, 26% below its 1961-90 average.

USGS, US GAO, USGS Water Data Labs

The cold winters of 1976-79

1979 is Illinois's coldest year in this site's 1950-2025 record. Its first months were the end of the winter of 1978-79, the coldest on record for the contiguous United States and the third brutal winter in a row across the Midwest. Illinois had its coldest January-February on record, 14°F below normal, and seventeen major winter storms.

Illinois State Water Survey, Missouri Climate Center, Monthly Weather Review (1978), Wikipedia

Pacific Decadal Oscillation flips to a warm phase

The Pacific Decadal Oscillation is a North Pacific pattern, and its influence on this side of the country is weak and indirect - mostly through how it nudges the winter jet stream downstream. Its 1976-77 flip marks a genuine shift in Pacific-basin climate, but for eastern states the Atlantic Multidecadal Oscillation is the ocean cycle that matters more.

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Global SO2 emissions peak

Global sulphur dioxide emissions peak at about 130 million tonnes a year, then begin a long decline as Europe, North America and Japan clean up their air. As the aerosol mask thins, the rate of global warming picks up through the 1980s.

Klimont et al. 2013

El Chichón eruption

A sulphur-rich eruption in Mexico whose aerosol cloud stayed in the stratosphere for more than two years, cooling the Northern Hemisphere by about 0.2°C.

Smithsonian Global Volcanism Program

1982-83 El Niño

El Niño has only a weak pull on Illinois's winter rainfall - it is not one of the seven states where ENSO reliably moves it. In 1982-83 Illinois's winter rainfall ran +43% against the 1961-90 average, with temperatures +3.7°C. On NOAA's RONI index, which strips out background ocean warming, this is the strongest El Niño since 1950.

ONI +2.1°C · RONI +2.4°C

NOAA CPC, NOAA CPC (2)ENSO Tracker

1988 Midwest drought / Yellowstone fires

1988 was Illinois's 4th-driest year (745 mm, 24% below its 1961-90 average) with 20.4 heatwave days, its 4th-highest.

NOAA

Daytime warming catches up with the night-led pattern

Across 1961-90, 81% of the world's land saw nights warming faster than days. Across 1991-2020 the pattern reversed: 70% saw days warming faster, as the industrial haze that had been dimming daytime sunshine was cleaned up. The study compares two fixed thirty-year windows; measured as a turning point, this site's own global series turns around 1982.

Zhong et al. 2023

1990-91: mild winters and warm years

1991 is Illinois's 10th-warmest year in this site's 1950-2025 record, following a mild 1990. Mild winters did most of it: January-February 1990 was the warmest start to a year the contiguous US had recorded, a record that stood until 1998.

News & Record / AP (1991), NOAA NCEI

Mount Pinatubo erupts

Pinatubo's cooling is only faintly visible in Illinois's own record. 1993, two years after the June 1991 eruption, ranks 18th-coldest of the 76 years there, though 1996 and 2014 have since been colder. A single large eruption can hold temperatures down for two or three years; it cannot reverse the trend.

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The Great Flood of 1993 (Mississippi River)

1993 is Illinois's wettest year in this site's 1950-2025 record - 1300 mm, 33% above its 1961-90 average. The flood was not a single storm but a whole summer of rain falling on ground that never dried out.

NASA Earth Observatory / NWS

Atlantic Multidecadal Oscillation flips to a warm phase

The Atlantic Multidecadal Oscillation flipped to a warm phase in 1995. Atlantic hurricanes reach this far inland mostly as remnants - Helene's floods in east Tennessee in 2024 among them - and warm AMO phases have also been linked to more frequent drought across the central United States. It is a natural cycle of roughly 60-70 years, so it shifts the baseline rather than driving the long-term trend.

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1997-98 El Niño

In 1997-98 Illinois's winter rainfall ran +15% against the 1961-90 average, with temperatures +4.3°C - though ENSO's pull on rainfall here is weak. It is often treated as the textbook strong El Niño.

ONI +2.4°C · RONI +2.3°C

NOAA CPC, NOAA NCEIENSO Tracker

Pacific Decadal Oscillation flips back to a cool phase

The Pacific Decadal Oscillation returned to a cool phase around 1998-99. Its direct effects are western - drier conditions across the south-west, changes in North Pacific fisheries - and its influence on this side of the country is weak and indirect. The Atlantic Multidecadal Oscillation, which flipped warm in 1995, is the ocean cycle with more bearing on eastern weather.

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The July 1999 heat wave

From 28 to 31 July 1999 Chicago reached over 100°F with heat indices up to 120°F, and Midway set an all-time record dew point of 82°F. NOAA's storm record lists 99 heat deaths in the Chicago area - far fewer than the 700-plus of July 1995, after the city put its new heat emergency plan into action.

NOAA NCEI Storm Events, Naughton et al. (2002)

Start of the sustained rise in US heatwave frequency

Illinois's own Heatwave Days (WSDI) count rose from a 1961-90 average of 5.7 days a year to 10.7 in 2016-25 - a clear rise, though more moderate than the 2.4x averaged across the 50 states.

World Bank CCKP (CCKP), NOAA Climate at a Glance

The 2012 drought and heat

Illinois recorded 33.1 heatwave days in 2012, more than in any other year of this site's 1950-2025 record, alongside its warmest annual mean and its 5th-driest year. The state climatologist ranked May-July the 3rd-driest such period in 126 years of Illinois records, and only the 2nd 'extreme' drought since 1965, after 1988-89.

NOAA NCEI, Summer 2012 Drought Update, NOAA NCEI, Drought, Illinois State Climatologist

Post-2013 acceleration in US temperatures

5 of Illinois's ten warmest years on record have occurred since 2013 - fewer than in most states - but 7 of the ten have come since 2000, with 2012 the warmest of all. The acceleration is clear in Illinois's data; its year-to-year swings are simply wide enough to spread the warmest years further apart than the national mean does.

NOAA Climate at a Glance

The polar-vortex winter of 2013-14

2014 is Illinois's 2nd-coldest year in this site's 1950-2025 record - its coldest since 1979. In early January 2014 the polar vortex slipped south and Arctic air spread across most of the country, and the cold returned in waves into March.

Wikipedia, CBS NewsPolar Vortex Tracker

2015-16 El Niño

In 2015-16 Illinois's winter rainfall ran +41% against the 1961-90 average, with temperatures +3.9°C - though ENSO's pull on rainfall here is weak. By the raw ONI index it was the strongest El Niño ever measured - a reminder that a strong index does not guarantee a strong local outcome.

ONI +2.6°C · RONI +2.3°C

NOAA CPCENSO Tracker

2015-2025: the eleven warmest years on record

Globally, the eleven warmest years on record have all happened since 2015. Illinois does not follow it: its hottest year is still 2012, the drought year. Five of its own eleven warmest fall in the 2015-2025 span, with 2012, 2006, 1998, 1991, 1987 and 1954 outside it. Illinois is one of 15 states where 2012 still tops the list.

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Low-sulphur shipping fuel rule (IMO 2020)

New international shipping rules cut the sulphur allowed in ship fuel from 3.5% to 0.5% almost overnight, removing about 8 million tonnes of sulphur dioxide a year. With less sulphate haze over the oceans, it may have added around 0.1°C of warming by 2023-24.

Gettelman et al. 2024

The 2020 Midwest derecho

The 10 August 2020 derecho crossed Illinois on its 770-mile run from South Dakota to Ohio, bringing damaging straight-line winds and crop losses, though the most extreme gusts and the heaviest agricultural damage were in Iowa to the west.

NWS Des Moines, NASA Earth Observatory

2023-24 El Niño

A strong El Niño arriving on top of long-term greenhouse warming, with the cut in shipping sulphur from the 2020 fuel rule adding a little more. Together they produced the record global temperatures of 2023 and 2024.

ONI +2.0°C · RONI +1.4°C

NOAA CPCENSO Tracker