Pennsylvania 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 summer of 1953

1953 gave Pennsylvania 14.1 heatwave days, its 3rd-highest of the 1950-2025 record, though it ranks only 14th-warmest by annual mean - a summer event, not a warm year. 40 Pennsylvanians died of heat in 1953, the third-highest state toll of that summer.

Northeast Regional Climate Center, LIFE

The 1962-66 Northeast drought

1963 was Pennsylvania's driest year in this site's 1950-2025 record - 811 mm, 24% below its 1961-90 average. It was not one dry year but five: 1962-66 as a whole averaged 16% below that baseline.

USGS, Seager et al. 2012, NYC Emergency Management

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, Pennsylvania averaged +1.20°C warmer across 2006-2025 than across the two decades before the Act, 1951-1970, the 16th-largest rise of the 49 states measured and above their median of +1.09°C. Cleaning the air was right for human health. It also uncovered warming that was already there.

US EPA

Hurricane Agnes

Pennsylvania took the worst of Agnes in June 1972: 8-10 inches of rain over four days, with isolated totals near 18 inches, and Susquehanna River discharges higher than any since European settlement, exceeding the 100-year recurrence level at many gauges. Wilkes-Barre and Harrisburg were both inundated. It was the most destructive hurricane in US history at the time.

USGS

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

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 Pennsylvania's winter rainfall - it is not one of the seven states where ENSO reliably moves it. In 1982-83 Pennsylvania's winter rainfall ran -28% against the 1961-90 average, with temperatures +2.8°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

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

1990 is Pennsylvania's 10th-warmest year in this site's 1950-2025 record, and 1991 its 9th-warmest. 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 does not show in Pennsylvania's own record: 1993, two years after the June 1991 eruption, was an unremarkable year there, 33rd-coldest of 76. A single large eruption can hold temperatures down for two or three years; it cannot reverse the trend.

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Atlantic Multidecadal Oscillation flips to a warm phase

The Atlantic Multidecadal Oscillation flipped to a warm phase in 1995, and for the Northeast its reach is through the Atlantic hurricane season: the warm phase since includes Irene in 2011 and Sandy in 2012. 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 Pennsylvania's winter rainfall ran +28% against the 1961-90 average, with temperatures +4.4°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 1999-2002 Eastern drought

2001 was Pennsylvania's 4th-driest year on record here - 872 mm, 18% below its 1961-90 average.

USGS New England Water Science Center, Vermont State Climate Office, NOAA Drought.gov

The July 1999 heat wave

Over the Independence Day weekend of 1999, highs near 100°F and heat indices around 110°F gripped eastern Pennsylvania for three days. NOAA's storm record links 74 deaths to the heat, most in and around Philadelphia.

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

The wet year of 2003

2003 was Pennsylvania's 5th-wettest year in this site's 1950-2025 record - +28% against its 1961-90 average. It is one of three exceptionally wet years the Mid-Atlantic has had this century, alongside 2018 and 2020.

NOAA NCEI

Start of the sustained rise in US heatwave frequency

Pennsylvania's own Heatwave Days (WSDI) count rose from a 1961-90 average of 4.5 days a year to 7.1 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 Northeast's record summer of 2010

2010 gave Pennsylvania 13.4 heatwave days, its 4th-highest of the 1950-2025 record, alongside its 15th-warmest year overall.

NOAA Climate.gov, NOAA NCEI, Northeast Regional Climate Center

Hurricane Sandy (Superstorm Sandy)

Pennsylvania was hit well inland: Sandy held together far past landfall, bringing damaging winds across the south-east, heavy rain, and heavy wet snow in the Laurel Highlands. Millions lost power. The storm's sheer diameter - tropical-storm-force winds spanning about 1,150 miles - is why an inland state was affected at all.

NWS Mount Holly, NJDEP, Britannica

2012: the warmest year on record for the contiguous US

2012 is Pennsylvania's 2nd-warmest year in this site's 1950-2025 record. Nationally it was the warmest year the contiguous United States had recorded, 3.2°F above the 20th-century average, with a record-warm March that broke more than 15,000 daily records; 19 states set record-warm years.

NOAA Climate.gov, NOAA Climate.gov (2)

Post-2013 acceleration in US temperatures

5 of Pennsylvania'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 2024 the warmest of all. The acceleration is clear in Pennsylvania'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 Pennsylvania's 11th-coldest year in this site's 1950-2025 record. 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 Pennsylvania's winter rainfall ran +26% against the 1961-90 average, with temperatures +3.7°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. Pennsylvania's own record is a looser fit - six of its eleven warmest fall in that span, with 2012, 2006, 1998, 1991 and 1990 outside it. Its warmest year is 2024.

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2018: the wettest year on record across the East

2018 was Pennsylvania's wettest year in this site's 1950-2025 record - 1627 mm, 53% above its 1961-90 average and comfortably clear of 2011 (1549 mm), the previous highest. Heavy rain in the lower Susquehanna Valley in late July was one of the events that did it; Pittsburgh recorded its wettest year on record.

NOAA NCEI, NOAA Climate.gov, NOAA NESDIS

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

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