Wyoming 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

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, Wyoming averaged +0.95°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

Pacific Decadal Oscillation flips to a warm phase

A warm Pacific Decadal Oscillation pushes the winter storm track north of the northern Rockies and Plains, tending to bring milder winters and lighter snowpack. The 1976-77 flip began about two decades of that phase.

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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 Wyoming's winter rainfall - it is not one of the seven states where ENSO reliably moves it. In 1982-83 Wyoming's winter rainfall was close to its 1961-90 average (-8%), with temperatures +2.3°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 Wyoming's 2nd-driest year (290 mm, 28% below its 1961-90 average) with 20.8 heatwave days, its 3rd-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

Mount Pinatubo erupts

Pinatubo's aerosol veil left a clear mark on Wyoming. 1993, two years after the June 1991 eruption, is Wyoming's 2nd-coldest year of the 76 on record and no year since has been colder. 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. It is an Atlantic cycle, but its reach is not confined to the coast: a warm AMO alongside a cool Pacific has been tied to more frequent drought across the western and central United States, including the widespread droughts of 1996 and 1999-2002. 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 Wyoming's winter rainfall was close to its 1961-90 average (+8%), with temperatures +1.5°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 flipped back to a cool phase around 1998-99, which tends to favour colder, snowier winters across the northern Rockies and Plains.

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Colorado River flow decline / Lake Mead crisis

Wyoming is an Upper Division state: the Green River rises here and is the Colorado's largest tributary by volume. Wyoming's stake in the river's decline is as a producer of water rather than a consumer of it - warming reduces what its snowpack delivers downstream even in years when snowfall itself is normal.

Udall & Overpeck 2017

The 2002 Western drought

2002 was Wyoming's 5th-driest year on record here - 304 mm, 25% below its 1961-90 average.

NOAA Drought.gov, NOAA NCEI

Start of the sustained rise in US heatwave frequency

Wyoming's own Heatwave Days (WSDI) count rose from a 1961-90 average of 5.9 days a year to 8.7 in 2016-25 - a modest rise, and one of the smallest in the country against a 50-state average of 2.4x.

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

The 2012 drought and heat

2012 was Wyoming's driest year in this site's 1950-2025 record (278 mm), its warmest year on record, and 14.8 heatwave days.

NOAA NCEI, Summer 2012 Drought Update, NOAA NCEI, Drought

Post-2013 acceleration in US temperatures

6 of Wyoming's ten warmest years on record have occurred since 2013, and 9 of the ten since 2000, with 2012 the warmest of all.

NOAA Climate at a Glance

2015-16 El Niño

In 2015-16 Wyoming's winter rainfall was close to its 1961-90 average (-6%), with temperatures +2.2°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. Wyoming does not follow it: its hottest year is still 2012, the drought year. Six of its own eleven warmest fall in the 2015-2025 span, with 2012, 2007, 2006, 2001 and 1981 outside it. Wyoming is one of 15 states where 2012 still tops the list.

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The 2015 western snow drought

For Wyoming, 2015 was its 3rd-warmest year, its highest heatwave-day count (23.4 days) and its 8th-lowest snow year of 44.

Mote et al. 2016, NOAA Drought.gov, USDA

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 2022 Yellowstone flood

Most of Yellowstone National Park lies in Wyoming, and in June 2022 rain falling on an unusually deep late snowpack put the Yellowstone River about 5 ft above anything in 82 years of records, closing the park entirely. Wyoming's 2022 rainfall was 8% BELOW its 1961-90 average - the flood came from the snowpack, which was the 8th largest in this site's 44-year snow record, melting under warm rain.

NASA Earth Observatory, WyoFile

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