Will shifts in the polar vortex cause extreme day-to-day temperature fluctuations to become more common?

Posted on December 29, 2025 by WeatherGuys Editor

The polar vortex is a large area of low pressure in the lower stratosphere that is bordered on its southern edge by the polar night jet — so-called because it develops as the sun sets at high latitudes after the autumnal equinox, creating large and deep pools of cold air. The characteristics of this stratospheric polar vortex have a substantial influence on wintertime temperatures in the lowest part of the underlying troposphere, which is where we all live.

The science behind the polar vortex. (Image credit: NOAA)

The nature of the polar vortex changes throughout the winter. When the vortex circulation is largely west-to-east around the pole, it tends to contain the most extreme cold air masses at high latitudes. When it is characterized by high amplitude waves, often associated with a weaker vortex, it can initiate rapid transport of warm air poleward in some locations and frigid air equatorward in others. Such waves, or lobes, of the polar vortex can pinwheel over the Northern Hemisphere, sending cold air southward in association with weather systems tied to the underlying tropospheric jet stream.

Our global climate is warming because of human activity. Near-surface Arctic temperatures are rising more than twice as fast as those at lower latitudes because of the retreat of snow and ice, which reduces the amount of reflected solar radiation at high latitudes. This is known as “Arctic amplification,” and it reduces the mid-tropospheric temperature contrasts that support a strong, circular polar vortex.

Some research suggests a weaker temperature gradient allows the jet stream to meander more easily, promoting the kind of wave amplification that disrupts the polar vortex. If so, this would indirectly increase the likelihood of a midwinter polar vortex sending cold air south.

So, while global warming is reducing average cold temperatures overall, disturbances in the high-latitude circulation can still create sharp, temporary cold air outbreaks.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Phenomena, Seasons

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Why should we care about NCAR?

Posted on December 22, 2025 by WeatherGuys Editor

News that the Office of Management and Budget in the Trump White House will close the National Center for Atmospheric Research, or NCAR, came earlier this month.

Aerial view of the NCAR-Wyoming Supercomputing Center (Photo credit: NCAR/NWSC)
Aerial view of the NCAR-Wyoming Supercomputing Center (Photo credit: NCAR/NWSC)

NCAR, established in 1960, has provided a unique public/ private partnership in the intervening several decades that has accelerated research and innovation in the weather and climate sciences. It is no exaggeration to say that without NCAR, weather forecasting — where a seven-day forecast made in 2025 is as accurate as a two-day forecast made in 1985 — would not be nearly as advanced as it is today.

Why, you might ask, has the administration put such a remarkable institution as NCAR in its untutored sights? Because it has become, in the opinion of Russell Vought, the Director of OMB, “one of the largest sources of climate alarmism in the country.”

It is a rich, though tragic, irony that people like Vought, who complain about “climate alarmism,” are the very ones about whom we should all be alarmed. There is no question that human beings, mostly through the burning of fossil fuels, have altered the chemical composition of the atmosphere so as to promote a gradual, but irrefutable, warming of the planet. That warming has ramifications on the weather that the atmosphere will deliver in the future.

NCAR has exceptional scientists and support staff dedicated to better understanding the behavior of the atmosphere in both the near-and long-term future. Closing it down puts our nation at risk not only from weather-related natural disasters but from the climate-related disasters that will follow and that have the potential to sow instability across an already troubled world.

This is a short sighted and irresponsible move on the part of our great nation — a nation that, under the current administration, seems blithely determined to surrender global leadership on weather and climate science.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, History, Meteorology, Weather Dangers

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What is the status of the ozone hole?

Posted on December 15, 2025 by WeatherGuys Editor

The ozone hole is a region of exceptionally depleted ozone in the stratosphere over the Antarctic. Each year for the past few decades during the Southern Hemisphere spring (August-October), chemical reactions involving chlorine and bromine cause ozone to be rapidly destroyed. This depleted region is known as the “ozone hole.”

This map shows the size and shape of the ozone hole over the South Pole on the day of its 2025 maximum extent. Moderate ozone losses (orange) are visible amid areas of more potent ozone losses (red). Scientists describe the ozone “hole” as the area in which ozone concentrations drop below the historical threshold of 220 Dobson units. (Image credit: Lauren Dauphine of the NASA Earth Observatory, using data courtesy of NASA Ozone Watch and GEOS-5 data from the Global Modeling and Assimilation Office at NASA GSFC)

Chlorofluorocarbons, or CFCs, are ozone-depleting chemicals that do not occur naturally. They were invented by chemists in 1928 and used as propellants in spray cans, in Styrofoam puffing agents and as coolants for refrigerators and air conditioners. In 1974, these human-made gases were first linked to ozone destruction. The United States banned the use of CFCs in 1978.

On May 16, 1985, British Antarctic Survey scientists published research that revealed a significant drop in ozone levels above Antarctica. In response to this discovery of an “ozone hole,” representatives from 23 nations met in Montreal, Canada. The resulting 1987 Montreal Protocol called for a 50% reduction in the usage and production of CFCs by the year 1999. This and subsequent international agreements, combined with the introduction of substitute chemicals, largely eliminated the use of CFCs worldwide. Although the global use of these chemicals has declined, they are very stable molecules and will stay in the atmosphere for over 50 years.

Satellite instruments provide routine global monitoring of ozone. The size of the Antarctic ozone hole is gradually decreasing. In 2025, the ozone hole was the fifth smallest since monitoring began in 1992. Because of the long life of CFCs, full recovery of the ozone hole to 1980 levels may not happen until 2070.

As the ozone hole continues to decline in size, we should reflect on the fact that it was only possible because of good science, industry adjustments and collective global cooperation over decades of sustained effort.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Uncategorized

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How unusual was our recent snow and what might it portend?

Posted on December 8, 2025 by WeatherGuys Editor

With the exception of a freaky 12.1 inches of slushy snow that fell on Madison on March 25, 2023, the most recent 10-inch snowfall in the city was Dec. 20, 2012 — nearly 13 years ago!

Snowfall 11/29/2025 on Madison’s west side. (Photo credit: M Vasys, CIMSS)

So, it’s been a very long time since we have been visited by the kind of snowfall we saw on Nov. 29-30 — the total over the two days was 11.7 inches. More than that, the 9.3-inch accumulation officially registered at Dane County Regional Airport on Nov. 29 was the largest single-day November snowfall total ever, eclipsing the former record of 8.5 inches that fell on Nov. 30, 1940. So, not only did we finally see a hefty snowfall event after more than a decade of waiting, but we also set an early-season record as well.

Of course, none of this really bears on the complexion of the rest of the winter season. In fact, the month of December 2000 was both unusually cold and snowy, with an average temperature that was 14.1 degrees below normal accompanied by 35 inches of snowfall that was 23 inches above normal for the month. It appeared we were off to a great start for a cold, snowy winter. However, January and February followed with 12.1 inches and 5 inches below normal snowfall, respectively, and a middling 2 degrees above and 3.3 degrees below average in temperature.

So, only time will tell if this early wintry start will translate into a prolonged, robust winter season.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Uncategorized

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What is the largest snowflake?

Posted on December 1, 2025 by WeatherGuys Editor

An ice crystal can grow if the air around it has a relative humidity near 100%. The ice particle grows by water vapor deposition. Growth by deposition is generally slow. If you find nicely shaped snowflakes, they likely were produced by vapor deposition. A snowflake can be an individual ice crystal or an aggregate of ice crystals.

Temperature dependance on formation of ice crystals. (Image credit: Ackerman and Knox, “Meteorology – Understanding the Atmosphere”)

There are four basic shapes of ice crystals: the hexagonal plate, the needle, the column and the dendrite. The dendrites are hexagonal with elongated branches, or fingers, of ice; they most closely resemble what we think of as snowflakes. The temperature at which the crystal grows determines the shape.

Aggregation is the process by which ice crystals collide, get entangled or stick together and form a single larger ice particle. The probability that two crystals will stick together depends on the shape of the crystals. If two dendrites collide, it is likely that their branches will become entangled and the two crystals will stick together. When two plates collide there is a good chance that they will simply bounce off one another. Temperature also plays a role in aggregation. If the temperature of one crystal is slightly above freezing, it may be encased in a thin film of liquid water. If this particle collides with another crystal, the thin film of water may freeze at the point of contact and bond the two particles into one.

The record size for an aggregate snowflake occurred in January 1887 in Fort Keough, Montana, when some flakes were measured at 15 inches in diameter. That is about the size of a family sized pizza pie!

The world’s largest solitary ice crystal measured 10 millimeters, or 0.394 inches, from tip to tip. This dendritic-shaped crystal was photographed by Kenneth G Libbrecht on Dec. 30, 2003, during a gentle snowfall in Cochrane, Ontario.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Phenomena, Severe Weather

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