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 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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Despite record warmth, winter is coming

Posted on November 24, 2025 by WeatherGuys Editor

It is a fair bet that we would get near universal agreement that the weekend of November 15-16 was incredibly nice, even the slightly cooler Sunday that followed a truly remarkable Saturday. Both Milwaukee and Madison set all-time record high temperatures for November 15 at 69 and 68 degrees Fahrenheit, respectively.

This chart shows the daily and total snowfall amounts at the Dane County Airport compared with the normal total snowfall. The vertical lines and left axis represent the daily snowfall; the horizontal lines and right axis represent the snow depth, yearly total, and normal total snowfall. (Image credit: Wisconsin State Climatology Office)

Apart from the fact that each city recorded highs that were at least 20 degrees lower the very next day (46 for Milwaukee and 48 for Madison), the weekend was also noteworthy for another meteorological reason: At least for Madison, November 15 is the date on which the probability of precipitation falling as snow first reaches 50%. This means that, from here on out, if precipitation is in the forecast, there is at least an even chance that it will fall as snow. Only 10 days later comes the average date of our first 1-inch snowfall.

So if the thought occurred to you as you enjoyed an almost summery day on November 15 that we were way over our meteorological skis, you were exactly right.

Currently the tropical Pacific Ocean is undergoing a weak La Nina event, which means the ocean temperatures in the central tropical Pacific are slightly cooler than normal. Such an anomaly does not have a very consistent impact on the winter weather over our region, unlike its opposite, the El Nino.

In fact, the latest outlook for this winter (December, January and February), released by the Climate Diagnostics Center on October 16, suggests that we have “equal chances” of being warmer or colder than normal in southern Wisconsin. The same forecast suggests “leaning above” for our precipitation outlook for the winter.

Of course, neither of these outlooks precludes us being visited by either a big snow or a frigid cold air outbreak during our coming winter. However, it may be that the winter will feature only limited appearances by such events.

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, Meteorology, Seasons

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What is lake effect snow?

Posted on November 17, 2025 by WeatherGuys Editor

Lake effect snow forms when cold air passes over the warmer water of a lake. As cold air moves over water, the lower layers are warmed and moistened by the lake below. This makes the air mass unstable. Evaporation increases the moisture content of the air mass, which is then precipitated in the form of snow on the land downwind.

The figure above shows the average annual snowfall for the Great Lakes region. In general,snow depth increases northward. This is expected because temperature usually decreases poleward. The other distinct feature is the difference in the amount of snow along the shoreline. (Image credit: Meteorology; Understanding the Atmosphere by Ackerman and Knox)

Maximum heat and moisture exchanges occur when the air is cold and the temperature difference between the air and the water is large. This condition tends to occur during early winter; this is when the most lake effect snow is produced. A long path across warm water by the air mass results in heavy precipitation over the land.

The longer the path, or “fetch,” the more the evaporation will increase along with greater potential for large snowfall amounts over the land on the downwind side of the lake. Hills can amplify the snowfall amounts by providing additional lifting. The location of a snowbelt along a particular lake is a function of the temperature difference between the air mass and the water, the fetch, and the terrain on the leeward side of the lake.

Lake effect snows are good for the economy of a region, particularly ski resorts. They also provide water for reservoirs and rivers. Too much lake effect snow can be hazardous, however; on October 12 to 13, 2006, Buffalo, New York was blitzed with 22.6 inches of snow in less than 1 day. Because trees had not yet shed their autumn leaves, the snow weighed down and broke tree branches. Nearly 1 million residents lost electrical power at the height of the storm because of falling trees and power lines.

Lake effect snow can bombard a location as long as all the ingredients—cold winds, warm water, and a long fetch—are present.

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, Seasons, Severe Weather

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