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Monthly Archives: December 2025
Will shifts in the polar vortex cause extreme day-to-day temperature fluctuations to become more common?
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 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. Continue reading
Category: Meteorology, Phenomena, Seasons
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Why should we care about NCAR?
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.
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. Continue reading
Category: Climate, History, Meteorology, Weather Dangers
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What is the status of the ozone hole?
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.”
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. Continue reading
How unusual was our recent snow and what might it portend?
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!
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. Continue reading
What is the largest snowflake?
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.
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. Continue reading
