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Category Archives: Meteorology
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 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
Despite record warmth, winter is coming
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.
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. Continue reading
What is lake effect snow?
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.
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. Continue reading
