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Category Archives: Meteorology
Why does the morning low temperature sometimes occur after sunrise?
Those who keep a particularly vigilant eye on the weather might have noticed that on certain mornings, in all seasons, the lowest temperature is often recorded just after sunrise.
One of us had a morning paper route as a boy and was puzzled for years about this seemingly counterintuitive phenomenon. How could the temperature continue to fall for the few minutes after sunrise on a cold winter morning? Continue reading
Category: Meteorology, Phenomena
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Why is Wisconsin winter weather so cloudy and grey?
Climatologically, the winter months are the cloudiest time of the year in Wisconsin. Wisconsin’s average monthly cloud cover for December, January, and February for the years 2006 to 2024 was 65.3%, 61.1% and 50.5% respectively. There are a few factors that contribute to this climate observation.
Wisconsin sits in a latitude band where synoptic scale storm systems often pass nearby. This scale is associated with large weather patterns like mid-latitude cyclones and anticyclones. During the winter, the mid-latitude storm tracks tend to be along the polar front and the upper tropospheric jet stream. These winter storms have large stratiform cloud shields that move across the Midwest, producing more persistent cloudy conditions. Even when storm centers miss the state, these cloud shields can linger for days afterward. Continue reading
Category: Climate, Meteorology, Seasons
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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
