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Monthly Archives: December 2023
What happens on the winter solstice?
Astronomical winter begins at the winter solstice, which happened this year for the Northern Hemisphere at 9:27 p.m. Central Standard Time on Thursday, Dec. 21. Winter south of the equator begins in June.
The Northern Hemisphere winter solstice occurs at the time the sun is at its southernmost point in the sky. At this time, the sun is overhead at noon on the Tropic of Capricorn, approximately 23.4 degrees south of the equator. Continue reading
What is the probability of a white Christmas in Madison?
Believe it or not, the National Weather Service has criteria for declaring a Christmas white: If there is at least 1 inch of snow on the ground at 6 a.m. on Christmas Day, the year has registered a white Christmas.
As might be expected, the probability of this condition being met varies widely across the state of Wisconsin. In the far north, the probability exceeds 90% and approaches a certainty in some locations, while in the south the historic probability runs at about 40%, which might strike Madisonians as unexpectedly low. Continue reading
Why is the wind often calmer at night than during the day?
The cycle of daytime heating and night time cooling explains why, under most circumstances, calm winds are near the surface at night.
The wind usually increases with height above Earth’s surface. The wind several thousand feet above the ground is almost always stronger than that experienced near the ground. Friction causes the wind close to the ground to move more slowly. Friction decelerates the wind in the same way a rough road surface slows down a bicycle. Continue reading
Category: Meteorology, Phenomena
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Does fall give any hints about the intensity of the coming winter?
Each winter we keep track of the areal extent of air colder than 23 degrees Fahrenheit at the 850 mb pressure level (about 1 mile above sea-level) around the entire Northern Hemisphere. This measure allows us to characterize the intensity of the winter season with respect to the lower tropospheric temperature.
Over the past 75 seasons there has been a systematic decrease in the December-January-February, or DJF, average areal extent of about 4.6%, and this is an unequivocal sign of global warming, measured in the winter season. Continue reading
