The Weather Guys

What happens on the winter solstice?

Posted on December 26, 2023 by WeatherGuys Editor

The winter solstice marks the shortest day and longest night of the year. In the Northern Hemisphere, it occurs when the sun is directly over the Tropic of Capricorn, which is located at 23.5° south of the equator and runs through Australia, Chile, southern Brazil, and northern South Africa. (Image credit: NOAA/National Weather Service)

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.

The December solstice marks the longest night and shortest daylight of the year for those living in the Northern Hemisphere. For the Southern Hemisphere, it marks the shortest night and longest daylight. Although daylight hours increase during the winter months, temperatures continue to fall because Earth cools down and heats up slowly. The overall energy balance is still negative, with more heat lost at night than is regained during the daytime.

Meteorologists mark the start of our Northern Hemisphere winter as Dec. 1, with winter covering the three months of December, January, and February. These are the coldest months of the year. This means the time when daylight gets longer occurs during the Northern Hemisphere meteorological winter. Slowly at first, but at increasing daily intervals, as spring approaches.

For the Southern Hemisphere, meteorological winter covers the months of June, July, and August. For the Southern Hemisphere, the winter solstice occurs around June 21.

A day is not just the period of daylight. A day defined relative to the sun is called a solar day; it is the time from one local noon to the next. A sundial, correctly orientated, will indicate the precise moment of local solar noon, when the sun reaches its highest point in the sky each day. The longest solar days, as measured from one solar noon to the next, occur each year around the December solstice, no matter where you live.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. 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 the probability of a white Christmas in Madison?

Posted on December 18, 2023 by WeatherGuys Editor

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.

Last year, we had a fairly fresh 6 inches of snow on the ground on Christmas morning, courtesy of a crippling storm that affected the eastern half of the United States just days before the holiday. In fact, during the past 25 years we have done better than average in terms of white Christmases, as 15 of the past 25 years — 60% — have qualified. While 2004 met exactly that 1-inch threshold, both 2008 and 2000 welcomed Christmas Day with 15 inches of snow at Dane County Regional Airport, the latter in what was the snowiest December in Madison history.

Unfortunately, it appears this year will not qualify. Just a week out there is nothing promising in the forecast — that is, no storms on the horizon. This immediate circumstance, coupled with a robust El Niño ongoing in the equatorial Pacific Ocean may well doom any chance of a white Christmas this year — and may extend the unwelcome snowlessness through a good deal of the winter.

Typical winter weather patterns during El Niño. Credit: NOAA

So, we can hope for some snow before next Monday, but it is probably best to proceed toward the holiday with a realistic outlook in that regard.

Category: History, Seasons

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Why is the wind often calmer at night than during the day?

Posted on December 11, 2023 by WeatherGuys Editor

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.

When the sun sets, the ground loses the solar energy from the sun and continues to lose energy via emission of longwave radiation. This cooling ground conducts heat away from the air near the ground, causing that air layer to cool down faster than the layers higher in the atmosphere. This creates a stable area with cool air near the ground and warmer air above. As the word suggests, “stable” means it is difficult to move the air layer, keeping the fast-moving air above from mixing down to the surface. This is called a decoupling: This layer is no longer influenced by what is occurring above it.

Calm conditions due to decoupling are evident in this sunset photo of Lake Mendota taken by Jonathan Gero in 2010.

On many calm nights, there is still wind blowing far overhead. When the sun is up, it warms the surface of Earth, which in turn warms the atmosphere above it. The warm air rises and the displaced air is replaced by the air above. These warm thermals mix up the air, bringing the faster moving air from above down near the surface. As the daytime heating goes on, more air from above is mixed down and the wind speed picks up. This results in turbulence and mixing of the air near the ground.

Of course, if there is a low-pressure area or fronts in the region, the winds will likely blow day or night.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Phenomena

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Does fall give any hints about the intensity of the coming winter?

Posted on December 4, 2023 by WeatherGuys Editor

Average temperatures were above normal and precipitation was below normal during the September through November timeframe this year in Madison. (Image credit: NWS-Sullivan)

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.

Another interesting aspect of this observational record arises from examining the buildup of the cold air leading into the winter. So, we have also kept track of the areal extent of such air from Sept. 1 to Nov. 30, SON, over the same 75 seasons. This year’s entry into that time series turns out to have been the sixth-warmest of all time — that is, it had the sixth-smallest areal average for SON.

A warm SON by this measure does not always mean a warm DJF in the following season. In fact, if one ranks the 75 SON seasons from warmest to coldest and then ranks the DJF seasons from warmest to coldest, there are years in which a warm SON is followed by a cold DJF and vice versa. Such years would be characterized by what could be termed a large “rank shift” — that is the ranking in one list is very different from the ranking in the other for the same season.

The largest rank shift in which the winter was colder than the fall occurred in 2002-03, when the fall was the 18th-warmest but the following winter was only the 58th-warmest, a rank shift of 39 places. Close on its heels but in the other direction was 2011-12, in which the fall was cold (70th-warmest) and was followed by the 17th-warmest DJF of all time — a rank shift of 53 places.

Current research is investigating whether or not there are systematic signs of a circulation change across the hemisphere that might give an early indication of whether or not a given year may experience such a dramatic change from fall to winter.

Category: Climate, Seasons

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How can there be frost on the ground when my thermometer reads 36 degrees?

Posted on November 27, 2023 by WeatherGuys Editor

Temperature inversions near the ground enable phenomena like frost and fog on cool fall mornings. Photo credit: John Lalande

A reader asked us about an observation she made at her home last week. She reports that her thermometer read 36 degrees Fahrenheit, but there was clearly frost on the front lawn.

This set of circumstances does not mean that her thermometer is faulty and in need of replacement. Instead, it reflects a nearly daily reality that goes undetected for most of the year until the cold season. It turns out that the air does not radiate heat away nearly as well as the solid ground beneath it. As a consequence of this difference, given 13 hours of nighttime with clear skies, the ground radiates a lot more energy away (and cools rapidly) while the air above struggles to cool as efficiently. Over those many hours, this difference results in a big difference between the ground temperature and the air temperature even as little as 5 or 6 feet above the ground.

On almost every clear, windless morning throughout the year you could measure such a temperature structure, known as an inversion, in the lowest 6 or so feet of the atmosphere. In the late fall, the colder ground may be below 32 degrees and permit frost to form while the air just above it, at the thermometer level, could be several degrees warmer than that. Hence, it is not at all unusual for a set of seemingly contradictory observations, like those reported by our reader, to occur at this time of year.

Steve Ackerman and Jonathan Martin, professors in the uW-Madison department of atmospheric and oceanic sciences, are guests on WHa radio (970 aM) at 11:45 a.m. the last Monday of each month. send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.