The Weather Guys

The winter solstice (In Latin, sol, “Sun,” and stice, “come to a stop”) is the day of the year with the fewest hours of daylight. In 2015, this occurs for the Northern Hemisphere on Dec. 21 at 10:48 p.m. CST.

As Earth orbits the sun, its axis of rotation is tilted at an angle of 23.5 degrees from its orbital plane. Because Earth’s axis of spin always points in the same direction — toward the North Star — the orientation of Earth’s axis to the Sun is always changing as Earth orbits around the Sun.

As this orientation changes throughout the year, so does the distribution of sunlight on Earth’s surface at any given latitude. This links the amount of solar energy reaching a location to the time of year and causes some months of the year to always be warmer than others — in other words, the seasons.

On the Northern Hemisphere’s winter solstice, the northern spin axis is pointed away from the Sun and latitudes north of the Arctic Circle (66.5°N) have 24 hours of darkness.

The winter solstice is often referred to as the first day of winter. But there are other definitions of winter.

For example, the beginning of winter might be defined on the calendar day, on average, when precipitation has an equal chance of falling as rain or snow. For Madison, that calendar day is Nov. 15.

Meteorologists often define the three months of winter as December, January and February — the coldest months of the year.

Kayla Powell of Lacrosse makes her way past a collection of ice heaves on Lake Mendota in February. With this month’s warm temperatures, it could be a while before the lake freezes again. Photo credit: John Hart, State Journal archives

The climatological ice-on date for Mendota is Dec. 19 but the actual ice-on date in any given winter is highly variable. Given our warm autumn this year, it is likely we will experience a later than normal ice-on date.

Right now the lake’s water temperature is about 7C (45 degrees Fahrenheit) and, importantly, that temperature is nearly uniform throughout the water column. Such uniformity in temperature started about Oct. 1 this year, according to data from the Integrated Nowcast/Forecast Operation System (INFOS) for the Yahara Lakes (www.infosyahara.org/temp_mendota). At that time, the water temperature was about 15C (59 Fahrenheit). Thus, in the past two months, the lake water has cooled by 8C (14 degrees Fahrenheit).

Such cooling requires an enormous amount of energy to be extracted from the lake. Knowing the size of the lake (39.85 million square meters), its average depth (12.7 meters) and the density of water (1000 kg per cubic meter), we know that 507 billion kg of water reside in Lake Mendota. Given the 14-degree temperature change and the fact that 4200 joules of energy must be extracted from each kilogram of water to lower its temperature 1C, we know that 1.7 quadrillion joules of energy have been extracted from the lake since Oct. 1.

Since there are 3.6 million joules in every kilowatt hour (kwH) and the Madison metro area uses about 79 million kwH of energy on a typical January day, we can calculate that the amount of energy thus far extracted from Lake Mendota to get it to its current temperature is enough to power the entire Madison metro area for nearly 60 winter days.

Additionally, it is not until the water temperature cools another 3C (requiring extraction of the equivalent of another 23 days of energy) that the lake is even ready for the additional surface cooling that eventually leads to freezing. It is easy to overlook the fact that truly enormous amounts of energy are being transferred from water to atmosphere before our eyes.

Atop an Antarctic mountain

No, it is never too cold to snow.

It snows in Antarctica — where temperatures are minus 70 degrees — though only a few tenths of an inch.

To get snow, the always-present water vapor in the atmosphere has to be converted to ice crystals. How much water vapor is in the atmosphere depends on the air temperature.

The maximum amount of water vapor in the atmosphere is a function of the air temperature. So the colder the air, the less water vapor is present in the atmosphere. Therefore, large snowfall amounts are not associated with extremely cold temperatures.

Snowfall requires vertical motion in the atmosphere, or lifting. Warm moist air above the surface may be lifted by a front. As the air rises it expands and cools because air pressure decreases with altitude.

As the air rises, expands and cools, the relative humidity in the atmosphere increases and eventually cloud particles form. If the air is below freezing they will be ice crystals. As the lifting continues the crystals grow and fall from the cloud, and if they reach the surface, that is snow.

Snowfall can occur at very cold conditions as long as there is some source of moisture and some lifting. Very cold days in Wisconsin often are associated with high-pressure weather systems where downward motions prevail, preventing the formation of clouds and precipitation.

Heavy snowfalls generally occur around 15 degrees or warmer, though still below freezing, since the warmer air generally has more water vapor.

Suka the polar bear enjoys her first Wisconsin snowfall in the Arctic Passage exhibit at Vilas Zoo on Saturday. Accurate and precise measurement of snow accumulation Is a difficult task. The trick is finding a good place to measure and a firm surface upon which to set your ruler. Photo credit: Amber Arnold, Wisconsin State Journal

The trick in measuring snow consistently is simply finding a good place to measure and a firm surface upon which to set your ruler.

At official stations, a snow board (a square piece of wood 16 inches on a side and painted white) is employed.

Other surface options are wooden decks, picnic tables, and cars. Your measurement location should be 20 to 30 feet away from the house with an unobstructed view of the sky.

Sidewalks are not recommended as they tend to accelerate melting of the snow.

Grass is also suboptimal as snow tends to sit up on top of the blades of grass, while the ruler goes down to the ground.

When the wind blows and the snow drifts, accurately measuring snow accumulation is even more challenging as drifting becomes a problem.

To deal with drifting snow, measurements must be made at various places and then averaged to get what is considered a representative measurement.

The liquid equivalent of accumulated snow can be measured with the standard rain gauge — an 8-inch diameter cylinder that collects precipitation. The conversion from liquid equivalent to snow depth varies with every storm and is broadly temperature dependent.

Thus, it is really difficult to re-create a snowfall distribution from liquid equivalent observations over an area even as small as Dane County.

Discussions are underway currently to terminate the snowfall recording at Dane County Regional Airport.

This would be a terrible blow to the climate record of Madison, and we sincerely hope a new agreement can be worked out between the FAA and other interested parties.

We will at least have official records through this snow season.

Our storm on Wednesday night and Thursday of last week was the first strong storm of the autumn/winter season. As you found yourself caught in the strong winds, you may well have wondered how do storms like this one come to be.

That has been the central motivating question in meteorological science for most of the past 100 years.

During that time meteorologists have learned a great deal about how such storms are formed. In most instances, two or more days before the storm is noticed at the surface of the Earth, processes are at work in the upper troposphere.

Specifically, at the height of the jet stream (about 6 miles above the surface), weak downward vertical motions begin to drag the layer of the atmosphere known as the tropopause downward into the middle troposphere, the lowest layer of the Earth’s atmosphere where all of its weather occurs.

This process eventually results in the creation of a mid-level vortex, a region of counterclockwise rotating winds, at about 3 miles above the ground. Once generated, this vortex is then moved around by the atmospheric winds in its vicinity.

At the forward side of this moving vortex, the air is forced to rise. Such upward vertical motion evacuates air from the lower troposphere, lowering the pressure at the surface. Simultaneously, the upward vertical motion produces clouds and precipitation.

So long as the mid-level vortex continues to intensify and move, so too does the surface cyclone. In many cases, the mid-level vortex eventually becomes quasi-stationary and positioned directly above the surface cyclone. This usually marks the end of the intensification for the storm though it can still deliver high impact weather at this stage.