What is thundersnow?

Posted on February 8, 2016 by WeatherGuys Editor
A pedestrian on the UW-Madison campus makes his way through a line of snow-covered trees Tuesday as a winter storm moves through the area. The storm was accompanied by thundersnow - lightning and thunder that occur during a snowstorm. (Photo credit: John Hart, State Journal)

A pedestrian on the UW-Madison campus makes his way through a line of snow-covered trees Tuesday as a winter storm moves through the area. The storm was accompanied by thundersnow – lightning and thunder that occur during a snowstorm. (Photo credit: John Hart, State Journal)

Lightning is always accompanied by thunder. Lightning is a huge electrical discharge. Static charges form in a storm composed of ice crystals and liquid water drops. Turbulent winds inside the storm cause particles to rub against one another, causing electrons to be stripped off, making the particles either negatively or positively charged.

The charges get grouped in the cloud, often negatively charged near the bottom of the cloud and positively charged up high. This is an electric field, and because air is a good insulator the electrical fields become incredibly strong. Eventually a lightning bolt happens to neutralize the electric field.

The lightning bolt rapidly heats the air around it, to as hot as 50,000 degrees. This rapid warming causes the air to quickly expand and generate a sound wave.

That sound wave is thunder. Sound can interact with objects in multiple ways. Snowfall, and snow on the ground, tends to muffle the sound.

Thunder is common in the summer when cumulonimbus clouds have strong updrafts of warm, moist air that can rise upward to 40,000 feet or more. The air is above freezing near the ground and far below freezing near the top of the updrafts. This provides for lots of opportunity to generate an electric field.

Thunder is not common in winter as the storms are shallower than the summer convection. Most of the precipitation in winter occurs below about 20,000 feet and the turbulent motions are weaker. The colder temperatures also make for few liquid water drops in the winter storms.

Category: Phenomena, Severe Weather

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What is Groundhog Day and do the forecasts work?

Posted on February 1, 2016 by WeatherGuys Editor
Then-mayor Jon Freund, left, leans in for Jimmy the Groundhog's prognostication on Feb. 2, 2015 -- just before he was famously bitten on the ear by the groundhog during Sun Prairie's Groundhog Day celebration. The roots of Groundhog Day date back to the 6th century. (Photo credit: Associated Press)

Then-mayor Jon Freund, left, leans in for Jimmy the Groundhog’s prognostication on Feb. 2, 2015 — just before he was famously bitten on the ear by the groundhog during Sun Prairie’s Groundhog Day celebration. The roots of Groundhog Day date back to the 6th century. (Photo credit: Associated Press)

Groundhog Day is an example of predicting the weather based on folklore. If the groundhog comes out of its hole and sees its shadow, we are in store for 40 more days of winter. Of course, after Feb. 2, there are only 47 days left of astronomical winter – which ends on or about March 21.

The roots of Groundhog Day go back to the 6th century. Feb. 2 is 40 days after Christmas and is known as Candlemas. On this day, candles that are used for the rest of the year are blessed. This is also about the mid-point in winter, in meteorological not astronomical terms.

The forecast rhyme goes:

If Candlemas Day is bright and clear,

There’ll be two winters in that year;

But if Candlemas Day is mild or brings rain,

Winter is gone and will not come again.

If the day is bright and clear, the groundhog “sees” his shadow and we have more winter. Of course, the weather conditions on Feb. 2 at single locations like Sun Prairie or Punxsutawney, Pennsylvania, tells us nothing about the weather for the rest of the winter season.

As for accuracy — the “predictions” made by the various rodents involved in this annual event are correct about 40 percent of the time — vastly inferior to what is delivered by modern science. Right or wrong, they are fun community celebrations.

Category: Climate, Meteorology, Seasons

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Was the Blizzard of 2016 forecasted well in advance?

Posted on January 27, 2016 by WeatherGuys Editor
A wide corridor of snow can be seen in this stunning nighttime view of the U.S. east coast in the wake of the winter storm from Suomi NPP VIIRS Day/Night Band at 0647 UTC, 24 Jan 2016, courtesy of CIMSS/SSEC at UW-Madison

A wide corridor of snow can be seen in this stunning nighttime view of the U.S. east coast in the wake of the winter storm from Suomi NPP VIIRS Day/Night Band at 0647 UTC, 24 Jan 2016, courtesy of CIMSS/SSEC at UW-Madison

The blizzard that affected over 80 million Americans from the Deep South to New England over the weekend was a historic storm in many ways.

Perhaps most obviously, the snowfall totals that it delivered in the so-called Megalopolis (the stretch of cities from Washington, D.C., northeast to Boston) equaled or surpassed records in many locations.

Final snowstorm totals varied across major cities on the East Coast, the National Weather Service reported, from 22.4 inches at Philadelphia International Airport, to 27.9 inches at LaGuardia International Airport and a staggering 36 inches in Maugansville, Maryland.

Other locations that don’t often see large snowfall totals — such as eastern Kentucky, which received more than 15 inches of snow — were also hammered by this storm. Additionally, Weather Storm Jonas was the largest snowstorm on record for Harrisburg, Pennsylvania and Baltimore, Maryland and at JFK Airport in New York City.

The fact that the storm was so accurately predicted many days in advance is also noteworthy. Though not the first such accurate forecast in history, it is a high-profile example of the quiet revolution in weather forecasting that has taken place over the last 30 or so years.

The combination of increased observing capabilities (much of that increase in the form of satellite data), increased understanding of the physical processes that produce the weather, and increased computing power has greatly improved the weather prediction enterprise that now informs so much of our decision making in advance of, and in the face of, such extreme weather emergencies.

Such forecasts are the fruits of sustained commitment and investment in basic research in satellite meteorology (of which UW-Madison is both the originator and the world leader), dynamic meteorology and computer technology provided by research grants over decades.

Category: Meteorology, Severe Weather, Weather Dangers

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Are hurricanes in January typical occurrences?

Posted on January 19, 2016 by WeatherGuys Editor
This photo, taken from video provided by NASA on Friday, shows Hurricane Alex seen from the International Space Station. The rare January hurricane in the Atlantic closed in on the Azores on Friday with authorities in the Portuguese islands warning of waves up to 60 feet high, wind gusts up to 100 mph and torrential rain.

This photo, taken from video provided by NASA on Friday, shows Hurricane Alex seen from the International Space Station. The rare January hurricane in the Atlantic closed in on the Azores on Friday with authorities in the Portuguese islands warning of waves up to 60 feet high, wind gusts up to 100 mph and torrential rain.

While rare, there have been other hurricanes in the month of January. This is the fourth storm since record keeping started in 1851. Before Alex, the most recent January hurricane, Alice, was in 1955. Another unique aspect of Alice was that it formed on Dec. 30, 1954 and lasted until Jan. 6, 1955.

Why are January hurricanes rare?

Hurricanes form over warm waters. The evaporation of the warm ocean waters condenses to form clouds and precipitation releasing latent heat energy that helps to maintain the storm. A general rule of thumb is that hurricanes will not form unless the water temperature is at least 80 degrees.

Evaporation is a function of the temperature difference between the air and water. The larger the temperature difference the greater the evaporation. The Northern Atlantic Ocean is typically not that warm in January.

Alex formed over waters whose temperatures were about 68 degrees, certainly below our rule of thumb. But, about 6 miles above, the air was very cold, at minus 76. The 144-degree difference between the air and water temperature, along with small vertical wind shear, helped the storm to develop into a hurricane.

Unfortunately, this early storm does not allow us to make a forecast about the upcoming season.

Category: Meteorology, Phenomena, Severe Weather

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What’s really going on in that slush puddle?

Posted on January 11, 2016 by WeatherGuys Editor
Though a slush puddle may look like a static, unchanging, boring and ugly mixture of dirty ice and water, it is actually an extremely dynamic cauldron of various phases of the magical water substance, constantly undergoing all manner of phase changes. Photo credit: Steve Apps, State Journal archives.

Though a slush puddle may look like a static, unchanging, boring and ugly mixture of dirty ice and water, it is actually an extremely dynamic cauldron of various phases of the magical water substance, constantly undergoing all manner of phase changes. Photo credit: Steve Apps, State Journal archives.

The recent snow and ice followed by a bit of melting filled sidewalks and some city streets with puddles of slush — that wondrous winter concoction of liquid water and ice.

Though it is not possible to see, surrounding any puddle of slush is also a cloud of countless invisible water vapor molecules. In fact, of all the many chemical constituents of the Earth’s atmosphere, only the water substance can naturally occur in all three of its phases – solid, liquid and gas – at Earth temperatures.

And, to add an even greater level of grandeur to this fact, the ice and the liquid water in the slush puddle are constantly subject to sublimation (transitioning from ice to vapor) and evaporation, respectively, while the invisible vapor is constantly liable to deposition (transitioning from vapor to ice with no intermediate liquid stage) and condensation.

So even though the slush puddle looks as if it is a static, unchanging, boring and ugly mixture of dirty ice and water, it is actually an extremely dynamic cauldron of various phases of the magical water substance, constantly undergoing all manner of phases.

These phase changes involve substantial amounts of energy transfer as well. In fact, for a gram of liquid water to evaporate to vapor, 600 calories of energy are required from the environment. When a gram of water vapor condenses, that same amount of energy is given back to the environment.

A scientist’s view of the world need be no less lyrical than a poet’s — we hope you will never see slush puddles the same again.

Category: Meteorology, Seasons

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