Why has it snowed nearly every day recently?

Posted on February 13, 2013 by Weather Guys Editor

Lately our fair city has been the recipient of continuous dustings of light snow evoking images of a Brueghel painting. In fact, Thursday was the ninth consecutive day with measurable snow, tying a record set only one other time (Feb. 23-March 3, 2007) in Madison’s history.

One might wonder if it is unusual to have such a parade of frequent light snowfalls. As it turns out, the large-scale setup that has brought our current snow streak is quite normal for the winter around here. Nearly every winter the western Great Lakes states experience extended periods in which the airflow in the middle part of the troposphere (about two to three miles above the surface) is from the northwest, with origins in arctic Canada.

Embedded within this northwesterly flow are barely perceptible vortices that are carried rapidly southeast. Such vortices are enhanced by passage over the Canadian Rockies in Alberta. These mid-level vortices are capable of creating rising air which, in turn, produces clouds and precipitation.

Since the air that is forced to rise is cold and quite dry, there is usually not sufficient water available to these so-called Alberta Clippers to create big snowfalls — just lots of small ones. It has long been our sense that the total seasonal snowfall in Madison (approximately 40 inches each year) is largely the result of many small snowfalls and very few more substantial ones. This is consistent with the prevalence of cold northwesterly flow that characterizes our winters.

Category: Seasons
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What are ice pellets?

Posted on February 4, 2013 by Weather Guys Editor

Ice pellets are a form of precipitation. They are small, translucent or clear balls of ice. Ice pellets are rain drops that have frozen before they hit the ground. When they hit the ground, they bounce. Ice pellets are also called sleet and can be accompanied by freezing rain.

In winter, precipitation usually begins falling out of a cloud as ice particles. If the temperature underneath a cloud stays below freezing all the way to the ground, the ice crystals never melt and snow falls. If the temperature is above freezing below the cloud bottom to the ground, the frozen particles melt into liquid droplets that reach the surface and this is called rain.

Ice storms occur when precipitation particles melt and then fall through a layer of cold air near the ground. Sleet results when the layer of subfreezing air at the surface extends upward far enough so that raindrop freezes into a little ball of ice. Freezing rain forms when a very shallow layer of cold air at the surface causes freshly melted raindrops to freeze on contact with exposed objects on the ground, whose temperature is below freezing.

Another type of precipitation is graupel. Ice crystals may encounter small drops of water whose temperatures are below freezing.  These supercooled liquid droplets freeze when they come into contact with the snow crystal. When this process continues so that the shape of the original snow crystal is no longer identifiable, the resulting ice particle is called graupel.  Graupel is brittle and will fall apart when it strikes the ground.

On Sunday afternoon, Jan. 27, Madison experienced rain, freezing rain, sleet and snowfall all in one day.

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How cold has this winter been, historically?

Posted on January 28, 2013 by Weather Guys Editor

Wednesday is the 62nd anniversary of the coldest day ever in Madison — Jan. 30, 1951 — when the temperature dropped to minus 37 F.

As we described in a column a couple of years ago, to get the air that desperately cold in Madison a healthy snow cover is necessary. This January, we had a warm spell near mid-month that effectively melted most of the snow remaining from our December snowstorm. Thus, even though last week’s cold was the worst of the season thus far, it was a lot less cold than it might have been had it occurred while we had a deep, fresh snow cover.

Looking ahead for the rest of the season, we are nearing winter’s coldest point, climatologically (approximately Feb. 1), and until or unless there is fresh snow on the ground we are likely to escape the kind of cold that keeps the temperature below zero throughout the day. In fact, in the last five years there have been only two days on which the daily high temperature has been below zero — Jan. 15, 2009, and Jan. 19, 2008 — both with a high of minus 2 F in Madison.

In a related item of local interest pertaining to the severity of the winter thus far, state climatologist Dr. John Young officially declared two weeks ago that Lake Mendota froze over on Jan. 14, the third-latest date in an unbroken 159 years of observation.

This late date was a combination of the very warm summer and relatively warm autumn that allowed anomalous amounts of heat to be stored in the lake as winter approached. Then, given the lack of truly cold air in our area until recently, it has taken longer for the lake to give up its excess heat. These factors led to the very late freezing date this year.

Category: Climate
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What is clear air turbulence?

Posted on January 22, 2013 by Weather Guys Editor

Clear-air turbulence, abbreviated “CAT,” occurs high in the atmosphere at the cruising altitude of passenger jets. CAT happens in clear sky conditions and where the wind direction and speed changes quickly with height. If you flew on a plane that experiences a jarring bumpiness and yet there are blue skies out the window, then you’ve experienced CAT. It is most common over mountains, near fronts and around the jet stream. It is also more common in winter than summer.

CAT is a major challenge for weather forecasters, particularly those who support aviation industries. Rules-of-thumb exist to steer airplanes around likely areas of CAT; however, airplanes fly into turbulence on a daily basis. In fact, a common way of knowing that CAT is present is for a plane to accidently fly through it and then report the experience.

CAT results in tens of millions of dollars in aircraft damage, as well as passenger and crew injuries. On Jan. 16, three people were injured on a flight over southern Illinois and Indiana because of severe turbulence. On Dec. 28, 1997, a flight from Tokyo to Honolulu experienced heavy turbulence over the Pacific Ocean, killing one person as the result of severe head trauma and injuring at least 102 passengers. Now you know why the airlines tell you to keep your seat belt fastened tightly at all times.

What does CAT look like? By definition, you can’t see it but new methods of identifying CAT from satellite observations are being developed and tested at the UW-Madison. Hopefully we’ll have some smooth flying in the not-too-distant future.

Category: Phenomena
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How does weather variability affect our economy?

Posted on January 17, 2013 by Weather Guys Editor

Weather has a big impact on our economy at all levels — local, state and national. In our local economy, weather influences various business decisions, such as when to plant or harvest, when to pour concrete or shingle a roof in construction projects, or in predicting peak demand for electricity or gas for home heating.

Weather impacts can be complicated and affect various sectors of the economy in different ways. For example, a snowstorm may disrupt the economy negatively by driving up heating and snow removal costs, while boosting the economy through increased attendance at ski and snowmobile resorts. A prolonged dry spell can negatively affect agriculture while having a positive impact on construction projects by helping them to remain on schedule. Weather also affects the economy by impacting both supply and demand for the products and services of a particular industry. For example, consider the ice fishing industry where a good season is determined by the total days of ice on our lakes. A warm winter thus has a negative impact on that sector of the economy by reducing the supply of good lake ice.

A recent study by the National Center for Atmospheric Research explored the economy’s weather sensitivity, using weather variability as a factor affecting our gross domestic product. The study concluded that the impact of routine weather events on the U.S. annual economy is 3.4 percent of U.S. gross domestic product — or about $485 billion (in 2008 dollars) for the 2008 U.S. economy! Overall, precipitation variations had a larger effect on the economy than temperature.

Category: Meteorology
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