How unusual is this kind of cold ?

Posted on January 7, 2014 by Weather Guys Editor

We are at the beginning of the most intense cold air outbreak of the winter thus far with high temperatures today not likely to rise above zero. It may be similarly cold tomorrow.

Having two consecutive days in a row with daytime high temperatures below zero is noteworthy in Madison. The last time that occurred in town was in early February 1996 (Feb. 2 and 3).

In fact, a daily high temperature colder than minus 5 has only occurred 12 times in the last 33 years, while a daily high lower than minus 10 has only been recorded on four occasions in that time period.

In addition, the morning low on Monday is likely to be near 20 below zero. The last time Madison saw a morning low temperature that low was on December 25, 2000, when the low was minus 21, part of a 12-day stretch during which 11 days had morning lows below zero.

Overnight lows of minus 20 or colder are not common in Madison, though not unprecedented. Since 1980 it has happened 26 times and since 1990, 12 times: Jan. 15, 16, and 18-20, and Dec. 31, 1994; Jan. 31, Feb. 2, 3 and 4, 1996; Jan. 5, 1999; and Dec. 25, 2000.

The low of minus 29 recorded on Feb. 3, 1996, tied for the third coldest of all time in Madison.

Whether the current cold outbreak meets or exceeds any of these records will be quite beside the point as we all try to bear the brunt of it over the next couple of days.

Category: Climate

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How does freezing rain form?

Posted on December 31, 2013 by Weather Guys Editor

Southern Wisconsin experienced freezing rain and freezing drizzle just before the start of the holidays.

When particles falling from clouds reach the surface as precipitation, they do so primarily as rain, snow, freezing rain or sleet.

Where we live, most precipitation particles are frozen at one time during their formation. What determines the precipitation type at the surface is the temperature between the cloud bottom and the ground.

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 subfreezing air near the ground.

Freezing rain forms when a very shallow layer of cold air is at the surface, causing the freshly melted raindrops to freeze on contact with exposed objects on the ground, which has a temperature below freezing.

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.

When sleet hits the surface, it bounces and does not coat objects with a sheet of ice. Freezing rain covers everything in a sheet of ice, creating shimmering and treacherous road conditions.

Freezing rain is the precipitation type with the highest rate of accidents and death during the weather event. The number of deaths due to accidents is larger for snow events, but that is because snow is more common than freezing rain. Southern Wisconsin averages only about 20 hours of freezing rain a year.

Category: Meteorology

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How does this month’s cold and snow stack up?

Posted on December 23, 2013 by Weather Guys Editor

Winter officially began at 11:11 a.m. on Saturday with the winter solstice, but everyone knows it has felt like winter for quite some time as we have endured a cold (and snowy) first two-thirds of December.

In fact, through the first 18 days of the month, the average temperature has been 6.2 degrees colder than normal and we have had 9.9 inches of snow accumulate over 10 snowy days (days on which at least a trace of snow has fallen).

December is nearly our snowiest month on average, logging 12.6 inches in a normal year over nine snowy days. (January logs 13.1 inches.) The 5-plus inches of snow that blanketed the Madison area Sunday puts us over the monthly average.

Of course, almost no matter what happens the rest of this month, we will not approach the record-setting cold and snow of December 2000. During that month, the average high temperature was only 22.1 degrees (10 degrees below normal) and we accumulated 35 inches of snow over 22 snowy days!

That month was the second-snowiest December on record (topped only by December 2008 with 40.4 inches) and the fourth-coldest December ever — a rare combination indeed.

Unfortunately, the start of the season offers no historical clues about how it will progress. After our cold and snowy December 2000, the rest of the season (January and February) was much warmer than normal and only 17.2 inches of additional snow fell.

After the record snowy December 2008, which came on the heels of our snowiest winter ever in 2007-08 (101.4 inches), we had another 32 inches of snow in January and February, well above the normal for those two months (21.8 inches).

Thus, it appears that anything could happen as the rest of the winter unfolds.

Category: Seasons

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How do the Yahara lakes freeze?

Posted on December 16, 2013 by Weather Guys Editor

Lakes freeze from the surface downward to the lake bottom. Ice floats because it is less dense than liquid water.

The density of liquid water depends on the water temperature. The density of water is highest at a temperature of about 40 degrees Fahrenheit. So, why is that important?

As winter sets in, lakes lose energy to the atmosphere as the water near the surface cools. The density of the water near the surface increases and this surface water sinks because it is more dense than the warmer water below.

Warmer water under the surface rises to replace this sinking water because of its smaller density. When all the lake water reaches a temperature of 40 degrees, further cooling of the surface water temperature makes it colder than 40 degrees and, because it is now less dense than the water around it, it will stay on the surface of the lake and continue to cool.

Once this surface water decreases to 32 degrees, the water freezes. The freezing then spreads downward into the lake and the ice thickens. This is why you find liquid water below the ice, unless the body of water is very shallow.

The water temperature below the ice is about 40 degrees. Fortunately fish can live in temperatures this cold.

Freezing also first occurs along the shoreline, where the water is shallow. Before ice can form on the surface, the entire water column must first reach a temperature of 40 degrees, and this is likely to first occur along the shoreline.

Lake Mendota typically freezes over on Dec. 20; while Lake Monona’s typical freeze date is around Dec. 15. This year, Lake Monona froze over on Dec. 10. With the cold weather southern Wisconsin has been experiencing, it is likely that Lake Mendota will also freeze over a bit earlier.

Category: Phenomena

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Is this early December cold spell unusual?

Posted on December 9, 2013 by Weather Guys Editor

Though the first really nasty cold snap of the year is already underway, by the middle of this week we should be in the depths of a deep freeze, flirting with a daily high temperature no higher than 20 degrees Fahrenheit.

Such cold maximum temperatures, though not unprecedented for early December, are also not common in Madison. In fact, only 18 of the last 40 Decembers in Madison have had at least one daily maximum temperature of 10 degrees Fahrenheit or colder.

Upon dividing the month into early December (Dec. 1-10), mid-December (Dec. 11-20) , and late December (Dec. 21-31), we find that seven of these years had at least one such cold day in early December (1976, 1977, 1978, 1985, 1991, 1995, and 2009). Six of the years had the first such day in mid-December (1983, 1989, 1999, 2000, 2004, and 2008) while the remaining 5 didn’t get that cold until late December (1973, 1980, 1990, 1993, and 1996).

So about every third December we can expect to get this cold before Dec. 21, and we might just do that again this week. Our chances will be enhanced thanks to the several inches of snow that blanketed the area on Sunday.

With a fresh snowcover on the ground, a number of effects that can deepen a cold spell come immediately into play. First, the snowcover is an excellent insulator and, if as deep as 3 inches, it effectively eliminates the vast amount of heat stored in the ground from helping to warm the air.

Second, snow is very good at cooling overnight, and that helps to lower the surface air temperature. Third, the snow is very good at reflecting sunlight during the day, which helps to minimize the daytime heating of the air near the ground.

Category: Seasons

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