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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How did the 2013 hurricane season end?

Posted on December 2, 2013 by Weather Guys Editor

The 2013 Atlantic hurricane season officially ended on Saturday. It was the least active year in 30 years. There were only 13 tropical storms, two hurricanes and no major hurricanes. The two hurricanes that formed were very weak. In an average year, there are 12 tropical storms; six or seven go on to become hurricanes and two of those reach major hurricane intensity.

A tropical storm is a rotating weather system with low pressure, strong winds and a storm center with heavy rain producing thunderstorms arranged in a spiral pattern. When the highest sustained winds in a tropical storm reach 74 mph, the storm is reclassified as a hurricane if it is over the Atlantic Ocean or a typhoon if over the Pacific Ocean. When the sustained winds reach 97 mph the storm is considered a major hurricane. Typhoons with winds above 150 mph are called super typhoons.

In late May and early June of 2013, the seasonal forecasts were predicting seven to nine hurricanes. Why was the forecast so far off? For one reason, seasonal forecasts are difficult to make. While the spring and early summer surface pressure and sea surface temperature suggested an above-average season, there were conditions that were not forecast and likely helped this hurricane season to be below average.

During the hurricane season, drier-than-normal air blew off the deserts of Africa into the tropical waters where hurricanes form. This dry air can weaken or inhibit the formation of hurricanes. In addition to this dry air, the average temperature structure was less conducive to thunderstorm development. These are likely two conditions that made this a below normal season. The coming year will bring forth research studies seeking to provide a complete explanation of this unexpected below-average season.

Category: Tropical

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Are tornadoes typical in the Midwest during this time of year?

Posted on November 25, 2013 by Weather Guys Editor

No, although in the U.S. tornadoes have occurred in nearly every state and in every month of the year. Wisconsin has reported a tornado in every month except February. There have been nine confirmed tornadoes in Wisconsin during November between the years 1844 to 2012.

Tornadoes form in regions of the atmosphere that have abundant warm and moist air near the surface with drier air above, a change in wind speed and direction with height. It is generally accepted that tornado season begins in the springtime. Because vertical wind shear is closely related to the presence of a jet stream, tornado season in the U.S. moves north and south during the year.

The National Weather Service issued a Particularly Dangerous Situation (or PDS) tornado watch for Janesville and southern Wisconsin on Nov. 17. A weather watch indicates that possible hazardous weather may occur. A watch is intended to provide people with enough time to set safety plans in motion for possible hazardous weather. A PDS is issued to indicate that there is an enhanced risk of very severe and life-threatening weather.

The intense thunderstorms that moved across the Midwest on Nov. 17 hit Illinois the hardest. The severe weather that day killed at least eight people and injured dozens. The weather included tornadoes, large hail and strong winds. The threat of severe weather from these storms caused a delay in the Chicago Bears-Baltimore Ravens football game at Soldier Field.

Category: Severe Weather

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Why does the wind diminish after sunset?

Posted on November 18, 2013 by Weather Guys Editor

A reader wrote us last week with two questions prompted by careful observation: 1. Why does the wind speed tend to decrease after sunset? 2. Why does this effect seem less pronounced in late autumn and winter?

As it turns out, the answers are somewhat related to one another.

The wind speed tends to decrease after sunset because at night the surface of the Earth cools much more rapidly than does the air above the surface.

As a result of this difference in cooling ability, it doesn’t take long for the ground to become colder than the air above it.

The air in close contact with the ground — say in the lowest 300 feet of the atmosphere — then becomes colder than the air above it.

This circumstance leads to the development of what is known as a temperature inversion. Inversions dramatically reduce the amount of mixing that occurs between different vertical layers of the atmosphere. As a consequence, once the inversion sets up (after sunset), it is much harder for fast-moving air above the ground to mix down to the surface, where it could appear as a gust of wind.

During the day it is very easy for the air to mix and cause surface gusts.

If there is a low pressure area or storm in the region the winds will blow day or night. Late autumn and winter bring the strongest storms of the year to our area. These storms have cloud and temperature structures that can often overrule the tendency for inversions to set up at night.

The much stronger winds near the strong storms, coupled with a tendency to avoid inversions near storms, mean that windy nights are more common in the cold season.

Category: Meteorology

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