What is Groundhog Day and do the forecasts work?

Long before computers, the Weather Channel and the Internet, humans needed weather forecasts. Farmers and sailors particularly needed to know if storms were approaching. Over time, various folklore forecasts, often in the form of short rhymes, were devised and passed down through the generations. Although memorable, the folklore forecasts are of uneven quality — some good, others bad.

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 sixth 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 a single location — like Punxsutawney, Pa., or Sun Prairie — tell us very little about the weather for the rest of the winter season.

As for accuracy, the predictions 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.

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How do you measure snow accumulation in high winds?

Accurate and precise measurement of snow accumulation is a difficult task. The measurement tools are simple: a ruler or yardstick that measures in inches and tenths of an inch.

The trick in measuring snow consistently is simply finding a good place to measure and a firm surface for your ruler to set on. It is recommended to use a snow board, which is a square piece of wood 16 inches on a side and painted white. 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 good sites as they tend to accelerate melting of the snow. Neither is grass a good site as snow tends to sit up on top of the blades of grass, while the ruler goes down to the ground. With this setup, measuring new snow accumulation is straightforward, if there’s no wind. When the wind blows and the snow drifts, measuring snow is a challenge.

To deal with drifting snow, make measurements at a various places in your yard and then average them to get what is considered a representative measurement.

Snow is also measured with the standard rain gauge — an 8-inch diameter cylinder that collects precipitation. In winter the gauge can be equipped with a known amount of antifreeze so that the snow melts when it enters. This measurement yields the liquid water equivalent of the snowfall.

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Do the tropics influence the weather in Madison?

It may seem implausible at first glance, but current research in the Department of Atmospheric and Oceanic Sciences at UW-Madison is exploring connections between tropical cyclones (hurricanes) near the Philippines and extreme weather events in southern Wisconsin.

The connection appears to derive from unusual jet stream structures forced by the outflow from the hurricane at high levels in the atmosphere.

Hurricanes are well-organized collections of tropical thunderstorms that take warm, moist air from the lowest levels of the tropical atmosphere and very quickly thrust it nine miles or more above the surface. This rapid upward motion substantially distorts the shape of the tropopause which, in turn, intensifies the normal jet stream located on the northern periphery of the vast hurricane outflow.

The intensified jet, often a very long-lived structure, can become a major weather event produced in the central United States several days later. Currently, we think this type of connection is most likely to occur in spring and fall.

Our incredible windstorm in October 2010 was a clear example of this tropical connection to Wisconsin weather. We are just beginning to explore these ideas in detail and will keep you informed of our results.

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What causes wind gusts?

A wind gust is a sudden, brief increase in the speed of the wind followed by a lull. According to National Weather Service observing practice, gusts are reported when the peak wind speed reaches at least 18 mph and the variation in wind speed between the peaks and lulls is at least about 10 mph. The strongest wind gust recorded occurred at Australia’s Barrow Island, with a gust of 253 mph that occurred during tropical cyclone Olivia on April 10, 1996.

Gusts at the ground are caused by either turbulence due to friction, wind shear or by solar heating of the ground. These three mechanisms can force the wind to quickly change speed as well as direction.

In the case of friction, gusts are generated when wind blows around buildings, trees or other obstacles. This type of gustiness is generally largest near tall buildings and alley ways and least over large water bodies.

But air over water can still be gusty. This can be caused by wind shear. A wind shear is a change in the wind over a distance. This can be a change in wind direction, wind speed or both.

On sunny days wind gusts can be generated by rising air currents when the ground is heated. This can generate a thermal of warm air that rises, with air from above sinking to replace the rising thermal. This descending air can cause wind gusts.

The duration of a gust is usually less than 20 seconds. Whatever the mechanism, wind gusts often do things that you’d rather didn’t happen — like destroy an umbrella or blow your new hat blown away.

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What were the top Wisconsin Weather Events for 2011?

Each season in 2011 had a memorable weather event, some with negative impacts. There were two good snow storms in February. The Groundhog Day blizzard had wind gusts of up to 60 mph and snowfall depths ranging from 1 to 2 feet. That resulted in snow drifts of 6 to 10 feet. The second storm occurred on February 20-21 with snow accumulations between 8 and 15 inches. Freezing rain and sleet also fell across southern Wisconsin.

There were 38 documented tornadoes, making the year the third busiest on record. A record-breaking tornado event occurred on April 10 with at least 15 tornadoes, including four strong tornadoes. This was the largest total number of tornadoes in one April day. There were 11 confirmed tornadoes on May 22.

Summer weather included straight line winds and a heat wave. The heat wave of July 17-21 covered most of the state with heat indices of 100 degrees to 117 degrees. A tornado-water spout was observed over Lake Mendota on August 8. Waterspouts were also observed east of Milwaukee Harbor on Sept. 24. A severe thunderstorm hit western Waukesha County on June 21 with 100+ mph winds, which leveled or damaged thousands of trees.

Labor Day saw relatively cool temperatures with lows in the 30s. Throughout October, November and December, Wisconsin experienced temperatures that were above our 30-year climatic mean.

During the year, 10 people were directly killed by weather events: one person from a blizzard, one from lightning, one from a tornado, one from non-tornadic thunderstorm winds, one from flooding and five during the July heat wave.

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