How can you forecast frost?

Posted on November 21, 2011 by Weather Guys Editor

There are a series of questions you can ask yourself to aid in predicting the formation of frost for your own backyard:

• What is the temperature? If at sunset the temperature is close to freezing then there is a better chance for the formation of frost. Remember the temperature reported by your local weather station is measured 1.5 meters above the ground. So if the forecast is for a temperature of 39 degrees, the ground may be close to freezing.

• How fast is the wind blowing? A strong wind inhibits the cooling of the surface, and thus frost.

• What sort of cloud cover is there? Clouds are good emitters of infrared energy so they mitigate against the energy losses on the ground at night. If it is cloudy, and will stay cloudy, then the likelihood of frost is reduced. If it is to be a clear night then frost is more likely to develop.

• What is the condition of the ground? If the soil is warm and moist, then conduction will transfer heat upwards from below the surface and inhibit the development of frost. Also know your backyard. Cold air will drain into local low spots making them colder.

• How long will the night be? The longer the night, the more time the ground has to cool and so the colder it will get.

• What is the dew point? This is a very important question to ask yourself. A rule of thumb: If the dew point is above 45 degrees at sunset then you are probably OK. Below 40 degrees you will probably see a frost if the other conditions are good.

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What does the recent big storm in Alaska mean for us?

Posted on November 14, 2011 by Weather Guys Editor

The massive storm that struck the west coast of Alaska last Tuesday and Wednesday was truly an amazing meteorological event. The entire Bering Sea coast was under the threat of hurricane-force winds, with many areas facing heavy snow and zero visibility.

Importantly, this storm is able to exert hurricane force winds over a much larger area than the typical tropical storm. Thus, the aerial extent of the strong winds in this beast far exceeds that in all but the strongest hurricanes.

Many of the towns along the west coast are vulnerable to coastal erosion under such conditions, especially this year when the sea ice, usually in place by this time of year and working as a natural breakwater, has not yet been established.

In fact, a similarly strong storm struck the coast in mid-November 1974 and was accompanied by high tides timed perfectly with its arrival. In that case, the substantial sea ice was able to spare places like Nome and Kivalina from devastating damage from coastal flooding and erosion.

To the extent that the late establishment of the sea ice barrier is related to the increase in global average temperature, storms like this one will pose greater threats to population centers on the Alaskan west coast in a warmer climate.

For us in Wisconsin, an echo of this storm may yet be heard. In its wake in Alaska, the first really frigid air from over the North Pole will slide into Alaska and the Yukon by the end of this week. That cold air will then funnel southeastward during the following week potentially setting the stage for a cold second half of November in the central United States.

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How often do the ‘gales of November’ come early?

Posted on November 7, 2011 by Weather Guys Editor

Some historic storms have occurred around Veterans Day (formally known as Armistice Day). Friday marks the centennial of a winter storm that blew across the Midwest on “11-11-11.”

The storm developed over southern Wyoming the morning of Nov. 10, 1911, and traveled east, reaching southwestern Iowa by the morning of the 11th. The storm’s center traveled northeast across Wisconsin, reaching central Wisconsin by noon and Michigan’s Upper Peninsula by evening.

An F-4 tornado on the five-category Fujita Scale traveled to the northeast across southwestern Rock County, leveling farms, killing nine people and injuring 50. Within an hour of the tornado, survivors were digging out in blizzard conditions and near-zero temperatures.

The rapid drop in temperature was impressive. At the U.S. Weather Bureau’s Madison office, the temperature at noon on the 11th reached 70 degrees, but by midnight, the temperature fell to 20 degrees. Morning and afternoon thunderstorms gave way to snow flurries.

Another memorable storm is the Armistice Day Storm that ravaged the Upper Great Lakes region Nov. 11 and 12, 1940. This storm took down the Tacoma Narrows Bridge that spanned Puget Sound before heading to the Midwest. By the time this storm ended, it killed more than 150 people, sank at least 3 ships, stranded hundreds of train passengers and isolated Midwestern towns with 20-foot snow drifts.

On Nov. 10, 1975, a storm developed that was a near repeat of the 1940 Armistice Day Storm. This storm sent the freighter SS Edmund Fitzgerald to the bottom of Lake Superior with all 29 hands. Lightfoot’s 1976 ballad helped make the incident famous disaster in Great Lakes shipping history.

All this history suggests we should get ready for a potentially exciting month of weather.

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What is typical Halloween weather?

Posted on October 31, 2011 by Weather Guys Editor

We tend to remember those Halloweens with bad weather. Slogging through rain or snow in costumes with trick-or-treat bags is tough. But according to Dr. Ed Hopkins of the Wisconsin State Climatology Office, in the last 30 years (1981-2010), Madison has had only 10 Halloween days when measurable precipitation (0.01 inches or greater) fell, which translates to a probability of 33 percent that rain falls on that date. Halloween 1985 was memorable in Madison for its 1.05 inches of rain. The most precipitation for Oct. 31 was 1.21 inches in 1960.

Of greater interest is the amount of rain that falls during the late afternoon and early evening when most trick-or-treaters are making their rounds. In 1979, 0.31 inches of rain fell from about 4 p.m. until 7 p.m., of which nearly a quarter of an inch fell during the hour between 4 p.m. and 5 p.m. Since 1996, little significant rain has fallen during the afternoon and evening.

Air temperatures have varied widely during the late afternoon and early evening hours, including one Halloween low of 28 degrees in 1996. Over the last decade, Madison trick-or-treaters have experienced only two holidays with the temperature near freezing. While trace amounts of snow fell in 1993 and 1995, no measurable snow (0.1 inches or more) has fallen on Halloween since records have been kept at the Dane County Regional Airport.

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Why was it so windy last week?

Posted on October 24, 2011 by Weather Guys Editor

The wind is air in motion. Moving anything requires a force. Violent destructive winds, as well as gentle summer breezes, result from a complex interplay of different forces. One of these forces results from a pressure gradient, or how fast pressure changes over distance. Strong winds almost always result from large pressure gradients. Recently, southern Wisconsin has been under the influence of a weather pattern that has strong pressure gradients, and thus strong winds.

Near the Earth’s surface, the friction plays a pivotal role in wind, acting to slow the wind. Over the open lakes the wind will be faster than through a stand of trees, where it will be slowed by friction. In the presence of buildings, the air can be funneled between buildings and pick up speed.

The air flows from high atmospheric pressures to low pressures. The Coriolis force pulls the wind to the right so that winds blow counterclockwise around lows and clockwise around highs in the Northern Hemisphere. Friction slows down the wind and weakens the Coriolis force. So, if you stand with your back to the wind, and then turn about 30 percent to your right, low pressure will be on your left-hand side

Thunderstorms also can cause strong winds. Thunderstorms have upward air motions, called updrafts. There are also downdrafts, or sinking air in a storm. Such downdrafts carry air from high elevations in the atmosphere rapidly to the ground. Since wind speed is nearly always much larger at high elevations, the downdrafts carry very high-velocity air to the surface creating the winds of more than 100 mph.

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