How were recent heavy rains so well-predicted so many days in advance?

Posted on October 22, 2012 by Weather Guys Editor

Though we have been relatively dry for much of the autumn, on the weekend of Oct. 13-14 we received a soaking rain of 0.86 inches on Saturday followed by 1.74 inches on Sunday. Two aspects of this heavy rain event are noteworthy to us. First, though Madison averages an inch of rain in a single day about six times each year, the 1.74 inches that fell on Sunday was the most in a single calendar day in Madison since 3.61 inches of rain fell on July 22, 2010. That long stretch includes two full summers (2011 and 2012) in which we never received such a rain.

Second, the rainy weekend was clearly in the forecast for almost seven days in advance. In other words, at the end of the prior weekend, it was clear that next weekend was going to be a washout.

That forecasting success is a particular example of a revolution that has quietly occurred in the science of numerical weather prediction (forecasting with the aid of high-speed computers) over the last 30 years. It used to be that the one- to two-day forecast was the apex of forecasting skill but the combination of more powerful computers and continued research into the underlying science of weather systems has led to our present ability to forecast the weather sometimes several days in advance.

It is exciting to imagine where the state of the science may be in another 30 years.

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What is an air mass?

Posted on October 15, 2012 by Weather Guys Editor

An air mass is a large body of air whose properties of temperature and humidity are similar in any horizontal direction. Air masses can cover hundreds of thousands of square miles. Air masses are formed when air stagnates for long periods of time over a uniform surface. The characteristic temperature and moisture of air masses are determined by the surface over which they form. An air mass acquires these attributes through heat and moisture exchanges with the surface.

Air masses are classified according to the temperature and moisture characteristics where they develop. Cold air masses originate in polar regions and are therefore called polar air masses. Warm air masses usually form in tropical or subtropical regions and are called tropical air masses. Moist air masses form over oceans and are referred to as maritime air masses. Dry air masses that form over land surfaces are called continental air masses.

In a typical year, weather changes resulting from the movement of air masses kill more people in the United States than all other weather phenomena combined. For instance, people from warmer winter climates (like Dallas, Texas) might be at greater risk from a cold air outbreak than people living in colder climates (like Madison). Conversely, Madisonians may be at greater risk to heat waves than residents of Dallas.

Socioeconomic status, cultural backgrounds and the size of the elderly population can also put a city’s residents at greater or lesser risk. For example, households with heaters in the north and air conditioners in the south may reflect socioeconomic status, whereas cultural backgrounds may influence the amount of time people spend outdoors.

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What are cooling degree days?

Posted on October 8, 2012 by Weather Guys Editor

A degree day is a measure of heating or cooling. A degree day is defined as a departure of the mean daily temperature from a given standard: one degree day for each degree of departure above (or below) the base temperature during one day. The degree day is related to the outside temperature and is not related to time.

A cooling degree day is every degree that the mean temperature is above 65 degrees during a day. So, if the high temperature for the day is 95, and the minimum is 51, the average temperature for the day is 73. That would be 8 cooling degree days (73-65).

The amount of energy required to maintain a building’s temperature in the summer is proportional to the accumulated cooling degree days. Cooling degree days, or CDD for short, are used to estimate fuel consumption needs.

The typical accumulated CDD for Madison is 664. The recent hot summer has, so far, registered 1065 CDDs — well above normal.

Of course, cooling a building depends on total energy gains and losses of the building and not just the mean temperature outside. Other factors, such as cloudiness, insulation and the number and efficiency of electrical appliances running will influence energy needs.

We also compute growing degree days, GDD, which is used by farmers to predict plant growth such as when a flower will bloom or when a crop should reach maturity. During winter, we keep track of heating degree days, or HHD. Each degree that the mean temperature is below 65 degrees is one heating degree day.

The Wisconsin State Climate office publishes this data at go.madison.com/madisonclimate.

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How is the ozone hole doing?

Posted on October 1, 2012 by Weather Guys Editor

Currently, the ozone hole is not as large as it was in 2011, but it is larger than it was in 2010. The ozone hole refers to the rapid depletion of stratospheric ozone over Antarctica. This ozone is located in a layer about 15 miles above the surface.

Human activity has contributed to the deterioration of the ozone layer by adding chlorofluorocarbons, or CFCs, to the atmosphere. These chemicals were invented by chemists to be used as propellants in spray cans, as Styrofoam puffing agents and as coolants. These chemicals are very stable and once injected into the atmosphere can remain for decades and are transported all over the globe, including over Antarctica. Once they are high over Antarctica, CFCs can significantly deplete the stratospheric ozone layer.

During Antarctica’s winter, a vortex of winds develops around the pole and isolates the polar stratosphere. Very cold temperatures can then develop (109 degrees below zero Fahrenheit), which helps to form thin clouds made of ice, nitric acid and sulphuric acid mixtures. Stratospheric temperatures in the mid-latitudes, where we live, do not get this cold. As the sun rises in the Southern Hemisphere in springtime (e.g., late September), a crucial ingredient is added — UV radiation. The combination of UV radiation, stratospheric clouds and chemicals results in a rapid destruction of ozone.

This September we celebrate the 25th anniversary of the Montreal Protocol, an international treaty to end the production of ozone-destroying chemicals. The Montreal Protocol is working, as ozone depletion due to human effects is starting to decrease. Unfortunately, even with the Montreal Protocol, the ozone hole is expected to stay around until about 2050.

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Did we hit record for 90-degree days?

Posted on September 25, 2012 by Weather Guys Editor

Though it isn’t unprecedented to get another day above 90 degrees this late in the year (the all-time latest such day in Madison’s history is Oct. 14, 1975), it is very likely our run at the record of 90-degree days in a season will end at 39 — agonizingly one day shy of the record 40 set in 1955.

Looking back, it was the lack of really warm days in August (only seven days over 90) and thus far in September (only two) that foiled our pursuit of the record. September 1955 had five such days with the 40th day at or above 90 that season occurring on Sept. 18.

The weather was very interesting in the week preceding that record-setting warm day. On Sept. 9, 1955, the high temperature was 97 and yet the low temperature just two nights later was a chilly 34. The very next night (Sept. 12) the low temperature dropped to 31 but did not go below freezing again until the night of Oct. 18.

So far this season (as of Sept. 20), we have not had an overnight low temperature below freezing. It appears this summer, incredibly warm as it was, will come up just short of a rather formidable record.

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