Wednesday, October 27, 2010
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Short-term weather lore holds a kernel of truth

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[October 27, 2010]  CHAMPAIGN -- Before the Internet, Weather Channel and NOAA radios, our ancestors relied on nature to tell its tale of upcoming weather. Moss growing on the south side of trees and squirrels hiding their nuts deep underground were thought to foretell a severe winter ahead.

Some natural prognostications like these are grounded in truth, given our current knowledge of meteorology, but others are purely fiction, according to Jim Angel, state climatologist, of the Illinois State Water Survey.

Short-term weather forecasts based on nature observations are more likely to be accurate than long-term seasonal predictions. In fact, there may be some merit to the notion that bad weather is coming when cattle lie down in the pasture and birds fly low.

"Many animals have a better sense of hearing and smell than we do, so when humidity, air pressure and wind direction change right before a storm, as well as the distant rumble of thunder, some animals may become restless," Angel said. "They can pick up on weather changes hours before we can."

Predictions based on the appearance of the sky are thought to be particularly valuable, since certain clouds are associated with certain weather conditions, according to Angel. Clouds described as mare's tails and mackerel scales are very high-level cirrus and cirrocumulus clouds that can precede an approaching warm front, with rain not far behind.

Likewise, a halo around the moon is actually the refraction of moonlight through the ice crystals that make up high-level cirrus clouds, indicating an approaching low pressure system bringing rain or snow.

Long-term forecasts, such as winter weather predictions, are much more uncertain.

"Centuries ago, it was important to determine how severe the winter would be so that adequate wood and supplies would be stored for the duration," Angel said. "The early settlers' lives may have depended on their predictions, so they were grasping at anything to forecast the coming weather. However, the size of the brown band on woolly worms, the groundhog seeing its shadow or spoon-shaped persimmon seeds are just happenstance."

Even with today's modern technology, the theoretical limit of daily weather forecasts is about two weeks. Within the six- to 14-day range, forecast errors can be large enough to limit their usefulness.

That is why forecasters typically discuss only general patterns of weather behavior beyond five days, usually in terms of probability or odds. For example, the eight- to 14-day forecast may show the eastern U.S. with an increased chance of below-normal temperatures.

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The same is true for seasonal forecasts, which are driven by both long-term trends and specific weather patterns such as El Niņo.

For the upcoming winter, forecasters look at historical records to decipher a pattern. The Midwest is under the La Niņa effect, which is characterized by unusually cold waters in the eastern tropical Pacific Ocean.

The National Weather Service's winter forecast for Illinois is an increased chance for above-normal temperatures in the southern two-thirds of Illinois, and equal chances of above-, below- and normal temperatures in the northern third of Illinois. All of Illinois has an increased chance of above-normal precipitation.

What does the woolly worm predict?


The Illinois State Water Survey, at the University of Illinois at Urbana-Champaign, is a division of the Institute of Natural Resource Sustainability and is the primary agency in Illinois concerned with water and atmospheric resources.

[Text from file received from the Illinois State Water Survey]

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