SalemNews.com, Salem, MA

Special Report

April 17, 2013

Everybody talks about the weather, but it's not easy to predict

(Continued)

When the almanacs risk testable predictions, they often fail. The Old Farmer's Almanac predicted that the Atlantic corridor would average 47.5 degrees for March 2013, off by nearly four degrees. That might seem like a reasonably good guess, except that anyone with access to historical averages — that is, anyone with an Internet connection — can usually get within a few degrees by sticking near the mean. The almanac's prediction for February was 29 degrees, nine degrees below the actual temperature, and the forecast for January was off by five degrees.

Critics of the almanacs are nearly as old as the almanacs themselves. A forecaster at the U.S. Weather Bureau complained about the almanacs' inaccuracy in 1905, and a Harvard professor did the same in a public address in 1926. But we simply can't let go of the dream of weather omniscience.

Generally speaking, forecasters who make predictions months in advance rely on analog techniques, which means they look for patterns in the current weather, then find similar patterns in prior years. Their predictions are based on what happened in the past. The problem is that this technique has never been shown to work particularly well. The atmosphere is a complicated place, and it's very difficult to say that a single past year, or even a combination of past years, is enough like this year to make accurate predictions.

The National Weather Service relies more on dynamic forecasting. The agency's experts observe the weather systems at work in the atmosphere, then use mathematical and physical models to calculate what will happen next. The technique is far more reliable, but it has its limits.

"We sustain higher accuracy out to two to three days in advance; then it starts dropping off faster at days six through eight," says Louis Uccellini, director of the National Weather Service.

Text Only | Photo Reprints
Special Report

Local News
  • 140765_SN_DLE_FESSENDEN4 Special Spaces transforms two downstairs rooms into playroom for Riley Fessenden

    BEVERLY -- What happens when the perfect kid-friendly playroom appears in your house overnight? Beverly's Fessenden kids were mostly speechless when they returned from seeing "The Little Mermaid" on Saturday and got their first look at the playroom t

    July 28, 2014 10 Photos

  • Beverly Hospital courier loses patients’ lab forms

    BEVERLY -- A courier for Beverly Hospital last month lost lab request forms for 54 patients that included names, health insurance identification numbers and, in some cases, Social Security numbers. The courier misplaced the forms, which were in a zip

    July 28, 2014

  • Big plans for three parks in the Point neighborhood SALEM -- One will be overhauled, another created from scratch, and a third will get a spiffed-up community garden. It's a good time to be a park in the Point neighborhood. The densely packed area just south of the city's downtown is on the verge of i

    July 28, 2014

  • Peabody could see school choice profit next year PEABODY -- After three years, the city's schools are projected not only to break even on school choice, but actually make a $30,000 profit. School officials expect to lose just under $300,000 in state aid next year for students who opt to attend scho

    July 28, 2014

  • 140723_SN_DLE_SALVARMY2 Dennis and Susan Knight take over Salem Salvation Army operation

    SALEM -- It was Christmas Eve in Sanford, Maine, but 8-year-old Dennis Knight and his three sisters had good reason to be short on holiday spirit: Their mom and dad had warned them there would be no Christmas that year. The family couldn't afford it.

    July 28, 2014 3 Photos