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
  • Ipswich selectmen act to stop clam raiders IPSWICH -- The Southwest is not the only border under siege. Ipswich is taking emergency action to deal with clammers coming across the Essex and Gloucester border on Sundays to harvest on the richest clamming beach in Ipswich. It's not as serious, o

    July 22, 2014

  • Scuba diver identified

    The district attorney's office has identified the man who died Tuesday in an apparent drowning while scuba diving approximately 2 miles off of Marblehead Harbor. He is Gregory Cole, 58, of Westfield.

    Foul play is not suspected.  The medical examiner will conduct an autopsy to determine the cause and manner of death.  The facts and circumstances leading up to his death remain under investigation.

    July 23, 2014

  • New Brimbal Avenue design to be presented Thursday

    BEVERLY -- Residents will get their first chance to weigh in on the new design proposal for the Brimbal Avenue/Route 128 interchange on Thursday. City and state officials, along with designer Jacobs Engineering, will host a community meeting Thursday

    July 22, 2014 6 Stories

  • Bail set at $250K for teacher slaying suspect

    July 23, 2014

  • Marblehead diver rescue 2 Body of diver recovered off Marblehead

    State Police divers found the body of a 59-year-old man in the waters off Marblehead at 2:30 p.m. Tuesday, according to state police.

    July 22, 2014 2 Photos 1 Story