JULIE LANE PHOTO | Bill Clark III is in his ninth year as South Ferry’s weather prognosticator, judging seasons by wind conditions, using the flag behind him as a tool for his predictions.

While the Mayan calendar was predicting the end of the world just before the winter solstice, Bill Clark was poised to predict Shelter Island weather for the next three months, convinced there would still be a need for his prognostications.

And unlike meteorologists who use sophisticated technology for their frequent wrong day-to-day weather predictions, Mr. Clark is standing by his own predictions made based on wind currents as they caressed Old Glory flying over South Ferry docks. Despite last week’s frigid temperatures and Monday’s snow, Mr. Clark told us his December observations showed it would be a relatively mild winter. If what we have seen to date continues through late March, he’ll be proven right — again.

Clark family members, who have been offering weather prognostications for at least four or five generations and maybe much longer, claim an 85 percent accuracy rate.

“We can’t predict every cold snap,” Mr. Clark said. But he felt confident enough during last week’s freeze to predict it would be short lived and this week would see higher than usual temperatures. After Monday, he was right.

“I’m learning as I go,” said Mr. Clark, who took over the family duties from his uncle and South Ferry president Cliff Clark nine years ago.

Family members don’t even know how the prognostications started or for how many generations they have been done. But Bill Clark does know his Uncle Cliff, his grandfather and his great-grandfather before that followed the family tradition. He’s the sixth generation of his family to serve as a South Ferry captain.

Whether the solstice predictions started on Shelter Island or were begun by family members who lived up-Island or in Connecticut has been lost to time, Mr. Clark said.

So how does Bill Clark carry out his responsibilities?

After using a computer to identify the exact time of each of the seasonal solstices, he goes out at the appointed hour to observe the wind direction and speed as it’s causing the flag to ripple.

“If the wind is really ripping, it’s a better indicator,” Mr. Clark said.

In December he observed light winds, perhaps about 20 mph, blowing from the southeast during a light rain. That prompted the prediction of a warmer than usual winter. Easterly winds generally point to a wetter season ahead while westward blowing winds are predictors of a dryer season. North winds, as you might expect, carry colder temperatures and northeast winds are generally predictors of a snowy winter season, he said.

The December 2011 winds were southwesterly and, sure enough, it was a mild dry winter, Mr. Clark said.

Sometimes the flag just seems to flutter around and he has to be patient and wait until it settles into a pattern, he said. As for daily weather expectations, he finds tides are a good predictor and watches for even slight changes in their direction or speed.

Why does it all seem to work?

“We don’t know exactly,” he said. But it seems to be related to jet streams that set up and carry weather trends from place to place, he said. “

He takes kidding from friends if he hits a wrong prediction, but figures with an 85 percent accuracy rate, and a wrong patch a couple of years ago, he should be right on target in the near future.