Within hours of the last strains of "Auld Lang Syne," hundreds of people wearing little more than a swimsuit and a smile will scramble into the icy Atlantic Ocean.

Among those taking the plunge will be Michael Avino, 68, a member of the Coney Island Polar Bear Club until he moved to Atlantic City 10 years ago after retiring as a New York City detective.

"I've been doing this for 50 years," Avino said. "It keeps you young. Nowadays we do it for charity, which is nice, too."

He and about 300 others are expected to turn out tomorrow morning for Atlantic City's Polar Bear Plunge, sponsored by the Make-A-Wish Foundation, in front of the Tropicana Casino Resort.

A seashore staple for at least a century, the plunges are, as Avino noted, mostly about charity now, but they also are a way for towns to bring in tourists during the long off-season.

At the Jersey Shore, at least four towns - Atlantic City, Brigantine, Ocean City, and Ventnor - are offering their own Polar Bear Plunge on New Year's Day. Other Shore towns have scheduled their cold-water fund-raisers for later in the winter.

Throughout the region, charities such as the Alzheimer's Association of the Delaware Valley have jumped on the bandwagon - if not exactly into the brine - with events that include the second annual Polar Bear Plunge into the Schuylkill on March 8.

The beach may change, but the drill is the same everywhere: Run onto the beach, strip off that warm overcoat to a bare-bones bathing suit, and run as fast as you can into the surf.

Don't linger on the sand. Don't dip your toe in first. Just dive in as soon as you get in deep enough to cover yourself in the coldest water you can imagine. Then fight the urge to collapse right then and there, and run, run, run back onto the sand, where you might have a wingman waiting with a warm, dry towel and your coat.

The water can be astoundingly cold and the ambient temperatures just as foreboding. The ocean is about 40 degrees, and weather forecasters are calling for 42 degrees and rain, with a chance of snow, for tomorrow.

Some events across the United States raise hundreds of thousands of dollars for charities that include cancer research and juvenile diabetes. In Sea Isle City, an event that attracted only a handful of people when it began 20 years ago, draws more than 2,500 who take the plunge and 35,000 who watch, according to organizers.

But other than the notion of doing it for a charitable cause, the question is: Why? There has to be an easier way to raise cold cash.

"You know, it is kind of fun," says 85-year-old Irene Jameson, who until two years ago, when she was forced into retirement at the wishes of her children, ran and participated in Sea Isle City's Polar Bear Plunge for 18 consecutive years.

Jameson, has written Memoirs of the Polar Bear Queen, with the proceeds being donated to charity once the self-published tome comes back from the printer.

She said that as her town's public-relations coordinator, she had come up with the idea to hold a Polar Bear Plunge in Sea Isle each February to extend the resort's "shoulder seasons."

Jameson said she had heard about such an event from her son, who was a policeman in Delaware. In that state's beach towns, law enforcement employees and others have been turning out by the hundreds for more than 30 years to raise money for causes.

But the tradition of the polar bear swimmer dates back to at least the early 20th century, when impresario Bernarr Macfadden founded the Coney Island Polar Bear Club in 1903. Macfadden believed cold-water swimming offered health benefits.

The promotion of that idea apparently spawned Polar Bear groups all over the globe, from Russia to South Korea and points in between, including Venice Beach, Calif., where a penguin rather than a polar bear is the mascot and the water isn't all that cold.

Until recently, in a kind of polar bear underground that before the Internet or texting operated strictly by word-of-mouth, loosely organized groups of swimmers would gather at designated waterfronts, always on New Year's, to share their cold-water activity. Then event organizers began to see the activity as a way to attract a crowd - and tourist money.

The plunges have evolved in another way, too: Once largely gatherings of men, most events now attract a mix of people, and more young women than ever, Jameson said.

She and Avino said some training was involved.

"I run every day on the Boardwalk, especially when it's cold outside, to get used to the cold," Avino said.

Jameson used to start a routine weeks before the plunge, she recalled. She would rinse in cold water when she showered for several weeks, then work her way up to cold showers. If it snowed before the event, she would walk around in the snow on her sundeck - barefoot.

"But I was never one to hang out on the beach a long time before the plunge. You get cold, and then your feet get cold that way," Jameson said. "You run out there, strip down, and then run right into the water and get in and get out. You can't hang around too long before or after. That's the secret."

But some experts contend cold-water immersion is never a good idea. And people with cardiovascular disease or heart conditions should take heed, said Carol Sames, an exercise physiologist at Upstate Medical University in Syracuse, N.Y.

When the body hits the cold water, the heart rate spikes and increases blood pressure and vasoconstriction, or a narrowing of the blood vessels. Unlike exercise, which raises the heart rate and oxygen demand gradually, plunging into cold water creates an immediate impact in the heart that can be dangerous, Sames said.

For people without heart conditions, the activity can be invigorating, provided swimmers get in and get out, some experts say.

"Some people will jump in and actually swim. That's where you start to run into problems, even if you are a tremendous swimmer," Sames told the Post-Standard of Syracuse. "All of that shivering is going to start to lead to fatigue. You will almost get that lethargy going on."

Contact staff writer Jacqueline L. Urgo at 609-823-9629 or jurgo@phillynews.com.