No one reacts to the changes in weather with greater hysteria than the people who frequent Prospect Park’s Great Lawn. A tiny spike in temperature will bring a rush of picnic packers and soccer players. Should a chill shoot through the air, the crowd covering the grass will thin out, leaving behind only the most intrepid dog walkers and tai chi practitioners.

And then there are the days when the rain comes down in sheets, or the days it’s so cold that icicles form in your nostrils and the Great Lawn is as wide open as an empty cornfield.

But those days don’t mean a thing to Fred Nelson Jr., who practices his tap-dancing moves in the park’s Meadowport Arch every single day of the year. He wakes up in his Prospect Heights apartment every morning, prepares a breakfast of fruit or applesauce, and heads out to set up his portable tap-dancing studio – consisting of nothing more than a worn-down block of wood – in the elegant sandstone tunnel that connects the Great Lawn to the main loop.

Sometimes he stays for only a couple of hours, but some days he’s out there for six, eight, even 10 hours, shuffling his feet, swinging his arms, and trying to keep count of the rhythm in his head. He’s not a showy dancer, but the acoustics are wondrous, with the click-click-clicks ringing all the way to the other end of the block-long tunnel.

Mr. Nelson is 6-foot-2 but looks even taller, with his long, skinny limbs and thin dreadlocks hanging down to his shoulders. A soft-spoken, graceful man, he keeps to himself, and in the park he interacts only with those who approach him first. He has no cup for money, and when people try to slip him a token of appreciation he politely declines. It’s his goal to master the art of tap dancing, and he reckons he has another year and a half to go. Sometimes he takes private lessons at a studio in the Theater District, but it’s the daily practice that’s going to get him where he wants to be.

He works the night shift as a doorman at a building on the Upper East Side where, he said, he often dances on the job. Even after a full day sweating in the park, he’s never tired.

There’s a foot-size black scuffmark on the arch’s wall from his daily stretching exercises. “Sometimes they paint over it,” he said, “but I always put it back there.”

Mr. Nelson started dancing in the park about four years ago, on a grassy spot by the top of the arch, within sight of Grand Army Plaza. One rainy day he used the tunnel, but when the weather cleared, he continued to practice down there.

“The echo was so nice I got sort of hooked,” he said.

It’s damp and dark where he practices, sometimes so dark that the view out of either end is just a blinding white.

“Light’s just something that I have to give up,” Mr. Nelson said. “I give it up for sound quality. Sometimes it’ll hit me in some kind of way. Sometimes I’m looking out at people doing all these things, and I’m here in the dark, pounding away, but it’s like a discipline, and if I want what I’m after, then I’ve got to make certain sacrifices, so I don’t really mind.”

Mr. Nelson, who said his age is north of 50, started tap dancing eight years ago. At first he was dancing primarily at the studio in Manhattan, but his passion blossomed four years ago, and he started to need to dance all the time. A shoulder injury he incurred swinging a tennis racquet required that he wear a back brace, and he wasn’t certain he’d be able to continue dancing.

“It was like something was being taken from me,” he said. “Once it was about to be taken I realized the value and what it meant to me.”

So he started coming to the park every day with his wooden board, and he was free to dance for as long as he liked.

Picking up tap dancing after age 50 has been difficult for Mr. Nelson.

“You spend all your life moving one way and then you have to unlearn it,” he said.

A native of the Detroit area, Mr. Nelson grew up living for R&B dancing. He stayed on top of all of the new moves, and at a party he was always one of the first people to hit the dance floor.

These days, he’s less interested in performing than in working on counting and orchestration exercises. He doesn’t stray from his 1-by-2-foot wooden block, and as he stays in place and his feet click away, the top half of his body often looks slack, disconnected from whatever is going on down below. He repeats his exercises, over and over, hour after hour, like a child doing scales on the piano. And yet, as unexciting or endless as his daily routine may be, he doesn’t seem to mind.

“Here I basically work for getting the sound,” he said. “Maybe if I keep trying I can create Fred’s sound.”

That he often practices twice a day, sometimes for a combined eight hours, seems natural to him.

“I feel good about it, ” he said. “I’m not under the impression I’m super great or I’m going to make a lot of money, but there’s a spiritual side to it. I know somebody else might not find any meaning whatsoever from it, but it speaks to me and I’m glad it does.”

It all started eight years ago, when Mr. Nelson was riding on the subway with his then 13-year-old son. A troupe of tap dancers came on the subway and he watched his son become entranced by the sight.

Mr. Nelson went out and bought a pair of tap shoes for his son – and something within him told him to buy himself a pair, so he could supervise his son’s education. He took his son to a tap school, but after a few classes the teenager told his father he wasn’t interested in becoming a tap dancer.

“My son said, ‘Why don’t you do it?”‘ Mr. Nelson said. “So I kind of thought about that for a while, and I realized it was something I wanted.”

In his four years as king of the tunnel, he has come to know every square inch of sandstone, every mouse and butterfly that he shares the arch with. “I know the squirrels and they know me,” he said. “And I know the mouse that runs in and out of there” – his index finger pointing out a hole at the foot of the wall.

“Sometimes when it’s just me out here, I say, ‘Am I crazy? What am I doing?’ But something drives me to do it. It’s like if you’re trying to cross a river you’re more than halfway across, instead of turning back and going to the other side you might as well finish the journey.”