Andrew Williams poses without his shirt behind his backpack Thursday at a park near his home in Warren, Pa. Williams plans to participate Sunday in Naked Hiking Day, an annual event celebrated on the first day of summer by a relatively few enthusiasts.

Hagerstown, Md. ? Every year on the first day of summer, a few outdoor enthusiasts nationwide expose virtually all of themselves to insects, scrapes and thorns for the pleasure of bonding with nature au naturel.

They call it Naked Hiking Day.

“There’s no way to explain it until you experience it,” said Andrew Williams, 28, a machinist from Warren, Pa., who first hiked naked six or seven years ago. “It’s not about being lewd and crude and all that. It’s just enjoyment.”

This year, the summer solstice falls on a weekend — this Sunday. Father’s Day.

Hikers who prefer clothes are not happy.

Rangers and police warn that people caught outdoors in the altogether could be charged with indecent exposure. Managers of the Appalachian Trail, where the tradition is sometimes observed by those trekking from Georgia to Maine, also discourage nudity.

“It’s just rude,” said Brian King, spokesman for the Appalachian Trail Conservancy in Harpers Ferry, W.Va. “People are out there hiking with their kids and families, and there are Girl Scouts and Boy Scouts.”

Law enforcement authorities say they see so few nude hikers, even on Naked Hiking Day, that they don’t consider it a big problem.

“It’s rare, probably because we have a lot of remote territory out there where one would not be detected,” said Capt. Woody Lipps of Virginia’s George Washington and Jefferson national forests, which contain parts of the Appalachian Trail.

Nude-hiking advocates say they aren’t out to startle people.

“I go out of my way to avoid being seen,” said Shane Steinkamp, 40, a New Orleans software engineer who blogs about his naked hikes in Mississippi’s Homochitto and De Soto national forests on his Web site, theplacewithnoname.com.

“I value the freedom to be able to walk in freedom, but I also accept the responsibility that I should not surprise folks who may take my enjoyment of nature in the wrong way,” Steinkamp wrote in an e-mail.

For Williams, the path to nakedness in the woods started at home in rural northwestern Pennsylvania.

“I would walk around the house like that, and walking inside the house turned into walking outside the house, and then short little adventures through the woods. So the next thing I know, I’ve got a backpack strapped on and I’m walking down the trail,” he said.

Both men said they don shoes or boots and a hat if conditions warrant. Sunscreen and insect repellent are also recommended, although Jonathan Jones, a 33-year-old dental hygienist from Waynesboro, Va., said there’s an advantage to hiking naked in a buggy area: wood ticks on the skin are easier to spot.