Desiree Mitchell thought it would be funny to ride the Metro in January with nothing on her legs but a pair of underwear that read “Burning Up” on the rear.

“Normally, if I wore these it would be freezing,” she said.

Instead it was nearly 70 degrees above and below ground on Sunday when about 50 people shed their pants for the District’s 13th annual “No Pants Subway Ride.”

If the summerlike temperature made it less strange for a bunch of riders to be scantily clad on the Orange Line, they still attracted both praise and dire warnings about seat-transmitted diseases.

After getting caught taking a photo of April Norwood, 38, of Alexandria, who was wearing cherry briefs, a woman with an African accent offered a compliment and solicited advice on acquiring a similar behind.

“I told her I worked hard for it,” Norwood said.

It was her fourth ride, with friend Yasmin Anderson, of Upper Marlboro, who wore black briefs with a homemade “I’m a bad influence but damn I’m fun” patch. Anderson, 34, was a little disappointed by the warm weather, recalling fondly the snow boots they wore with their undies last year. It made it a stranger, sillier thing to do, she said.

“We all have that one crazy friend,” Norwood explained. “She’s my Thelma; I’m her Louise.”

Luckily, they were on public transportation. As the name suggests, the event is a New York import. Started in 2002 by the group Improv Everywhere, it’s now (loosely) organized in Washington by Capitol Improv and Cupid’s Charity, which has its own pantless race in February to raise money for neurofibromatosis research.

“They like not wearing pants on the subway; we like taking our pants off for charity,” explained volunteer Ashley Casper, 32, who wore “Cupid” briefs.

Early versions of the ride were meant to be scattered and subversive. Participants were instructed to avoid one another and pretend they had simply forgotten their pants. A pants salesman would then travel through the train and sell them bottoms.

But pantlessness proved too popular for the strictures of absurdist performance art, especially with the advent of social media. Police got involved, pants were lost, and now the D.C. ride is well-advertised and coordinated with proper authorities.

There was a halfhearted attempt Sunday to maintain the mystery. Casper kicked off the ride outside L’Enfant Plaza with an entreaty to act normal. It did not take; as the group got on the Orange Line to Metro Center there were “Woos!” and laughs and many, many selfies. Mitchell wondered loudly whether, as at a strip club, someone should rub the poles down with alcohol.

At the next platform, a young man approached and asked a question. Mary England, a 31-year-old from Baltimore wearing a rainbow bodysuit over green briefs, responded by suggesting he take his pants off. He gave his name as “Reggie,” and a group soon circled, encouraging him to disrobe. But when Reggie’s jeans came off, underneath was long underwear.

“That was amazing,” England said. “That was taking off a mask to reveal another mask.”

In a city where everything is a cause, sometimes it’s nice to do something pointless, said Lisa McCleskey, 60, of Manassas, who wore black bathing suit bottoms. On top, she wore a pearl-crusted velvet blazer and sequined top, which she volunteered was half her outfit for a daughter’s recent wedding.

She and her husband moved to the area three years ago and have gone pantless every year since. Before, they participated in Seattle.

“It’s fun without being bad,” she said.

Other pairs included mothers and daughters, fathers and sons. Sage Raindancer, 46, wearing green briefs, said the warm weather was what convinced him to take the leap. His 5-year-old son was, surprisingly, the one more reluctant to take his clothes off in public.

Asked how he felt, the young boy whispered: “Happy and shy.”