Jeremy Levitt, a founder of Parts and Labor Design, a hospitality design firm, has a piece of advice about dress. “Go as if you’re having a hot toddy outside in front of a bonfire that doesn’t exist,” he said.

Mr. Levitt, who lives in Manhattan, has learned the hard way that his feet become blocks of ice when he sits outside for an hour or more. He’s taken to wearing warm shoes with a few layers of socks, and he and his wife and children also make sure to bring blankets.

With their beanies, winter jackets and gloves, outdoor diners have the look of winter athletes. Erika Chou, a Manhattan restaurateur who operates the spots Kimika and Wayla, has seen people come in “with a full snowboarding outfit, like Burton head to toe,” she said.