Summer officially starts this year on June 21, but that’s only the solstice, the day when the sun reaches its highest position in the sky. Down on street level, summer really begins on the first humid, sun-streaked day, when even the thought of sipping a hot cup of coffee is too much to bear. It’s as if, just as birds know instinctively when to migrate, we wake up one bright morning and agree that it’s iced coffee season.

Gregory Zamfotis, the owner of Gregorys Coffee in New York City, which is about to open its 24th location, starts tracking the temperature in early May. “I literally look at the weather forecast and send emails to my store leaders,” he said. Mr. Zamfotis estimated that 75 percent of the coffee he sells is hot and 25 percent is iced for most of the year. With the start of iced coffee season, those numbers flip, and 65 percent of the coffee he sells is iced.

That change can happen overnight. “You don’t want to get caught and run out by 9 a.m.,” he said.

The danger of running out is real. All of the iced coffee at Gregorys Coffee is cold-brewed, a process that takes 12 hours and yields a drink often described as smooth, round and lush. Mr. Zamfotis estimates that he sells 10,000 servings per day at the peak of the season.