There’s no way to rush cold-brew coffee. If you’re running a coffee shop, you need to anticipate demand. Every year, that demand is increasing: The United States is becoming a cold-brew nation.

In the past, coffee sales lagged during the summer and rose sharply during the holiday season. But cold brew now drives a surge in demand during warmer months, too, far more so than other iced coffee drinks. Coffee sales spike when the mercury rises. Cold brew is also attracting an entirely new audience for coffee: millennials, many of whom are making it their drink of choice.

“It’s pheromonal,” said James Freeman, founder of Blue Bottle Coffee. “And the feedback loop encourages more iced orders — watching other people order iced coffee inculcates the desire.” It’s the coffee version of “I’ll have what she’s having.”

What was once a regional curiosity largely limited to New Orleans and the South is now found throughout the country. The shift started about 10 years ago, when cold brew was adopted by innovative coffee shops like Blue Bottle (which sells what it calls New Orleans- style iced coffee, a milky cold brew flavored with chicory) and Stumptown Coffee Roasters (which sells nitro cold brew, infused with nitrogen so that it’s slightly fizzy, with the thick, creamy head of a good stout).

Cold brew was still a relatively niche market until 2015, when Starbucks introduced the drink in a number of stores; it is now available at every one of its more than 13,000 locations in the United States, 800 of which also offer nitro. It’s a coffee with both mass-market appeal and indie credibility. Today you can find cold brew at a coffee shop where everything is meticulously crafted by hand, and at a Dunkin’ Donuts.

The drink’s range is expanding even more rapidly when you count canned, bottled and packaged coffees, called “ready to drink” within the industry. You can get that New Orleans-style iced coffee in a school-lunch-size milk carton, or that nitro cold brew in what looks like a beer can.

Ready-to-drink, which has long been available at Whole Foods and other upscale markets, is now appearing everywhere.

What is cold brew? Essentially, it is a preparation. You steep coffee grounds in room-temperature water (which isn’t “cold,” strictly speaking) for six to 20 hours (depending on the recipe) to make a concentrate that can be diluted with water and served over ice. By giving up heat, you have to add time.

Cold brew is more than a slowed-down version of hot coffee; it’s a noticeably different product. Hot water brings out the acids in coffee, a characteristic that professional tasters call “brightness.” Cold water doesn’t, but still gets the full range of mouthfeel and sweetness. The absence of acidity is even more pronounced when compared with the iced coffee of the dark ages (of a few years ago), when it was almost always made with hot coffee chilled in the refrigerator. When hot coffee cools, even more acids develop, many of them unpleasantly harsh.

While hot coffee can be volatile, changing in flavor at different temperatures, cold brew is relatively stable, which makes it particularly well suited to being packaged. Joyride Coffee, a wholesaler that supplies offices and coffee shops in Boston, Los Angeles, New York, San Diego and San Francisco, sells 20-liter kegs of cold brew and nitrogenized cold brew that are tapped like beer. The kegs have a shelf life of 90 days.

Perhaps more important for the everyday drinker, cold brew is a perfect companion for milk. Rather than battling the aggressive acids in chilled hot coffee, the dairy tastes full and rich. Cold brew “is a sweet, round coffee milkshake without the sugar,” said Andrew Linnemann, a vice president on the Starbucks global coffee team. “Especially if you add a splash of milk.”

But cold brew has a poor reputation in some coffee circles. According to those critics, cold brew’s selling point — its absence of acidity — is a flaw. The best coffees in the world, the ones grown at high altitudes, command higher prices specifically because of their complex acidity: Brightness is a virtue. Why give up one of the defining characteristics of a great coffee?

In addition, detractors say, the long exposure to air during the steeping process can leave cold-brewed coffees tasting flat and oxidized. Some coffee shops treat cold brew as a dumping ground for lesser coffees — old beans that are losing their flavor or uninteresting beans that couldn’t stand up to conventional brewing.

“The main argument is the lack of acidity, that it’s very one-note,” said Jenny Bonchak, who started Slingshot Coffee Co. with her husband, Jonathan. “But that’s not how we want to drink coffee. We wanted something that was balanced and that was going to be juicy.”

Bonchak uses high-quality beans from Counter Culture Coffee, a roaster based in Durham, N.C., and Slingshot Coffee is praised by the kind of coffee nerds who usually wouldn’t touch cold brew with a 10-foot straw.

At All Day, a coffee shop in Miami, cold brew is the foundation of the menu. Camila Ramos, one of the owners, uses beans from Ruby Coffee Roasters in Nelsonville, Wis., for standard cold brew, and beans from Toby’s Estate in Brooklyn for nitro. There’s cold brew in the shop’s milky Thai-style iced coffee and in the fizzy and tart drink called Our Sweetheart #4, an exquisite mixture of coffee and rosemary limeade.

Ramos wasn’t always convinced by cold brew. Her conversion began when she was a competitive barista vying for a national championship, and she started experimenting with the process. “I often describe it as a ‘yellow’ flavor, weird, underextracted,” she said. “I had to challenge myself: Is cold brew actually a good thing, but it’s not been made well?”

She has since developed an exacting procedure for making cold-brew concentrate at All Day. The essential structure is the same as other recipes — coffee, water and time — but she details how long the beans should rest after roasting (21 to 28 days), how to agitate the coffee grounds in the water (with a wide spoon for five minutes) and which filter to use (the paper filter bags manufactured by Toddy).

During Miami’s nine-month summer, it’s her best-seller — remarkable for a town that loves its Cuban espresso. “From a logistical standpoint, we can get cold brew out the door in 10 seconds,” Ramos said. “If you want a coffee and you want it quick, cold brew is the answer.”