Jonathan Miller, the president of Miller Samuel, a New York appraisal and research firm, said that about 40 percent of condos in new developments in Manhattan were purchased by overseas buyers in 2015. Many of these wealthy foreign investors, he said, are in the city for only brief periods of time, viewing their apartments more as financial investments than as homes.

At the same time, the growing ranks of the American rich are concentrated in the biggest cities. New York is now home to 320,000 millionaires, according to Knight Frank. There are 5,600 people in New York worth $30 million or more, up 32 percent from 2005, and the number is expected to grow by nearly a third by 2025.

“This is the most mobile demographic on earth,” he said. “They’re not confined by their job or the costs of transportation. And what’s really changed recently is the overseas buyers using New York apartments as a safe deposit box for assets. That makes them even more detached from the city itself.”

Mr. Miller said that New York’s real estate was increasingly targeted at the wealthy. The average sale price of a Manhattan apartment hit $1.95 million in the fourth quarter of 2015, and those in new developments hit an average price of $3.3 million. Many of those new condo towers can feel strangely empty during the slow months for the rich.

The reason, he says, is that more of his clients these days have a “portfolio of homes” that they bounce around to from month to month. “For an overseas buyer, New York is the place they buy for when the wife wants to go on a shopping trip, or for a place for their kids to stay when they’re in school,” he said. “The world’s wealth is coming to New York.”

The American rich, he says, are moving from second-home ownership to more of a hub-and-spoke model. New York serves as a base for seasonal migrations to Miami, the Hamptons and Aspen.