The San Francisco Bay Area ranks as the second richest city in America, despite topping New York for billionaires, with more than 305,000 millionaires and 68 billionaires. The Bay Area's growth rate over the past 10 years has been even more impressive, with its millionaire population soaring 82%. The surge in investment and growth in artificial intelligence is expected to add another boost to the area.The fastest-growing U.S. city for the ultra wealthy among the top 10 is Austin, Texas, which has more than doubled its millionaire population over the past decade to nearly 33,000. Miami is up there too, with 87% growth in millionaires over the past decade — but with one-tenth the New York City total.The numbers show that the twin wealth hubs in the U.S. endure, despite wealth migration to the Sun Belt — which is roughly defined as the southern third of the U.S. known for its sunny weather and lower tax states."Despite the recent rise of major wealth hubs in Texas and Florida, the Bay Area and New York City are expected to remain America's wealthiest cities for many more decades to come," said Andrew Amoils, head of research at New World Wealth.