Why are Americans moving to warmer cities?

For the past few years, the country has been heading South. More Americans are moving to Texas, Arizona, and Florida, pouring people into the fastest-growing counties in the U.S. New Census data released last week shows which U.S. counties grew the most from 2015 to 2016. And nearly all of them are warm, sunny places.

Once again, the counties that surround Sunbelt cities like Phoenix, Orlando, Las Vegas, Houston, and Dallas are seeing the greatest influx of new residents. Arizona’s Maricopa County, where Phoenix is located, replaced Harris County, Texas, home of the Houston metropolitan area, as the fastest-growing county in the country. (The one exception to the trend is the exceptional growth of King County, Washington, where the Seattle region is adding more than 1,000 new residents per week.)

Surely all these sun-seekers are coming from some place, and it’s not hard to guess where. When you compare which counties are bleeding residents, they mostly surround northern, inland cities: St. Louis, Chicago, Detroit, Cleveland, Milwaukee, Pittsburgh, Buffalo. In other words, the Snowbelt.

Take Chicago, which lost more residents from 2015 to 2016 than any other big city in the country. A new study using Moving.com data shows that Chicagoans are moving to mostly four cities: Phoenix, Los Angeles, Atlanta, and Dallas. Goodbye lake effect, hello sunshine.

As a Brookings Institute analysis of domestic migration shows, this Snow-to-Sun migration is pretty much a direct correlation. The slowing economy and subsequent recession is what kept people from moving around too much a decade ago, but as the economy gets better, the fertility rate keeps dropping, and the population continues to age, these current trends should continue.

It’s actually not just the Snowbelt cities that are losing residents: Even big coastal cities like New York, San Francisco, and Los Angeles all saw slowed domestic migration over the last year. In the big, expensive cities, however, people are likely leaving due to a lack of affordable places to live.

The biggest factor affecting Snowbelt-to-Sunbelt migration is the flight of the boomers. Not only is this giant, aging portion of the population all leaving the workforce at once, they’re also leaving cities. That could also be attributed to the lack of walkable, affordable housing that allows seniors to age in place. The fastest growing metropolitan area in the country is The Villages, a master-planned, age-restricted community outside Orlando that only welcomes residents who are 55 and older.

The other challenge for the cities that will need to absorb this exponential growth is that they also happen to be ones that aren’t best equipped to deal with more residents. Coming of age along with interstate freeways, Sunbelt cities are stereotypically sprawling and built around the car. That will need to change quickly as these cities add infill density and transit systems that can support an era of smart, sustained growth.