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The grand conclusion? Americans love warm weather.

If you're like me, you spend the week of July Fourth finding excuses to get outside in the sun, and reading policy papers on urban growth in the 20th century. Fortunately those two things aren't as unrelated as they might seem. In fact, as we're reminded by a recent review of the causes of city population changes, few variables have predicted metro area growth in the past century as well as warm weather.

It's easy to overlook weather as a legitimate living factor, if only because the idea of moving somewhere just for the sunshine feels a bit too simplistic. And for sure, the reasons why certain cities grew faster than others in the 20th century are as numerous as they are complex. But when urban scholars have studied population changes during this period, they've routinely found a pleasant climate to be a powerful draw.

Take this chart linking state population growth and warmth from a 2005 paper by Harvard's Ed Glaeser [PDF]. Between 1920 and 1980, every 1 percent rise in January temperature led to an expected growth rate of 2.3 percent for a given state. The 25 states with mean January temperatures below 30 degrees grew 95 percent in population, on average, during this time; the 25 states above that mean temperature grew 309 percent.

What's been true for the states has largely held for metropolitan areas, too. During the second half of the 20th century, in particular, cities in the Northeast and the Midwest tended to lose (or not gain as many) residents compared to those in the South and the West. That much is clear from this map included in a 2007 paper by Jordan Rappaport of the Federal Reserve Bank of Kansas City [PDF], which shows the expected population growth of localities between 1970 and 2000 based on weather:

Now there seems like one very obvious reason why warm places would have thrived in the recent past: air conditioning. Indeed, the mass marketing of AC after World War II did make it possible for people to endure the hot summers of places like Phoenix in exchange for the lovely winters. But the growth of places with cool summers like Northern California, not to mention the fact that the weather-related shifts date back to 1920, suggests there's more to it than AC alone.

So if the proliferation of AC doesn't by itself explain why people moved to warm metro areas in recent times, what does? Well it could be that the decline of farming in the United States over the past hundred or so years played a role, or that the growing number of retirees has nudged people away from the cold. But those cultural shifts didn't quite coincide with the weather-related population changes, either.

Instead, Americans might have become sun-seekers in the 20th century largely because they also became wealthier during this time. The theory makes a great deal of sense. As a population exhausts its supply of mobile amenities, it naturally starts to value those that can't be delivered right to the door — a pleasant climate among them. In that sense, good weather is one of life's final luxuries. Rappaport concludes:

Overall, the empirics are most consistent with the explanation that individuals have been steadily increasing their valuation of nice weather's contribution to quality of life.

Now it bears repeating that nice weather, while easy to overlook as a factor for city growth, was definitely not the only reason people moved to certain places in recent times. A desire for space and suburbia pulled Americans toward some metro areas, and the decline of U.S. manufacturing pushed them away from others. The general skills and education of a population also influenced its residential choices. Above all, the expansion of transportation options facilitated all types of local migrations.

But even if sunshine isn't the exhaustive reason people settle somewhere, hopefully we all get a reminder this Fourth of July why it's often been a big one.

Top image: zhangyang13576997233 /Shutterstock.com

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