American geography is changing rapidly. People are moving to modern-day boomtowns and nobody seems to be replacing them in their home cities.

But why are certain locales so attractive to people while others can’t help but watch people leave? As you might anticipate from changes this drastic, it’s a combination of several factors working in tandem.

Living in the Sunlight

Most people don’t enjoy extreme heat any more than they enjoy extreme cold. But the numbers clearly show the fastest-growing cities in the country correlate to warmer climates.

Even less than a century ago, it would have been inconceivable that desert towns, like Phoenix and Las Vegas, and swampland settlements, like Houston and Jacksonville, could become such bustling metropolises in such inhospitable climates.

At the same time, most of the largest cities were north of the Ohio River and east of the Great Plains. While there were many reasons for this, one of them was certainly that it was easier to stay warm in the Rust Belt in the winter than it was to stay cool in the Sun Belt in the Summer.

Then they invented air conditioning.

The advent of air conditioning alone isn’t the reason for the Sun Belt boom, but it certainly made moving there more appealing. In our increasingly indoors-y society, people would rather go through 90-degree weather to get between buildings than trudge through 9-degree weather. But what kind of buildings would they be going to?

Earning a Living

For much of American history, the biggest cities were not only in geographically strategic locations, but tended to be in colder climates because that’s where the high-density jobs were. Some of these cities had more warm, insulated offices, while others had more factories and mills full of hot air, but most every city had some of both.

When blue-collar industrial work started fading and white-collar commercial work (and especially tech work) started booming, the landscape shifted. When these new companies decided where to set up shop, they could pick anywhere.

Many chose to build their (air-conditioned) offices in cities where it was nice and warm. Some chose to set their roots in specific colder cities, which nevertheless had a proven history of business savvy and innovation, like Chicago and New York. Almost none of them, however, picked the cities crowded with abandoned factories sticking up into gray skies.

The employment factor is not strictly a “hot weather/cold weather” thing, as there are plenty of prominent cases of smaller Southern cities currently hurting after all the factories closed.

On the flip side, some surprising names you might see on a “fastest-growing cities” list might be Fargo and Bismarck, North Dakota. (You’d better believe in wind power, because those energy-sector jobs are coming.)

But the cities that were the biggest and fell the hardest were disproportionately in the colder North, and the promising cities on the up-and-up are disproportionately in the South and Southwest.

A Taxing Issue

What do you do when your city has excellent (or at least tolerable) weather, but doesn’t have much incentive beyond that to draw in employers and workers? Why, you boast extremely low taxes, of course!

The cities and states that are growing the fastest have consistently lower personal and business taxes than cities that are losing denizens. Clearly, this has been working.

It’s working so well, in fact, that many experts insist tax rates alone can be credited as responsible for the current migration patterns. They may be right, but it’s worth noting that other experts have a more “developed” theory.

“Developed” Was in Quotes Because It Was a Hint

A different major theory is the greatest parasite feeding upon an ailing city is not its competition in California or Texas, but rather its own suburbs having gotten out of hand. This theory is not meant to oppose the “Sun minus Tax equals Business” idea, but instead is meant to turn some unturned stones.

Healthy, growing cities have well-developed, lively downtown areas and urban cores; sickly, shrinking cities have derelict city centers where very few well-off people can be found.

In the newest trend of young people moving back into the cities from the suburbs they were raised in, a successful city will have a thriving downtown area where regular people can live, work and play. A failing city will only be a symbolic nucleus to a bunch of satellite suburbs, which is where all the business is.

According to this theory, a city on the downswing is one where everybody exists in the suburbs and nobody has any reason to go downtown; a city on the rise is one where everybody exists downtown and nobody has any reason to go to the suburbs.

But this might seem wildly counterintuitive. After all, anybody who’s ever driven through Phoenix or Dallas or the Tampa Bay area can tell you it’s just miles and miles of enormous suburbs doing their own thing with a slightly larger main city in the middle. Yet those are always on the list of cities that are supposedly booming. How does that work?

That’s because those are also on the list of metropolitan areas that are booming. People often forget cities and their suburbs are distinct entities with symbiotic relationships. In a perfect situation, they help each other out, but sometimes they clash and can even impede one another.

This is why, if you look at a list of the largest cities in the United States, you’ll find some places in the top-20 that you’d never expect because they don’t have healthy suburbs to boost their credentials. (I’m looking at you, Indianapolis. You too, Jacksonville.)

If you compare that to a list of the largest metropolitan areas, you’ll still see a lot of overlap, but you’ll also see a lot of switching up in positions.

Outside of Texas, nobody really notices how large San Antonio (the U.S.’s 7th largest city), Austin (11th largest) and El Paso (22nd) are because they don’t have many suburbs to up their perceived size. Meanwhile in California, San Jose (10th) is consumed by the greater Bay Area and San Diego (8th) lives in Los Angeles’s shadow.

What is the point of all of this? What most people don’t realize is there are two separate booms going on right now: an urban migration and a suburban migration.

Some suburbs are doing great while their cities flounder, while some cities are living it up while their suburbs struggle to exist.

In all of this, the most successful areas in the country are where a big city and its suburbs are both playing to their strengths and working together, and the places having trouble are where a city proper is fighting with its satellite cities over people, jobs and entertainment.

Then again, I suppose bad weather can make anybody cranky.