I've been looking into this a bit more to figure some of this out.
Low Density and Low Latitudes
If you look at the core cities within the top 50 MSAs, only 19 have densities over 5,000 people per square mile. And of these, just two, LA and Miami, are below the 36'30" line of latitude. 19 others are below this line and have densities under 5,000 ppsm. Meanwhile, among the top 50 MSAs north of the 40th parallel, 11 of 14, or nearly 80%, have core cities with densities over 5,000. Chart below shows this, and please don't ask why I use airport code abbreviations.
|Amount||Latitude||Core City Density||Cities Represented||over > 5000 ppsm in 1950|
|ranked 1 to 50 and North of 40th||Metros with Core City > 5000 people per sq mi||LGA, BOS, PVD, BDL, BUF, PIT, CLE, MDW, MKE, MSP, SEA||LGA, BOS, PVD, BDL, BUF, PIT, CLE, MDW, MKE, MSP, SEA|
|ranked 1 to 50 and North of 40th||Metros with Core Under 5000 people per sq mi||PDX, DTW, SLC||PDX, DTW|
|Between 36’30” and 40||Metros with Core City > 5000 people per sq mi||DCA, BWI, PHL, STL, SFO, SJC||DCA, BWI, PHL, STL, SFO|
|Between 36’30” and 40||Metros with Core Under 5000 people per sq mi||RIC, ORF, CVG, SDF, MCI, DEN, SMF, IND, CMH||RIC, ORF, CVG, SDF, MCI, DEN, SMF, IND, CMH|
|ranked 1 to 50 and South of 36’30”||Metros with Core City > 5000 people per sq mi||LAX, MIA||MIA|
|ranked 1 to 50 and South of 36’30"||Metros with Core Under 5000 people per sq mi||RDU, CLT, ATL, BHM, TPA, MCO, JAX, MSY, BNA, MEM, OKC, DFW, HOU, AUS, SAT, PHX, SAN, LAS, ONT||ATL, TPA, BNA, PHX, JAX|
Now why is the 36'30" line so important?
As part of the Missouri Compromise, no new slave state could be admitted above this line. It was also the existing border between Virginia/North Carolina and Kentucky/Tennessee when the law was passed in 1821. But even then, just three of the country's 50 largest cities (Charleston, Savannah, and New Orleans) were South of 36-30. And this did nothing to stop growth in existing slave states, where Memphis shot up into the top 50 by 1860, having only been incorporated in 1826, while Nashville and the Georgia fall line cities of Augusta and Columbus grew into the top 100. (Atlanta did too, but it was still smaller than those two in 1860)
If we jump ahead to 1950, 41 of the 50 largest cities in the country had densities over 5,000 ppsm (aka people per square mile), far greater than the 19 of top 50 core MSA cities that have these densities today. But the nine that were under 5,000 ppsm, were all south of the 36'30". 1950 was a peak population year for many traditional cities, and the top 50 MSAs still had over 50% of their aggregation populations living in core cities. And this was before widespread air conditioning and suburbanization. It was also two years before Atlanta nearly tripled its land by annexing Buckhead and much of the western half of today's city, pushing its density back under 5,000.
Annexing Cities vs. Annexing Farms
Annexation below the 36'30" was very different than it had been in the North. When Philly annexed the Northern Liberties and Southwark in 1854, they were already existing cities in their own right. Same when Boston picked up Charlestown, Roxbury, and Brighton, and when New York added Brooklyn. Densities of the core jurisdictions kept rising. But Memphis, Charleston, Savannah, Nashville, Jacksonville, Phoenix, and Atlanta added rural places where new housing was planned, but often not yet built, leading to major reductions in densities. All seven cities were over 5,000 ppsm at some point before 1960, but haven't gotten back to that level since.
Annexation of adjacent rural areas has been far less common above 36'30", occurring primarily in Columbus, Indianapolis, and Louisville. One technique used to do this, county-city consolidation, first occurred through a de-annexation, when San Francisco County was broken off of San Mateo County in 1856.
One of the distinguishing features of these low density Southern cities was that they had no adjacent cities or towns to absorb like their larger Northern counterparts. There was no equivalent to Minneapolis-St. Anthony, Boston-Charlestown, Manhattan-Brooklyn, Cleveland-Ohio City, Washington-Georgetown, or even Seattle-Ballard. One reason is that the Northern cities were built directly on river banks, with different cities on each side, or as a string of towns near an ocean or a lake, each with their own port. Moreover, these places often had to come together for survival. Beyond just the cities it annexed, Boston had a long competition with nearby Salem for international shipping traffic. It needed to get bigger to finance port expansion. Minneapolis and St. Anthony fought for years across the Mississippi's only waterfall, and were forced to merge to help preserve it. Portland had to surpass nearby Oregon City, while Cleveland and Ohio City fought for dominance on the Cuyahoga before coming together.
Not all Northern annexations were about regional battles. Chicago acquired Lake View Township (today's Lincoln Park and North Side neighborhoods) because its residents wanted better sewer services. And they joined in 1889 with a density of about 5,000 ppsm. Meanwhile, when Atlanta acquired Buckhead and its Northwest neighborhoods 63 years later, it wasn't because voters wanted services, but the city wanted to add wealth, and the impacted citizens were told they had to join by the state. Moreover, this added 80 square miles of land with barely more than 1,000 ppsm to the city.
The high capital cost of sewer and water hookups remains a primary driver for annexation today, and has driven mergers of rural areas throughout the southern half of the country since WW2. Moreover, there is a strong history of urban density in low latitudes by the ocean, as LA and Miami have shown, though LA certainly wasn't that way 50 years ago. In 1910, prior to major annexations, Charleston and Savannah had over 10,000 ppsm. Memphis and Nashville were over 5,000 ppsm at this time as well. But most of today's major Southern and Southwestern metros are not on the Ocean nor on a river with heavy barge traffic, and in the cases of Atlanta and Charlotte, their downtowns aren't even on the river. Where would you see such a thing in the North?
Cities above 36'30", even those in the South, like Richmond and Norfolk, were built on fall lines or ocean ports, much like most Northern cities. But you look at a map, and you can see a city could have formed around the peninsula that's Charleston, the area between Lake Pontchartrain and the Mississippi that's New Orleans, or even along the fall line at Augusta, GA. But Orlando? Really, Mickey Mouse is that powerful? And then what about San Diego? Shouldn't it be larger than LA, where downtown is 15 miles from the port?
More on this in an upcoming post.