One of the problems with dividing California is that it needs to be divided into more than two pieces. The Central Valley is neither north nor south, geographically or culturally.
Yep. Mirrored on the land side by the Tehachapi range, which marks the border for certain less vagile fauna. Hence the traditional Tehachapi split. Geographically it is the best candidate.
Myself, I’ve always had a tendency to use a vague line starting at Big Sur to split the state. As Giraffe noted this mercifully confines Fresno to SoCal ;).
California stretches 1,240 km north-to-south covering nearly ten degrees of latitude. Here’s the state outline superimposed over France, for comparison. The coast simply gets cooler going north. Southern CA is swathed with desert while Northern CA is forested, with the very fertile Central Valley sort of in between. The Sierra Nevada mountain range runs nearly the length of the California-Nevada border. It’s a very diverse state socially and ecologically.
As long as we get Yosemite I’m good.
I saw an Absolut ad on a San Francisco building (while going to the Dopefest there) that labeled the two regions “California” and “South California.” It was entitled “In an Absolut World.”
The Grapevine.
Cool. I live in Provence. 
Why wouldn’t the logical boundary be. . . somewhere in the middle? Like, how any one can say the Grapevine just baffles me. I would say (looking at a map) somewhere around Fresno (even a little north of it).
Assuming that we’re just doing a “North / South” split, no “Central Coast” or “Central Valley”:
I, too, think of Big Sur as the furthest south point of “Northern California”; even the name (sur = “south”) suggests that it’s part of en entity that stretches northward from it.
One can also get down as far as Big Sur – but no further – from the SF Bay Area (and even from Fort Bragg on the Mendocino Coast or Nevada City in the Sierra foothills) using only local public transportation. This, to me, means that the cutoff line between North and South cannot be further north than Big Sur. [In addition, I’ve only ever heard anyone around Big Sur refer to its main thoroughfare as “Highway One”, never “The One” as those SoCal people have a tendency to do…]
Here is a map of the counties of California. Note that for the southernmost two-thirds or so, both the coast and the California / Nevada border run approximately Northwest - Southeast. If one wanted a simple “single-line” boundary, I’d propose running it perpendicular to the CA / NV border, bisecting Mono Lake (located near the “N” of MONO County on that map). Running approximately due southwest, such a “single-line” NorCal / SoCal border would reach the Pacific Coast just south of Big Sur.
Alternatively, if one wishes to maintain existing county boundaries, I’d put Monterey, San Benito, Merced, Mariposa, and Mono counties in NorCal, with the counties to the southeast of them (San Luis Obispo, Kings, Fresno, Inyo, etc) in SoCal. I’m undecided on Madera County; it contains the geographic center of California, and my “single-line border” (in the preceding paragraph) nearly bisects it, so it could go either way. Perhaps the residents could vote on it, or one could take polls as to whether a plurality refers to the main road through Madera as “Highway 99” (Norcalese) or “The 99” (SoCalese).
[Both of the above methods keep Yosemite in Norcal while banishing Fresno to SoCal, as nature intended. Sacramento and Stockton both need to be in NorCal too, despite what Hometownboy’s friend thought.]
Hey, if you’re going to call it “San Fran”, it’s just as well you’re not trying for inclusion in Northern California! 
San Luis Obispo is the problem child. It really has a Northern Cal ambience. It is also just barely North of Pt Conception, so biologically speaking, it should be in the North.
Both of my parents are originally from Southern California (dad’s from East LA, mom’s from Pasadena), but they moved to Northern California before I was born, and I grew up there. But most of my extended family still lives in Southern California (one of my aunts lived in Santa Monica for years, now her family is in S. Pasadena, most of the rest migrated south to Orange County). So I’ve spent plenty of time in both, and there is definitely a difference. Southern California is practically a desert (and to the east, it IS a desert), warmer, politically more conservative, more densely populated, and more Latino. The idea of a California of sun, sand, and surf, is almost entirely Southern California. Northern California is cooler, especially along the coasts where it rarely gets very warm at all, farther to the left politically, has redwood forests, and is less densely populated. This is where the idea of the “hippie” California comes from.
Los Angeles, Orange County, San Diego, Hollywood and the film industry are all in Southern California.
The Bay Area (including San Francisco, Oakland, and San Jose) Silicon Valley and the computer industry, and the Wine Country, are all in Northern California.
I’d put the line just south of Monterey, personally.
Former SLO resident here. SLO is neither NorCal or SoCal, it’s the Central Coast. SLO residents hate being lumped in Southerners.
SB is probably the boundary for SoCal. Gaviota and Lompoc up the 101 to Santa Maria, Pismo, SLO, Morro Bay, up to probably Atascadero and/or San Simeon is Central. North of there, that’s NorCal.
No idea how to deal with the interior of the state, though.
Nevada doesn’t have any water. Colorado has the water. And we don’t want Fresno, either.
Northern Californians are smart, earthy, generous, and hip. Southern Californians are dumber than rocks. Unfortunately, thanks to Hollywood, much of the world equates “U.S. Citizen” with “Southern Californian”.
Northern California has water. Southern California needs water. Southern California has more people and more legislative clout. Therefore there is a huge aqueduct bringing water from NoCal to LA. Another aqueduct supplies part of the Central Valley. If I were splitting California, I’d do it by rainfall.
Geographic map - - to show the Valley and the mountains.
My parents were born in SoCal, but by the time they retired, the area was so built up that they moved to NoCal for some elbow room. Fly over the LA area and the only green areas you’ll see are parks, golf courses, and cemetaries. I know that Greater LA isn’t all of SoCal, but it’s where most of the people are.
An introduction to some Southern California rocks:
Thanks to posts like the above-quoted, much of the world equates “Southern California” with “Hollywood.”
The same north/south dichotomy exists here in Idaho as well. Most Idahoans will concur that northern Idaho begins at the town of Riggins. Northern Idahoans often feel alienated and neglected by us in the southern part of the state. The climate there is cooler and wetter than in southern Idaho with more forests and mountains. The logging and mining industries are more prominent there, too. We have plenty of forests and mountains in southern Idaho, too, but this end of the state is more known for its high desert climate and agriculture. Northern Idaho is also still reeling from its stigma in being associated with KKK/skinheads/Neo Nazi’s/Aryan Nations/White Supremacists or whatever they’re called, even though the David Butler compound there was torn down years ago and the group was bankrupted. The capital and most of the larger cities in Idaho are in the southern half of the state, so there’s more weight carried here, legislatively and politically speaking. Only one highway (U.S. 95) connects the two ends of the state.
Northern California: redwood trees, wine country, pot-growers, spotted owls, Mt Shasta, Tahoe (barely), rock-covered beaches and cold ocean water, hippies, rainy winters, Berkeley.
Southern California: movie & TV industry, deserts, dry winters, sandy beaches & warm water, The OC, Malibu, Nixon & Reagan, USC.
Yeah, there’s a difference.
Gah!
Southern California does not equal L.A.
(And if you think the water is warm, you are in for a surprise.)
Culturally I think it’s mainly the rivalry between San Francisco and Los Angeles. I won’t go into it here; there must be a half dozen threads on that subject.
Geographically, I think if you are north of L.A. county but still in commuting distance, you live in SoCal.
Our ocean water is cold here too. You can’t go in without a wetsuit for a good part of the year.
L.A. has been voting strongly Democratic in recent elections; the fact that Reagan and Nixon came from here really doesn’t inform local culture or politics, IMO.