I was looking on Google maps at the Baja peninsula when I idly wondered how far the Southern California climate (dry, temperate, and sunny) reaches. Does the entire coast of the peninsula have the same climate?
The North American Desert article may be helpful.
It pretty much runs down the west side of the penninsula until the end of the Sierra de San Pedro Matirs, a little bit south of San Quintin. After that, it’s mostly desert all the way to the end. The reason for the Southern Californian “mediterranian climate” is that there’s the a string of very high mountain ranges (called the penninsular ranges) that are just a little bit inland. These are related to the San Andreas Fault, but that ceases to exist when it turns into a new mid-oceanic spreading center in the Gulf of California. The San Pedro Matirs are the southernmost of the high penninsular ranges, even though they’re not currently being held up by tectonics like, say, the San Jacintos in California.
While mountain ranges do have some effects in the region (occasional orographic rainfall), the Mediterranean climate has much more to do with the combination of latitude and nearby ocean – compare analogous zones in Chile, South Africa, Western Australia, and of course the Mediterranean. Rainy winters and dry summers (the opposite of what dominates much of Mexico, especially toward the southeast of the country).
GreasyJack is right on about the location of this transition, from Mediterranean to more or less pure desert (latitude of more purely subtropical high-pressure year-round dryness).
Hence, there are several high-quality vineyards in northern Baja.
Thanks guys; great answers!