I'm really big on geography and climate. San Diego, southern city in California, is at the start of a much more arid and dry climate as you go south from Santa Barbara, LA, etc.
The border is based on where the Spanish had Baja and Alta California before the U.S. took control, right?
Specifically, I want to know: Why is this where the border was declared? On a slope (not directly E-W), and did the climates have something to do with it (did Alta California have the 'better' weather in a sense)
Thanks!
Take a look at the 1847 Disturnell map, which shows the historical border between Alta and Baja California. It draws the line at "S. Miguel", an abbreviation for Mission San Miguel, which is a bit over 30 miles south of the current border. This line was drawn up between the Franciscans in the North and Dominicans in the South to delineate their respective territories and when New Spain was creating the province of "Nueva California", it was reused.
Another thing to take note of in the Disturnell map is how inaccurate the map is geographically. The gulf depicted reaches all the way to modern day Yuma, a full degree of latitude above its true location. As I mentioned in a long-ago answer, Mexico was deeply interested in preventing the US from getting a port on the gulf of California when the Treaty of Guadalupe Hidalgo was being negotiated, so this was a matter of serious importance. The US on the other hand needed river access to send ships into the interior of their new territory, and required at least the confluence. Both sides resolved to agree on two points of reference and establish a survey to resolve the exact border later. Quoting from the treaty:
And, in order to preclude all difficulty in tracing upon the ground the limit separating Upper from Lower California, it is agreed that the said limit shall consist of a straight line drawn from the middle of the Rio Gila, where it unites with the Colorado, to a point on the coast of the Pacific Ocean, distant one marine league due south of the southernmost point of the port of San Diego...
In order to designate the boundary line with due precision [...] the two Governments shall each appoint a commissioner and a surveyor, who, before the expiration of one year from the date of the exchange of ratifications of this treaty, shall meet at the port of San Diego, and proceed to run and mark the said boundary in its whole course...
One of the results of these subsequent surveys was the Gadsden Purchase, when the US realized just how far off the earlier maps were.
Ultimately, the simple answer to your question is that the border is just the line between the two points that were absolutely known to exist and be agreeable. They simply didn't have accurate maps to draw it differently.
[1] Shelburne, O. B. (2020). From Presidio to the Pecos River: Surveying the United States–Mexico Boundary along the Rio Grande, 1852 and 1853. University of Oklahoma Press.