Someone asked this among a group of perfectly intelligent UC Berkeley staffers and no one had a good answer, so I think this is a good exscuse to start using this blog in one of the ways I originally intended it: answering my own questions.  More to come…

Answer

First of all, not all of California has a Mediterranean climate. A Mediterranean climate is one in which summers are hot and dry and winters are mild and rainy, similar to the regions bordering the Mediterranean Sea. These conditions exist in CA from about Cape Medocino south to Baja, and east to the Sierra Foothills.

To understand why the climate is Mediterranean around here, you first need to know a basic fact about global climate. The equator receives more direct sunlight than other parts of the planet, which causes the air there to heat up and rise. The planet is spinning on its axis, so when air rises, it gets delfected north or south [uh, why?]. The rising air loses pressure, cools, and expands, losing the water vapor it gained at the equator in the form of equatorial rain. High up in the atmosphere, this dry, expanding air spreads out in all directions and falls back to Earth, and the air moving north and south falling around the 30th latitudes. In falling, the this dry air increases in pressure and heat again. Dry, warm air, falling around 30°, right where the Earth’s major deserts are … not a coincidence.

SO. Mediterranean climates occur on the poleward borders of these 30° desert zones. Basically, when I said the equator receives the most sunlight, what I meant was the region of the Earth that receives the most perpendicular sunlight, which oscilates between the tropics as the tilted planet revolves around the sun. So in the northern summer, the region of falling, warm air shifts north as the region w/ the most perpendicular insolation approaches the Tropic of Cancer. This pushes the hot dry weather up in to the Bay Area and beyond. In the winter the opposite happens, and we get cooler temperatures and rain.

So why isn’t the southeastern US similar, given that it’s at the same latitude? Uh, still working on that, though I imagine it has to do with oceanic gyres…

References

  • “Ideal gas law – Wikipedia, the free encyclopedia.” 3 Aug 2008 <http://en.wikipedia.org/wiki/Ideal_Gas_Law>.
  • “Mediterranean climate – Wikipedia, the free encyclopedia.” 3 Aug 2008 <http://en.wikipedia.org/wiki/Mediterranean_climate>.
  • Schoenherr, Allan A. A Natural History of California. University of California Press, 1995.
  • Skinner, Brian J./ P. Dynamic Earth An Introduction to Physical Geology 4th Text Only No CD. John Wiley & Sons Inc, 2000.

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