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San Francisco Peninsula

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About the San Francisco Peninsula

The San Francisco Peninsula is a small region occupying the northernmost part of the San Francisco Peninsula. It includes the entire city of San Francisco and extends a bit farther south, until the border with the Santa Cruz Mountains; on the bay side it extends slightly farther south, about to San Mateo.

This region is mostly flat and underlain by marine and sand dune deposits, with some metamorphic rock interspersed throughout, and some small outcrops and bluffs of serpentine.

The climate here can be classified as a warm-summer Mediterranean climate, but it is heavily influenced by the ocean, with its deep waters that remain cool year-round, and the nearby bay, with significantly warmer waters. There is very little seasonal variability of temperature, with average temperatures varying only about 11.4°F (6.4°C) between the warmest and coldest months. Precipitation, however, is strongly seasonal, with 4-5 months with no or almost no rainfall, yet significant rainfall during the winter months. Fog is common year-round, and relative humidity is high even in the dry season. This region also experiences small variations of climate over short distances; the ocean side is cooler and foggier.

This region was originally supported unique plant communities, including California oatgrass, purple needlegrass, coast live oak, and California bay. There were also some more typical plant communities on coastal dunes and salt marshes. This area is heavily urbanized and very little natural vegetation cover remains, but there are a few populations of rare and endangered plants. As is common in most areas, most of the remaining vegetation cover can be found on rugged upland areas, the largest area of which is San Bruno Mountain State & County Park, which, although it is contained within this region, shares a lot more in common with the Santa Cruz mountains than the surrounding bottomlands.

Along the coast, this area is bordered to the south and southwest by the Santa Cruz Mountains. East of that, it is bordered to the south by the slightly drier and slightly less rugged Leeward Hills, and farther east, by the even drier and flatter Bay Terraces/Lower Santa Clara Valley. Along the bay coastline, it is bordered to the south by the Bay Flats. The border with the Santa Cruz Mountain is abrupt and defined by topography, but the borders with the Bay Terraces and Flats are more gradual and represent transitions away from the ocean-influenced climate to the regions along the bay, separated from the ocean.

References

1. Pater, D.E., Bryce, S.A., Thorson, T.D., Kagan, J., Chappell, C., Omernik, J.M. "Ecoregions of Western Washington and Oregon (Poster)", U.S. Dept., of the Interior, U.S. Geological Survey, Reston, VA (0000) Web.