Terrain of the San Francisco +Bay Area

Terrain

San Francisco is in an area of very diverse topography. There are numerous micro-climates in the City. Winds are channeled over and around the City of San Francisco by the terrain, resulting in pronounced differences in the weather across relatively short distances.

San Francisco is approximately a seven mile by seven mile square, and sits at the northern end of a peninsula, straddling the Coast Range just south of where it is broken by the Golden Gate. San Francisco's steep topography is the boundary between the Pacific Ocean on the west, San Francisco Bay to the east and the Golden Gate to the north. The highest terrain is toward the south, where the elevations rise to over 900 feet, with Mount Davidson's peak of 938 feet the tallest, followed closely by Mount Sutro at 920 feet and both North and South Twin at 919 feet. In addition to the primary north- northwest to south-southeast ridgeline, a number of significant hills dominate the San Francisco horizon, as spurs off of the main axis. The city's steepness is shown by the fact that the 60 meter elevation contour is generally within one mile of sea level.

Climate

The sharp topography and maritime surroundings of San Francisco combine with the unique California climate to produce a number of extremely varied microclimates within its 46 square miles. California's location in the middle latitudes and on the west coast of the North American continent, places it in the relatively rare Mediterranean (Etesian) type climate (K”ppen type Cs). The only other regions of the earth sharing this climate type are the southwestern tip of Africa, the west coast of Chile, the west coast of Australia, and of course, the region surrounding the Mediterranean Sea (Critchfield, 1966). This type of climate type is generally characterized by moist mild winters and dry summers.

San Francisco's climate is further modified by the location of the City on the northern end of a peninsula, surrounded on three sides by the relatively cool waters of the Pacific Ocean and San Francisco Bay. The Pacific High and the northwest winds that are constrained by the Coast Range blow parallel to the coastline.

Summertime in San Francisco is characterized by cool marine air and persistent coastal stratus and fog, with average maximum temperatures between 60øF and 70øF, and minimal temperatures between 50øF and 55øF. The mornings will typically find the entire city overcast followed by clearing on the warmer bay side, but only partial clearing on the cooler ocean side. The summertime temperature gradient across the city is generally from northwest to southeast, with the warmer readings farthest from the coast and in the wind sheltered valleys east of the Coast Range bisector. These differences are enhanced further by a strong afternoon and evening seabreeze that is a result of the temperature (and consequently pressure) difference between the Pacific Ocean and the interior valleys of California. These westerly winds are channeled through the Golden Gate and lesser breaks in the high terrain of the Coast Range, reaching a maximum during the afternoon with speeds between 20 and 30 miles per hour being typical.

Rainfall from May through September is relatively rare, with an aggregate of less than an inch, or only about 5 percent of the yearly average total of approximately 21.5 inches. Off- season rains which do occur are usually the result of weak early or late season occluded fronts, or surges of subtropical moisture from the south that result in brief showers or thundershowers spreading into the area. Considerable moisture is due to drizzle when the marine layer deepens sufficiently. This is seldom enough to measure (i.e., less than .01 inch) on any given day, except along the immediate coast.

Winter temperatures in San Francisco are quite temperate, with highs between 55øF and 60øF and lows in the 45øF to 50øF range. The main source region of wintertime fog in San Francisco is the Great Valley.

Over 80 percent of San Francisco's seasonal rain falls between November and March, occurring over about 10 days per month. Winter thunderstorms occur on the average only twice per season in cold unstable post-frontal air masses.

Snow is extremely rare in San Francisco, with only 10 documented instances of measurable snow at the official observing site in the past 143 seasons. Snow has fallen on a number of other occasions, but usually only in trace amounts or at the higher elevations. Additionally, some of these occurrences are not true snow events but were the result of either small hail or ice pellets.

Spring and fall are transition periods for San Francisco. These seasons usually produce the most cloud-free days between the overcast days of summertime stratus and the rain laden clouds of winter. San Francisco's hottest days are typically during the spring and fall. The three hottest days in San Francisco occurred in September and October.

The occurrence of rainfall during the early spring and fall is infrequent, with only about 5 days per month on the average.