Marin County Basin

Marin County is wedge shaped, bounded on the west by the Pacific Ocean, on the east by the San Pablo Bay, on the south by the Golden Gate, and on the north by the Petaluma Gap. The county is mostly hilly. There are a few small towns on the west side, such as Stinson Beach, Bolinas, Inverness, Point Reyes Station, and Dillon Beach, but most of the population lives on the eastern side of the hills, in small, sheltered valleys. Areas along the west coast of Marin County are usually subject to cool marine air. In the summer months, the marine air is cooled as it passes over the offshore upwelling region, and forms a fog layer along the coast. In the winter, proximity to the ocean keeps the coastal regions relatively warm. Temperatures do not vary much over the year at these coastal areas: high 50s in the winter and low 60s in the summer. The warmest months are September and October, which are in the mid to high 60s.

The eastern side of Marin County has warmer weather and less fog. This is due less to the blocking effect of the hilly terrain to the west, but more to its distance from the ocean. Although there are a few mountains above 1500 feet, most of the terrain is only 800 to 1000 feet high. Much of time, this is not high enough to block the marine layer, which averages 1700 feet in depth. Because of the wedge shape of the county, area to the north are further from the ocean. This extra distance from the ocean allows the marine air mass to be heated before it arrives at eastern Marin County cities. In south Marin County, the travel distance is short, and the elevations lower, so there is a higher incidence of cool, unmodified, maritime air.

Cities next to the bay have their temperatures somewhat moderated. For example, San Rafael being near the bay, experiences average maximum winter temperatures in the high 50s to low 60s, and average maximum summer temperatures in the high 70s to low 80s. Inland areas, such as Kentfield, experience average maximum temperatures two degrees cooler in the winter and two degrees warmer in the summer. Average minimum temperatures in San Rafael are in the low 40's in winter and low 50's in summer. Minimum temperatures in Kentfield are two degrees cooler all year.

Wind speeds are highest along the west coast of Marin, about 8 to 10 mph. Although most of the terrain throughout central Marin County is not high enough to act as a barrier to the marine air flow, the complex terrain creates sufficient friction to slow the air flow. Downwind, at Hamilton AFB in east Marin County, the annual average wind speeds are only 5 mph. The prevailing wind directions throughout Marin County show less variation, and are generally from the NW.

The mountainous terrain in Marin County has higher rainfall amounts than most parts of the Bay Area (the southern Santa Cruz Mountains report higher rainfall amounts). Near Mt. Tamalpais, rainfall amounts are twice as high as the rest of the Bay Area, with San Rafael reporting an average of 37.5 inches per year and Kentfield reporting 49 inches per year. Further north, Hamilton AFB and Petaluma report 26 and 24 inches, respectively. Consistent with the Bay Area Mediterranean climate, 84% of the annual rainfall in Marin occurs November through March.

Air pollution potential is highest on the eastern side of Marin County. This is where the semisheltered valleys and largest population centers are located. Currently, most of the development has been along the bay, particularly in southern Marin. In the south, where distances to the ocean are short, the influence of the marine air keeps the pollution levels low. As development moves further north where the valleys are more sheltered from the sea breeze, it will encounter greater pollution potential.