Southwestern Alameda County

This region encompasses the low-lying area on the southeast side of the San Francisco Bay, from south of Hwy 580/Dublin Canyon to north of Milpitas. The region is bordered on the east by the 1600 foot East Bay Hills, and on the west by the Bay. Most of the area is very flat. The cities in this region are San Lorenzo, Hayward, Union City, Newark, and Fremont.

Situated between the western and eastern portions of the Coast Range, this region is protected from the direct effects of the marine air flow. Marine air entering through the Golden Gate is forced to diverge into northerly and southerly paths because of the blocking effect of the east bay hills. The southern flow is directed southeasterly down the bay, parallel to the hills, where eventually it passes over southwestern Alameda County. These sea breezes are strongest in the afternoon. The further from the ocean the marine air travels, the more it is modified. Thus, although the climate in this region is affected by sea breezes, it is affected less so than the regions closer to the Golden Gate, to the north.

The climate of southwestern Alameda County is also modified by its close proximity to the San Francisco Bay. Evaporation from the bay will cool the air in contact with it during warm weather, while during cold weather, the bay can act as a heat source. The normal northwest wind pattern will then carry this air onshore. During periods of flat pressure gradients, the bay can generate its own circulation system. This bay breeze, similar to the sea breeze, pushes cool air onshore during the daytime and draws air from the land offshore at night. Bay breezes are common in the morning, before the sea breeze begins.

Winds are predominantly out of the northwest quadrant in this region, particularly during summer months. In the winter, winds are equally likely out of the east. Cold air over land areas creates high pressure to the east, which forces air toward the west. Easterly surface flow into southern Alameda County passes through three major gaps: Hayward/Dublin Canyon, Niles Canyon, and Mission Pass. Areas north of the gaps then experience southeast winds, while areas south of the gaps experience northeast winds. Wind speeds are moderate in this region. Annual average wind speeds close to the bay are about 7 mph, while further inland at Fremont they are 6 mph.

Air temperatures are moderated by both the proximity to the bay and to the sea breeze. Temperatures in this region are slightly cooler in the winter and slightly warmer than east bay cities to the north. Average daily maximum temperatures in winter at Newark are in the high 50's to 60 degrees. During the summer months, average daily maximum temperatures are in the mid 60's. Average minimum temperatures are in the low 40's in winter and mid-50's in the summer.

Rainfall amounts in the region are lower than other east Bay sites to its north. Areas near the bay, such as Newark have lower rainfall amounts because of the rain shadow effect of the Santa Cruz Mountains. Newark annual rainfall is 14 inches. Areas closer to the hills have higher rainfall amounts because they are further from the Santa Cruz Mountains and because of orographic effects. That is, air that is forced to ascend the mountains will cool and condense, leading to increased rain.

Pollution potential is relatively high in this region during summer and fall months. When high pressure dominates the weather, low mixing depths and bay and ocean wind patterns can concentrate and carry pollutants from other cities to this area, adding to the locally emitted pollutants. The polluted air is then pushed up against the East Bay Hills. Flow eastward through the gaps is weak because winds in the Livermore Valley are usually from the east. Wintertime pollution levels are only moderate.