SFPUC 2001 Alameda Watershed Management Plan

III. ENVIRONMENTAL SETTING AND IMPACTSF. AIR QUALITY1.0 SETTING1.1 CLIMATE AND METEOROLOGYThe primary factors that determine air quality are the locations of air pollutant sources and theamount of pollutants emitted. Meteorological and topographical conditions are also importantfactors. Atmospheric conditions such as wind speed, wind direction, and air temperaturegradients interact with the physical features of the landscape to determine the movement anddispersal of air pollutants. The Alameda Watershed is located within two of the Bay Area AirQuality Management District (BAAQMD) subregions, the southwestern Alameda Countysubregion and the Santa Clara Valley subregion. Both of these subregions are located within theSan Francisco Bay Area Air Basin. The discussions below describe the general climate andmeteorology of each of these subregions.Southwestern Alameda CountyThe climate in southwestern Alameda County is affected by its proximity to San Francisco Bay.The Bay breezes push cool air onshore during the daytime and draw air from the land offshore atnight. Winds are predominantly out of the northwest during the summer months. In the winter,winds are equally likely to be from the east. Wind speeds are moderate in this subregion, withannual average wind speeds close to the Bay at about 7 miles per hour (mph) and further inlandat about 6 mph. During summer months, average maximum temperatures are in the mid 70s.Winter maximum temperatures are in the high 50s to low 60s. Average minimum temperaturesare in the low 40s in the winter and mid 50s in the summer. Pollution potential is relatively highin this subregion during the summer and fall. When high pressure dominates, low mixing depthsand Bay and ocean wind patterns can combine to carry pollutants from other cities to this area,adding to the locally emitted pollutant mix.Santa Clara ValleyThe Santa Clara Valley is bound by San Francisco Bay to the north and mountains to the east,south, and west. In the northern portion of the Santa Clara Valley, the maximum temperatureranges from the low 80s in the summer to the high 50s in the winter. The minimum temperatureranges from the high 50s in the summer to the low 40s in the winter. Winds in the valley aregreatly influenced by the terrain, resulting in a prevailing flow that roughly parallels the valley’snorthwest-southeast axis. A north-northwesterly sea breeze flows through the valley during theafternoon and early evening, and a light south-southeasterly flow occurs during the late eveningand early morning. In the summer, the southern end of the valley sometimes becomes a“convergence zone,” as air flowing from Monterey Bay gets channeled northward and meets withthe prevailing north-northwesterly winds. Wind speeds are greatest in the spring and summerand weakest in the fall and winter. Nighttime and early morning hours frequently haveNOP 96.223E: Alameda Watershed Management Plan III.F-1 ESA / 930385January 2001