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C) Air Flow Air flow and pollutant transport within the SJVAB is complex and largely influenced by its topographic features. In general, the Coastal Range hinders air flow from the west, the Sierra Mountains hinder air flow to the east and the Tehachapi and San Emi dio p 9 Mountains hinder the southerly flow of air in and out of the Valley. Air flow patterns in the Basin can generally be characterized by one of four directional types, the most frequent of which is upvalley in a north-northwesterly direction. However, seasonal variations do occur. The spring and summer months are typically characterized by strong northwesterly winds; whereas, calm light winds dominate the winter months (SJVUAPCD, 1994). i Climate Hot summers, mild winters, moderate rainfall, and moderate humidities characterize regional climatic conditions. The climate of the Basin is attributed primarily to the topography of the SJVAB and the mean position of the seasonal mid-latitude storm track. The topographic features of the SJVAB make it possible for various micro-climates to exist and are primarily responsible for the wide variations of rainfall, temperatures, and localized winds that occur throughout the region (SJVUAPCD, 1994). Temperature variations have an important influence on SJVAB wind flow, dispersion along mountain ridges, vertical mixing, and photochemistry. Since the moderating marine influence decreases with distance, monthly and annual spreads between temperatures are greatest inland and smallest nearest the coast. Average temperatures during the summer months for the SJVAB range from 50 to over 100 degrees Fahrenheit. The average high 9 9 g g temperature during winter months is between 50 and 60 degrees Fahrenheit, although highs of 35 and 40 degrees Fahrenheit can occur on days with persistent fog and low cloudiness. Temperatures below freezing (32 degree Fahrenheit) are unusual during winter months (SJVUAPCD, 1994). Precipitation is highly variable seasonally. Precipitation falling within the SJVAB is largely the result of the position of a semi-permanent subtropical high pressure belt located off the } Pacific coast (Pacific High). Precipitation during the summer months is rare, and is associated with an influx of moisture into the valley through the San Francisco area. During winter months, the Pacific High system moves southward allowing Pacific storms to move through the Valley (SJVUAPCD, 1994). Local Environment San Joaquin County is located at the northern end of the eight-county SJVAB. The climate is semi-arid with winter and spring rainfall averaging seven to ten inches per year. The rainfall is very seasonal with low heavy fog in the winter months. Snow is not a factor in annual precipitation. Temperatures within the project area range from a daily average January low of 36 degrees to a daily average July high of 94 degrees Fahrenheit (NOAA, 1992). Winter and spring winds may reach 70 miles per hour (MPH) on very rare occasions. Draft Environmental Impact Report 4-60 ER-96-3