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lower potential for pollutant accumulation than locations like Stockton (that are located closer to the center of the valley) because of the air movement through the Altamont Pass from the Pacific Ocean and the San Francisco Bay. There is, however, the potential for pollutants to be transported from Livermore Valley through the Altamont Pass to the San Joaquin Valley. • Temperature. Vertical temperature gradients influence the vertical stability of the atmosphere and vertical mixing of air pollutants. Unstable atmospheres have more vertical mixing than stable atmospheres. Typically, air temperatures decrease with altitude and facilitate mixing. However, a temperature inversion, which is a layer of warm air above a cooler layer of air, acts as a nearly impenetrable lid. Inversions severely limit vertical mixing of the atmosphere and thus decrease the vertical dilution of near-surface air pollutant emissions. Inversions occur frequently in the project area, typically at heights between ground level and about 150 meters above ground level. Summer inversions usually are caused by the compressional warming of air as it sinks toward the earth's surface under the influence of a semi-permanent high pressure zone known as the "Pacific High".When local or seasonal cooling of the earth's surface occurs, as it does most frequently during the fall and winter, ground-based inversions form. Both types of inversions can occur during the fall, contributing to high ozone and other air pollutant concentration levels. Horizontal temperature gradients create wind flows that disperse air pollutants. Horizontal temperature gradients are greater near the coast due to differential heating between land and water surfaces. This effect is diminished inland in proportion to the distance from the ocean. The project area is fairly sheltered from the effects of water bodies; thus, it experiences smaller temperature gradients and less efficient pollutant dispersion than coastal areas. There are occasions when the Pacific High is especially strong and the project area experiences some of the effects of the Pacific Ocean. The project area, however, does have larger horizontal temperature gradients (and thus more mixing)than farther inland locations in the San Joaquin Valley Air Basin because of the Altamont Pass. Temperature can also play an important role in the production of pollutants. In the winter, the potential for high carbon monoxide levels is related to minimum temperatures. Motor vehicles, the primary source of carbon monoxide, run less efficiently and produce more carbon monoxide when temperatures are lower. The lowest winter temperatures are usually found in the inland sheltered valleys because these areas are protected from the moderating influences of the ocean and bays. The lack of ocean influence in inland areas also leads to warmer temperatures inland during the summer months. Ozone is produced when hydrocarbons and nitrogen oxides react in the presence of sunlight and warm temperatures. Thus, the inland areas are more prone to ozone formation. The project area experiences the warmer summer and colder winter temperatures typical of inland areas.Temperatures in the area range from highs of about 34 degrees C (93 degrees F) in the summer to lows of 2 degrees C (36 degrees F) in the winter. Consequently, the project area has a relatively high potential for accumulation of air pollutants based on temperature extremes. • Precipitation.When precipitation occurs, air pollutants can be "washed out"of the atmosphere and/or prevented from entering the atmosphere. The summer climate of California is dominated by the Pacific High, located over the eastern Pacific Ocean. The Pacific High generally remains fixed offshore from May through September. Storms rarely affect California during summer and precipitation is negligible because of the persistent high- pressure cell. The long period of dry weather aggravates the problem of wind blown dust and road bed disturbance dust, resulting in generation of PM10 (particulate matter with a diameter of 10 micrometers or less). In winter, the Pacific High weakens and shifts southward and storms become more frequent. During the rainy weather periods, air pollution potential is very low. When clear conditions 8