EXHIBIT 4-2_55A_BRISTOL WIDENING TECHNICAL STUDIES

Home Browse Search

CLIMATE AND METEOROLOGY Throughout the basin, the vertical dispersion of air pollutants is restricted by the presence of a persistent temperature inver- sion near the surface (when temperature increases with increasing altitude, it reduces the mixing height). Winter inversions frequently weaken and erode by mid - morning, thereby preventing the accumulation of contaminants. On hot summer days, however, the inversion layer often remains, trapping pollutants in a limited mixing area until middle or late afternoon when the inversion layer lifts, erodes, or surface wands are sufficient to disperse the pollutants horizontally. Thus, a combination of low wind speeds and low inversions creates the highest pollutant concentrations. Four key elements are required to specify the meteorological conditions affecting the transport and dispersion of air pollut- ants. These include the wind direction, wind speed, atmospheric stability, and mixing height. Although regional meteorological conditions (such as temperature inversions, Santa Ana wind condi- tions, etc.) will dominate localized conditions for the most part, wind direction, wind speed, and localized turbulence gener- ated by site specific topographical conditions can play a key role in determining site specific ambient air quality. Wind direction and speed (which in turn affect atmospheric sta- bility) are probably the most important climatological, elements affecting the ambient air quality on -site. The on -shore dominant daytime wind pattern occurs between noon and 7:00 pm, following the peak travel period (5 am through 9 am) in the Los Angeles/ Orange County area. Consequently, during periods of low inver- sions and low wind speeds, the photochemical smog formed in these areas is transported downwind into Riverside County and San Bernardino County. During the fall and winter months the APEI is subject to moderate and strong Santa Ana winds. These dry warm northerly and north -- easterly winds typically last for several days and exhibit vel- ocities which exceed 40 mph at times.