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MacArthur Project Modif cation — Santa Ana - Air Quality Assessment <br />KPC Environmental, Inc. <br />The extent and severity of air pollution problems in the SCAB is a function of both <br />natural physical characteristics of the region (weather patterns, topography) and man- <br />made influences (traffic, development). Factors such as wind, sunlight, temperature, <br />humidity, rainfall, and topography all affect the accumulation and/or dispersion of <br />pollutants throughout the SCAB. <br />Historical Climate data is collected by the Western Regional Climate Center with the <br />closest monitoring station located at the Santa Ana Fire Station. Climate data collected <br />includes temperature and precipitation. <br />3.2.2 Temperature <br />The annual average temperature varies little throughout the SCAB, averaging 62- degrees <br />Fahrenheit, with the Santa Ana area annual maximum temperature averaging 75.7 - <br />degrees Fahrenheit and an annual minimum temperature averaging 51.9- degrees <br />Fahrenheit. The average high temperature in the project area is 68.6- degrees Fahrenheit <br />during the winter and 82.3- degrees Fahrenheit during the summer. Low temperatures can <br />range from around 43.8- degrees during winter nights to 59.9- degrees Fahrenheit during <br />summer nights. For site - specific analysis, temperatures selected represent the lowest <br />average temperature when assessing CO and Nox impacts and the highest average <br />temperature when assessing ROG. <br />3.2.3 Rainfall <br />Practically all of the annual rainfall in the SCAB occurs during the November- April <br />period. Summer rainfall normally is restricted to widely scattered thundershowers near <br />the coast and slightly heavier shower activity in the east and over the mountains. Annual <br />average rainfall in the Santa Ana area is 13.67 inches. <br />3.2.4 Humidity <br />Although the SCAB has a semi -arid climate, the air near the surface is surprisingly moist <br />because of the presence of a shallow marine layer on most days. Except for infrequent <br />periods when dry, continental air is brought into the SCAB by offshore winds, the ocean <br />effect is dominant. Periods with heavy fog are frequent; and low stratus clouds, <br />sometimes referred to as "high fog" are a characteristic climate feature. Annual average <br />relative humidity in the SCAB ranges from 70% coastal to 57% inland. <br />3.2.5 wind <br />Winds play a significant role in the air quality conditions of the SCAB. The area is <br />characterized by light average wind speeds which limit the capability to disperse air <br />contaminates horizontally. The average wind speed in the project area is between 5 and <br />12 miles per hour (mph) out of the West - Southwest. The dominant daily wind patterns <br />consist of sea breezes during the day with nighttime offshore breezes as the air drains off <br />the mountains that surround the region. <br />02/02/11 <br />