The Livermore Valley is a sheltered inland valley within the Diablo Range near the eastern border of the District. The western side of the valley is bounded by 1000 to 1500 foot hills with two gaps connecting it to the San Francisco Bay area, the Hayward Pass at the north and Niles Canyon at the south. The eastern side of the valley also has 1000 to 1500 foot hills, the Altamont Hills, with one major passage to the San Joaquin Valley called the Altamont Pass and several secondary passages; Kellogg Creek, Patterson Pass and Corral Hollow. To the north lie the Black Hills and 3849 foot Mount Diablo. A northwest to southeast channel connects the Diablo Valley to the Livermore Valley and splits the Diablo Range into eastern and western sections. The south side of the Livermore Valley rises up to mountains of approximately 3000 to 3500 feet in the Diablo Range.
For the winter season, with the exception of an occasional storm moving through the area, air flow is often dictated by a weak pressure pattern, allowing local conditions to steer it. At night and early morning, especially on clear, calm and cold nights, gravity drives cold air downward, like water, to drain off the hills and snake through gaps and passes. During the day if some surface heating over land takes place, a thermally developed pressure field can initiate weak flow from high to low, drawing air through these same paths of least resistance which may be in the opposite direction of late night and early morning flow. On the eastern side of the valley at the Lawrence Livermore National Laboratory (LLNL), the prevailing wind direction spans the north-northeast through east-northeast sectors, caused by drainage off the hills and flow out of the Altamont Pass. Flow is light during the late night and early morning hours, about 40% of the winds are less than 3 mph. A secondary, prevailing wind direction group, east-southeast through south-southwest, accounting for about 35% of the observations, is probably associated to daytime flow through the Altamont Pass on its way to the San Joaquin Valley and associated to winter storm passages. Winter minimum temperatures average some ten degrees lower than on the coast. At the National Weather Service station maximum temperatures range from the high 50's to the low 60's while minimum temperatures are from the mid to high 30's with extremes in the high teens and low 20's. The precipitation mean is 14 inches.
By the summer the strong Pacific High has usually moved into a position to dominate Bay Area weather. Sunshine is plentiful with clear skies most of the time. Cold water upwelling along the coast and hot inland temperatures can cause a strong onshore pressure gradient which translates into a strong, afternoon wind. At the LLNL over 70% of the wind is from the south-southwest to westsouthwest and by the afternoon 35% of the wind is about 11 mph. With a weak temperature inversion, air can flow over the hills with ease, but with a low and strong inversion air flow is weak, if there is any, and conforms to the twists and turns of the gaps and passes. At the National Weather Service station, maximum temperatures range from the high 80's to the low 90's with extremes in the 100's, while minimum temperatures are in the low 50's.
For the Livermore Valley the air pollution potential is high especially for photochemical pollutants. Dependent upon the meteorology for that particular summer and or fall, the frequency of elevated ozone levels at the AIR DISTRICT 's Livermore station can be significant, approaching, reaching or exceeding Santa Clara Valley levels. The valley not only traps locally generated pollutants but can be the receptor of ozone and ozone precursors from San Francisco, Alameda, Contra Costa and Santa Clara counties. This can happen near the end of an ozone episode when the sea breeze regains its strength and carries these pollutants inland. On northeasterly flow days, not uncommon in the early fall, ozone may be advected form the San Joaquin Valley to the Livermore Valley. During the winter the sheltering effect of the valley, its distance from the moderating marine air and the presence of a strong high pressure system, contribute to the development of a strong, surface based, temperature inversion. Within this stable layer local pollutants from automobiles, fireplaces and agricultural burning can concentrate, raising carbon monoxide and or particulate levels. With a growing population and no additional air quality controls, air pollution problems could become worse.