Why do you think the fog forms in Sacramento rather than in the foothills?

Answer 1

Fog often forms in Sacramento and not in the foothills, but that is not always true. During a night when the winds are calm and the sky is clear, fog will likely form in the Sacramento area during the cold season. The main reason is that the coldest layer of the atmosphere under these conditions are usually located right next to the ground. If the temperature drops far enough and there is enough water vapor, fog will start to form. This most often occurs along the Sacramento River and the American River where so happens that all three of the main weather gages are located near one of these rivers. The Sacramento International Airport weather instruments are located next to the Sac River, the Executive Airport instruments are located near the Sac River, and the official National Weat her Service gage is located near the American River. In order for the area to get widespread fog, you really need to get a good rain storm in to provide the moisture needed for widespread dense fog when the skies finally clear.

However, there are two situations that occurs that will leave the foothills with widespread fog and not the city of Sacramento. One is when the delta gets a strong delta breeze that brings in the marine layer into the valley. This marine layer result in the formation of a deck of low clouds over Sacramento. This cloud layer will intersect the foothills. When this occurs, where the cloud layer meets the foothills, you will find dense fog. Also, if dense fog forms and rapidly covers the Sacramento early in the evening hours. What usually happens overnight is that this fog layer will lift up to become a stratus cloud layer. Again, dense fog will then cover the foothills and little fog will be left in the city of Sacramento,CA.

Answer 2

Fog is formed when the relitive humidity at ground level reaches 100% and the air becomes satuated. In the Sacromento valley, most fog is "valley fog" (specifically Tule Fog) which is caused by cooler air settling into the vally while warmer air passes over the mountains above it (because warmer air is lighter). The cool air settling on the ground creates condensation, which satuates the air and creates fog.