As we all know, California is having a epic drought while also experiencing historically high temperatures. The high temperatures are compounding the drought by increasing the rate of evaporation. One of the factors involved that is often overlooked is the rate of fog. Fog works to cool the surface by providing shade until it is burned off. Less fog means the sunlight reaches the ground more, resulting in more heating. As it turns out, the amount of fog in California is decreasing.
It has already been noted how California's famous Tule fog is becoming increasingly rare. This is the thick, winter fog that can blanket the central region so thickly it actually waters the plants. But, fog in general is decreasing across the state. Over the course of the 20th century, coastal summertime fog worldwide was reported to decrease by 33%. California, in particular, has experienced a century-long decline in coastal fog.
What is the long-term outlook? That is not clear (foggy? Sorry). Warm ocean surface temperatures lead to a decrease in fog. But it is not certain the coastal waters will get warmer, even when the ocean overall is getting warmer, due to upwelling. It is possible the medium-term amount of fog will stay constant, or decrease slightly. But, the long-term outlook has to be bleak, even with upwelling. The depths are getting warmer and eventually the upwelling water will be too warm to lead to fog.
So, as the crops suffer the loss of the cooling fog (the grapes for one love fog), the rest of the state will also suffer due to the reduction in the shading, leading to higher surface temperatures, leading to greater evaporation, leading to even higher temperatures.
But, at least there will be fewer fog delays at the airports.