If San Francisco is known for one thing, it very well may be the bay fog. It is that smooth, almost cotton looking layer of moisture that rolls into the bay in the morning that when viewed from above, hides what is one of the great west coast cities from view.
And they built an airport right in the middle of it.
Why your flight was delayed.
It is far from uncommon that in morning when this fog rolls in, San Francisco airport comes to a grinding halt and most air carriers are either severely delayed due to approach availability or are forced to take the trek to their alternate airports, which often lie just a few miles inland where this fog loosens its grip on commerce. Sacramento is a frequent choice of alternate destination. The FAA decided that in an effort to improve safety, large air carriers running scheduled service that you can purchase tickets on need a certain amount of report visibility and cloud ceilings to even attempt an approach to landing.
“Captain, is that the runway or the bay bridge…? They look like brake lights…”
But why does this fog roll into the bay and put a hold on everyone’s arriving plans? Let’s imagine two parties. One of which is in a small studio apartment and the other is in large mansion. Each party is being attended by ten people. Someone brings a keg to both, but drunken coordination is never often hailed for its accuracy and as such events transpire and the keg explodes and sends beer flying everywhere. The same amount of beer is spilled in each party, but per unit area (let’s say a one square meter) you are much likely to find a person in the studio apartment. By extension, one could then state that the person to spilled beer ratio per unit area of the studio apartment is higher than that of the mansion. Imagine now an infinite keg of beer that just kept spilling and eventually fills up both the mansion and the apartment. Per unit area, the mansion can hold a lot more beer because one is likely to find fewer people in that square meter. The beer will run out of places to go per unit area quicker in the apartment than the mansion. Taking this analogy one step further, let’s replace the studio apartment with cold air and the mansion with warm air and the beer with water vapor. In the studio apartment/cold air the beer/water vapor will run out of places to go quicker. When moisture runs out of places to hang out in air, it changes forms and becomes visible moisture (fog/clouds). This happens in cold air quicker than warm air because, like the studio apartment, it is denser with molecules (although in this analogy, the apartment was denser with people).
The other reason your flight was delayed.
So the San Francisco bay can act like the venue for the party. The moist air that comes rolling in off the pacific ocean in the morning is like the spilled beer. The air over the land is colder, so it can’t hold that much beer. So, when that moist air rolls in to cold air, it doesn’t have a lot of places to hangout and thus must become visible moisture. This forms the fog, more specifically, advection fog, that caused your flight to end up in Sacramento.