"The wind itself is invisible, but its effects are not. When you're sailing, it's important to be aware of the strength and direction of the wind in order to harness its energy efficiently and sail safely."
One of the great things about San Francisco is that the weather is fairly stable and predictable year round. During the on-season, the wind will usually come from the same direction day in and day out. The prevailing wind direction is West, even though natural wind direction is East. The West winds prevail because they are much stronger. The way this works is that the San Francisco Bay water is a constant cool temperature (around 50-60 degrees). When the summer hits, this cool water keeps San Francisco very cool compared to the surrounding citites. Meanwhile, East of us, the temperatures reach twenty to thirty degrees warmer than San Francisco. This heat rises and leaves a pocket underneath that the cold air naturally wants to move into. Thus, the wind moves from West to East as the cold air rushes past San Francisco, over Oakland, and into the Eastern cities where temperatures are much higher. This is the pattern of San Francisco wind for most of the season. You will notice that when San Francisco is warm, the winds are significantly lower because the warm fronts kill the West wind. During these tiimes, a slight Eastern wind can be seen. This is the natrual wind that is created from the rotation of the earth and the flow of the oceans.
The other weather pattern we see in S.F. is what happens when high and low pressure fronts come through. These fronts will change the wind direction drastically. Since pressure fronts move in cirular motions, the wind will shift from the West, to the South, to the East, and to the North for instance. In the case of a storm, the pattern, in theory, places the beginning of the storm when the winds are South and the end when the wind is North. This is not a very good rule, but it is a nice basis for some sense of what is going on in the heavens.
