
You have probably heard the term "fair weather sailor". The fair weather sailor will tend to not leave the slip unless the weather is perfect: sunny, warm, not too much wind, not too little wind. But sometimes that's missing the fun.
Full disclosure, I am, for the most part, a fair weather sailor. But there are days when getting out on the water with a full rainstorm can be exhilarating. My favorite rainy day sails have been in the tradewinds, those tropical winds that invariably point towards some exotic beautiful island. On those days, the weather is actually quite nice but about every 30 minutes a squall comes through. These squalls do odd things to the wind but add fun to your sail. And it's warm enough that you don't really need full foul weather gear. To get to these squalls you really need to be in the middle of the ocean so it's not for everyone.
Luckily for the San Francisco sailor it rains here too. Granted, it's a cold dismal rain but it is rain. And we get to sail in it.
Let's take a step back and understand SF Bay weather from 30,000 feet. In the summer it is sunny and windy and cold. You can always count on the wind in the summer. In the winter, when it's sunny (and this happens a lot), there is no wind (at least no "knock your socks off, let's have fun" wind). To get wind in the winter you really need a storm. When we get those, we have a really good southerly breeze...if you catch the beginning or end of a storm, you can get that breeze without the rain but where's the fun in that?

To enjoy this winter sailing, it is imperative that you dress properly. A good set of breathable foul weather gear is essential. This does not have to be expensive, a friend of mine has never bought a new set of foulies in his life, there are deals out there. And foulies come in fashionable red or yellow so you can look good. But the key is that if your body stays dry you can stay warm. The layers underneath are important too, try to avoid cotton. Cotton absorbs moisture, go with either wool or a good wicking layer (runners and bikers deal with these same issues) to keep moisture off your body. I use cheap boots with expensive socks to keep my feet dry but some people swear by the more expensive boot like Dubarry's. Whatever you go with, just outfit yourself to keep dry and you can enjoy the day.
One of the keys to winter sailing is to know the weather. I'm not advocating going out sailing when there is a gale force storm but if it's reasonable, you will have the bay almost to yourself and an exciting day of sailing.