As has been noted, winter thermals here are sometimes better than
summer's -- which isn't saying much, but I had a great time one New
Year's hanging out over the East Bay Hills for about 90 minutes, never
going more than 4-5 miles in any direction, but having wonderful views
of the Bay and city. Cloud bases were about 3500' and Hayward was my
safety (and home) airport. OK, I was in a motor glider.

Mark Twain explained why (without intending) our winter thermals are
better than our summer ones: "The coldest winter I ever experienced
was a summer in San Francisco." The city can be colder in summer than
in winter if the fog blows in. We have a really cold current running
here. Consequently, summer thermals are largely absent until you get
out of the marine influence, which entails driving 1.5+ hours to the
places mentioned.

One possibility that hasn't been mentioned and might fit your
interests is Rolf Peterson's soaring operation at Livermore, less than
an hour drive from the city. OK, it's not really a soaring operation,
though it is listed in PASCO's bulletin. He owns a Grob 109B motor
glider and does instruction in it. If you want to see if he's
available call him. His number and email are listed under "Mt. Diablo
Soaring on PASCO's web site:

http://www.pacificsoaring.org/clubs.html

He's a great pilot, instructor and person. And, given the minimal
soaring in January (unless the Mt. Diablo wave is working), a motor
glider is a nice option. Even then a motor glider helps.

Hope you have a good time.

Martin