Wine Storage Temperature

The Snow Miser
I'm Mister White Christmas
I'm Mister Snow
I'm Mister Icicle
I'm Mister Ten Below
Friends call me Snow Miser
What ever I touch
Turns to snow in my clutch
I'm too much!

--The Snow Miser
I'm pretty sure that Rankin-Bass didn't include any wine drinking in their animated specials, but the recent chill in the air has got home winemakers asking questions. In particular, about how cold is too cold for their wine.
Rick R (I hope that doesn't stand for Roll) wrote to ask:

Dear Master Vintner,
I've been making wine in my basement for the last ten years. This year I started some renovations and had to move my wine to the garage. The renos aren't done yet, and it's really cold: despite having a space heater in the garage it's gone close to freezing several times in the last couple of weeks (I keep a thermometer out there).
None of the wine has actually frozen, and we drank quite a few bottles over the holidays, but I'm worried about long-term damage--is there anything I should do? I can't bring all of the wine inside the house--no room with the basement out of commission.

This is a great question. First things first: all of this advice applies to finished wine in the bottle. Master Vintner instructions are pretty explicit about keeping them in the correct temperature range during fermentation and clearing. Outside of that range they don't perform well and might not finish fermenting or clearing. I'm pretty sure Rick is only concerned about ready-to-drink bottles. To see where you really stand, get yourself a Min-Max thermometer and put in in the garage where the wine is standing. It will not only measure the temperature, it will record the minimum and maximum within whatever period you leave it there, usually the course of a 24 hour period. Check it once a day for a week to see what the real temperatures are.
The real danger is that your garage might get below about 26F. At that point your wine could freeze (higher alcohol wines will have a higher freezing point, lower wines the opposite. When the wine freezes it's actually the water portion that goes: the freezing point of pure alcohol is -173F (really!) and 40 proof goes at around -10F. As the water forms ice crystals they expand inside the bottle. If you're lucky, it'll just push the cork out a bit. If that doesn't break the seal, the wine will probably be okay--just thumb the corks back down when it thaws and hope for the best. If you put capsule over your corks check them by feel for wetness or bulging over the lip of the bottle.
If you're not lucky, the cork will come all the way out. At that point, unless you can (slowly!) warm the bottles and top-up and re-cork it has a good chance of suffering oxidative damage.
If you're very unlucky indeed, the bottles will shatter and that's the end of that. Sweep it up while it's still frozen (makes cleanup easier) and plan to make more wine in spring.
That isn't to say that it's all clear sailing if the corks stay in. You could see a deposit of tartrate crystals. Properly called potassium bitartrate, these are formed with tartaric acid bonds to potassium in the wine. Tartaric acid comes from the grapes, and potassium is either from soil minerals or from potassium metabisulfite additions.
They're not harmful, and not really unsightly--in white wines they look like undissolved sugar crystals, and in red tiny little ruby dust-bits. However, they do excite some people who think they're a sign of wine gone bad (they're not) or that the wine has glass chips in it. They're harmless and have no particular flavor. In fact, when they've dropped out the wine is more stable and often seems to have a refinement of flavor, due to a change in the pH.
Over the long term, temperature cycling is not ideal for the wine. Even at less than freezing temperatures the ullage (airspace) inside the bottles will expand as temperature rises and contract as it cools. This alternates between pressure and suction against the cork and over time it can start to draw air into the bottle, oxidizing the wine and ruining it. This is the reason that the stability of the cellar temperature is more important than the actual temperature itself.
And try to get those renos finished Garages are for cars, not wine!