55 Degrees and 60% Humidity.
Why? To get the real answer you'd have to ask the god of wine - Bacchus or Dionysus - with whichever you're on speaking terms. The rest of us can only assume that it's because that's the temperature and humidity of the caves wine was stored in for the millennia that it has been developed, nurtured, coerced, coddled, and enjoyed.
Is humidity important? If you have any real corks in your cellar, then Yes/Definitely/Absolutely. Don't let anybody tell you differently. If you have all screw tops or synthetic corks, then not really.
Both the temp & humidity should be constant - it shall not be varied.
There shall not be direct sunlight.
There shall be almost no vibration or movement of bottles.
Corks shall always be in contact with at least some of the wine within its bottle.
SO IT IS WRITTEN, SO IT SHALL BE.
- Jeff Hagen, VinGrotto.com