It’s not just low humidity that causes problems in the cold winter months. The dramatic temperature changes as a guitar or bass is taken from a warm house or studio into a cold car then into another warm destination can wreak havoc on the instrument’s finish. What happens is that the wooden instrument expands and contracts at a different rate than its finish expands and contracts, resulting in the finish cracking.
The solution is to make sure that any guitar or bass has thoroughly acclimated to the surrounding temperature before you open its case. If the instrument was in the cold long enough to really drop in temperature, let it sit for several hours or even longer to ensure the finish and the body are in equilibrium before opening the case.
If, when you carefully open the case, a “haze” appears on the finish, close the case immediately, the instrument is still too cold!