More than just a catchy song written in 1944 by Frank Loesser, “Baby, it’s cold outside” is also a mandate of sorts, to keep your wooden instruments and other gear safe and protected! Cold weather means two things: low humidity, which can cause cracking, popped frets, wood shrinking, neck warp, and other problems, and low temperatures, which can cause finish cracks and other problems.
In the first case, low humidity, make sure that your instruments are living their lives in a comfortable environment, humidity-wise. Keep your humidity in the 40-60% range, and your instruments will be contented and safe.
With regard to low temperatures, the biggest danger is transporting instruments. Allow plenty of time for your instrument to acclimate to room temperature before you open its case when taking it from the cold into a warm place. If the instrument isn’t acclimated, the wood will warm up at a different rate than the finish; this can cause the finish to crack or “check.” If your instrument was exposed to extremely cold weather for a long period of time, it may take up to 24 hours for it to completely acclimate. Be patient and let it thoroughly warm up before you open the case!