From time to time the question comes up about a guitar and humidity. Its always good to think about how wood and humidity interact.
Dry air and low humidity are responsible for many guitar repair issues especially seen on acoustic guitars or archtops, and even solid body guitars.
Splitting tops, shrinking fingerboards, neck changes, sharp fret ends are some of the most common problems that develop with guitars, mando's, and etc.
There is the school of thought that recommends keeping one's guitar in its case as the case will protect an instrument from drastic changes in temperature or humidity. In areas of the country (USA) or globally this is good advice. In winter weather where temperture changes are needed maintaining a consistent temperature and humditiy is significant. Controlling or monitoring these two elements is done by a variety of tools such as a humidity gauge (digital hygrometer) for humidity and a temp gauge for temperatures. There are also several kits that can be found in music stores and even a humidity pack issued by Planet Waves@ or a soundhole humidifier.
Most builders (luthiers) agree that 45% RH (relative humidity) +/-5 is ideal. A range of 40% to 50%.
Owners of new instruments need to be significantly aware of humidity. A new instrument needs settle time in an environment. Instruments purchased at a retail store or from a prior owner should be examined as many factors changed to the instrument since its birth or prior ownership. For example, travel changes, store conditions, and prior owner care impacts an instrument (any wooden instrument).
Tell-tale signs of a dry guitar (wood instrument) is corduroy-like texture on the finish that can be seen under good lighting. Finish ridges (for example) occurs when a guitar top looses moisture. This is especially seen in acoustics or archtops, and etc. Here the grain shrinks and the finish shrinks with the wood. This shrinkage is seen in instruments that resided in dry areas. Nevada would be a good example of a year round dry area as dry heat will dry up moisture. Thus whether in Nevada (USA) or when a heater is warming up air (in winter) one should be aware of humidity.
Acoustic/archtops tops are built to have a slight convex top radius, but if too dry the top will shrink in towards the body (concave). This will require an acclimation process to re-hydrate the wood. Some factories or manufacturers offer this service. A good repair shop can also provide this service.
In Florida where we have a high humidity level the biggest concern is too much moisture (80 - 90%). Florida's climate is not that scary except when its 100% humidity or during hurricanes where its a consistent 100% humidity. During these conditions locking up one's instrument in it's case is the suggestion.
Some (not all) of our shop customers' indicate that they keep their instruments in the case 24/7 unless being played. Also that the residence is air conditioned. We argue that (at least in Florida) being locked in a case with moist air is inviting moisture to infiltrate wood and even begin the process of mold. Simply, there's no air flow.
And we all know (or should know) that conditioning air (A/C) is removing moisture from the air in the building and conditioning it cool against hot humid Florida (or other hot spots) air. Notwithstanding, maintaining a vigil as to the environment's humidity is suggested.
In sum, we like areas like Florida. Yes Florida has a high humidty usually between 80-90%, but controlled by conditioned air (A/C). Thus, we like to keep our guitars in the open on a guitar stand and let the instrument breath. Of course watching the humidity level is always suggested. And as always, too much of one thing or another is not the right thing so in the end taking care of one's instrument is always the rule.