Winter is a dangerous time of year for acoustic guitars as excess dryness can lead to damage and deterioration of your guitar.
The villain in winter is excessively low relative humidity for our purposes lower than 40% relative to 75 degrees F (24 degrees C). Damage will result to instruments in your home that are exposed to a rate of relative humidity lower than these for any period of time.
The most reliable way to ascertain the relative humidity in your store or warehouse is to use a sling psycrometer. However the operation of a sling psycrometer is a little complicated for general use. A hygrometer (available at most hardware stores) will give you a rough estimate of the humidity.
Another way to tell if your location is particularly dry is to regularly inspect your instrument. A guitar will exhibit evidence of exposure to a destructive level of humidity in your home.
Sinking of the Top and Back
When the guitar is built the top and back are slightly arched. If the instrument begins to dry out the wood shrinks and the top and back will become flat or even concave. Exposed to low humidity for any period of time there may be cracks develop in the top and back or the bridge may lift or shear off and interior bracing may loosen.
Roughness in Finish
Low humidity will also cause the wood to shrink underneath the finish, which in turn causes the stable finish material to pop out of the pores in the wood resulting in a pebbled appearance of the finish.
Protrusion of the Fret ends
Lack of humidity will cause the fingerboard of the instrument to shrink, which will cause the ends of the stable metal frets to protrude from the edged of the fingerboard.
Cracks at Base of Fretboard Extension
In addition to protruding frets a guitar exposed to excessive dryness may exhibit the development of cracks in the top on each side of the fingerboards where it is glued to the top.
Gaps Around the Inlays
In an instrument with inlays you may observe a line or gap appear around the edge of the inlay as the wood shrinks away from the more impervious materials. This effect can sometimes be observed around the purfling, marquetry and rosette or sound hole rings.
Back Bow in the Neck
A back bow may develop in the neck caused by the shrinking of the neck and fingerboard while the frets remain immovable, forcing the neck back. This will result in string buzz.
Excessively low humidity will cause the top to shrink carrying the bridge down with it and couples to the developing back bow in the neck will result in a low action and buzzing up and down the fingerboard. A small drop will occur at the 14th fret on the steel string and the 12th fret on a classic guitar. Buzzing on the fretboard should alert you to the need to humidify your home or storage environment.
What to do.
You need to ensure that your guitar is maintained in an acceptable range of relative humidity.
Guitars in storage should be stored in their cases under appropriate humidity controls.