Martin Guitar Company 0-45s stephen stills ユーザーガイド

Your guitar is made of thin wood which is easily affected 
by temperature and humidity. This combination is the most
important single part of your guitar’s surroundings. Martin
keeps the factory at a constant 45-55 percent humidity and
72-77 degrees Fahrenheit. If either humidity or temperature
get far away from these factory conditions, your guitar is in
danger. A rapid change in temperature or exposure to cold
can cause small cracks in the finish. These are lacquer
checks. We recommend the use of a hygrometer/thermometer
to measure the relative humidity and temperature surrounding
your guitar. 

As humidity increases, moisture content of wood goes up
rapidly, causing it to expand and swell. A gradual increase 
in humidity won’t generally do permanent damage to your
instrument. When very high humidity is combined with high
temperature, glue joints could possibly become weakened 
and may even open slightly. If your guitar is exposed to high
temperature or humidity for any length of time, the glue
under the bridge could weaken causing the bridge to pull off.

Rapid changes in local humidity are what you want to guard
against. If, for instance, you place your guitar near a source
of dry heat, the humidity around it will drop much faster than
it would naturally, although a sudden dry spell can have the
same effect. If the moisture content of wood is forced down
in a hurry, portions of it shrink faster than others, causing