THE GUITAR & HUMIDITY
The guitar is made of different kinds of wood: spruce or cedar for the sound board, Indian or Brazilian rosewood, cypress or mahogany for the back and sides, ebony or rosewood for the fret board, and Spanish cedar or mahogany for the neck. These woods are sensitive to changes in humidity and will expand or contract according to how much humidity is in the air around them.
The ideal humidity for wood instruments is between 40 and 60 percent. Above 70 percent the instrument usually begins to lose its brilliance of tone. Below 35 percent a guitar runs the risk of the body cracking and the neck warping. This is the more serious menace and occurs mostly during the cold winter months in temperate climates and in arid regions. A good indicator of whether or not your guitar is in need of additional humidity is the fret board. If the ends of the frets stick out it means the ebony or rosewood has become too dry and has contracted. You can assume that the other woods have also shrunk and could crack under the stress.
There are a number of effective humidifying devices available to counteract this danger: The Oasis system, the Dampit, the Kyser system, and the D'Addario system all provide a humid atmosphere inside the guitar body. When using them it is important that you leave them inside the closed case when you take out the guitar to play it otherwise the the air in the case will become too dry.
An "old world" method of providing humidity to the neck and fret board is with a plastic traveling soap dish with a damp sponge inside and holes punched in the lid. If humidity is properly maintained your guitar should never suffer cracking or warping.