As a music artist, could you imagine your precious instrument sounding out of tuned?
The tone of a musical instrument with a pure timbre is significantly affected under high humidity conditions. Wood is an organic material which exchanges moisture with the surrounding air. It swells and contracts depending on its moisture content, and its moisture level depends directly on the moisture in the surrounding air. This change in shape and size puts stress on the instrument.
When it gets smaller, parts of the instrument like the top are under tension which will cause formation of cracks and failure of the joints and seams. When it gets larger, joints and seams can also fail, and at high moisture levels, wood's resistance to bending and permanent deformation goes way down. Heat and moisture were used by the maker to bend the ribs on your instrument, so you can understand how excessive moisture can result in permanent distortion of the top and a permanent sagging of the neck height. The dimensions and strength of the wood change with moisture content. Therefore, sound of instruments changes with moisture content.
These factors, weight, dimensions and strength are the very factors that instrument makers manipulate to control how their instruments sound in the first place!
According to Taylor Guitars, The wood in a guitar can swell tremendously, causing glue joints to fail, finish to lift, and neck angles to go bad. (See Taylor Guitars, "tech sheets".) The very best conditions for your instrument would be to keep the relative humidity of the environment at a consistent level all the time.