Caring for an acoustic instrument
Most acoustic guitars/ukes etc. are made from a natural material - wood.
Wood absorbs and gives off moisture according to its environment, making the wood expand or contract. If this happens too quickly, or the conditions are too extreme, then the guitar - especially the soundboard - can crack or distort.
Always transport the guitar in a case and allow it to come to temperature slowly in its case before taking it out. Modern hot and dry central heating can cause havoc with acoustic instruments.
More expensive "solid top" (i.e. non laminated) guitars are more at risk than cheaper instruments with plywood tops and sides.
Heat alone (e.g. direct sunlight) can cause problems on some guitars as the glue used in construction can be softened, maybe causing the bridge to lift for example.
Sunlight can also bleach or discolor the finish.
Rapid changes in humidity and temperature are more likely to cause problems than absolute conditions.
