What's with the violin and the weather?

Humid weather swells the wood causing parts to not fit, the peg box wood absorbs the moisture and can change shape. The violin top absorbs this moisture and makes the top swell up pushing the bridge higher making it harder to play. This higher arching of the top makes the sound post looser effecting the sound of the instrument, causing a more sluggish sound. Likewise during the dry winter months everything reverses, the top is lower and the post is tighter. Because of the dryness the sound is harsher, this dryness also makes the wood more susceptible to cracking. The dryness causes the pegs to stick more readily. Unless the violin is in extreme moisture areas, you usually won't get edges or other glued seams opening from the high humidity. However if it gets especially hot like leaving it in a car, the seams can all fall apart and the varnish can melt. On the other hand, the dry winter months are very hard on any glue seams.

Cheap instrument pegs are effected more by these extremes because the wood is of lesser quality and softer allowing for the wood to both absorb and dry out more quickly. The one thing that cheaper instruments are more stable in is the height of the top as generally the instruments are made with thicker materials and harder varnish making them less effected.