The amount of water in your home's wood varies greatly depending on the relative humidity (RH) level of your indoor air. If RH is too low, your indoor air will absorb water from the wood, causing it to shrink. If RH is too high, the wood will take on moisture, causing it to swell.
When indoor RH changes quickly, the dimensions of woodwork, floorboards and furniture change rapidly and warping and cracking can occur.
This is why maintaining optimum RH levels is important. Industry experts recommend keeping your home's relative humidity levels between 30-60% to preserve woodwork, cabinetry and furniture, among other things.
"Seasonal and even daily changes in humidity cause wood parts to swell and shrink, affecting tuning stability and touch. Extreme swings in humidity can eventually cause wood to crack and glue joints to fail...Maintaining the correct environment will preserve your piano investment for a lifetime of enjoyment." - Piano Technicians Guild