The Weather and Your Piano

The weather affects your piano in many ways.  Excess humidity can make a piano go out of tune by causing slight to expansion of the soundboard across the grain, which will lift the bridges  and very slightly tighten the strings causing the pitch to go sharp.  This is most obvious in the center of the piano, where the soundboard’s rise is most dramatic.
Ridges may appear along the grain of the soundboard where the large spruce panels are glued together.
Moisture can creep into the felt key and flange bushings.  The moisture content of the keys will change during wet seasons.  These things can cause sluggish action or key symptoms.  A piano can become unresponsive because hammers and keys don’t return to position rapidly enough for good repetition of notes.  Dynamic range becomes limited.
Mold can grow if too much moisture is present, causing damage to many internal parts, as well as health hazards to those allergic to certain types of organisms.

Conversely, dry weather—especially hot dry weather—will cause contraction of the soundboard and lowering of the piano’s overall pitch.  If there are cracks in the board, they will probably widen and become more visible.*  Any tendency to have buzzing, rattling, or other related sounds will be exacerbated.
Overly dry conditions can make tuning pins loose in their holes.  If proper steps aren’t taken, and the climate within the piano—or room where it sits—is not controlled, a piano can eventually be rendered untunable.
Key bushings contract, creating excess side-to-side key movement.  Actions become loose as flange bushings shrink.  A noisy action and keyboard that feels uncontrollable is a by-product. is
The San Francisco Bay Area is, for the most part, hospitable to our favorite instrument. The ideal relative humidity level (RH) for pianos is 42%.  The range here is from 40-60 percent most of the year.  In-piano climate control is recommended in some of the damper coastal areas, or in the hills where redwoods keep the temperature low and humidity levels fairly high.
*While it might seem a very severe problem, pianos with cracks in their soundboards can be repaired, if necessary.  Many pianos have less-than-perfect boards, yet produce beautiful tone with few or no  noticeable negative effects.