A tuned piano is under a significant amount of tension. (The average medium size piano has about 230 strings, each string having about 165 pounds of tension, with the combined pull of all strings equaling approximately eighteen tons!) As the piano goes through climate change, the wood expands and contracts. As it does so, the tension on the piano flexes, changing the way the tension is spread across the instrument (hence, changing the tuning).
Fluctuation in humidity levels will not only knock your piano out of tune, but it can also cause serious damage that affects the piano's structural integrity. Minor damage due to humidity fluctuation includes "sticking keys", sluggish action, and loose screws. More serious damage that can occur may include issues such as string corrosion, glue joint failures that cause loud buzzing, cracked bridges, cracked soundboards, and loose tuning pins.
If you have just moved a piano into your home, it is a good idea to let it sit for at least three weeks before having it tuned. This way the piano has time to acclimate to the new environment.
I always preform a quick visual assessment and tuning pin test on a piano that I am not familiar with. I do this before attempting to tune it, and I will alert you if I find any structural damage. Occasionally, certain problems are not evident until the piano is tuned or brought back to pitch, but this situation is rare.
I will happily help you to choose the ideal tuning schedule for your piano. There are certain times of year that are not as good for tuning stability because of a changing climate. HOWEVER if you have an event, stable climate or "pianolifesaver" system/"dampp-chaser", I'm not as concerned and will tune for you during these times. The climate can vary from year to year so this timing can be vague. It's a good idea to keep on your regular tuning schedule. The ideal situation is to tune the piano close to the same hygrometer reading, every time.
If you are short on time, please go to the 2 minute mark.
High humidity-pitch goes up
The sample on the left illustrates the the bow or "crown" in a soundboard during a time of high humidity (greatly exaggerated). There is more tension on the string so the pitch sounds higher.
Low humidity-pitch goes down
The sample on the right shows the soundboard contracted during a time of low humidity. There is less tension on the string, so the pitch sounds lower (again, greatly exaggerated).