Have you noticed any changes in the sound of your piano over the years? Weather is an important aspect of care in the proper maintenance of a piano. Both weather and temperature can affect the tune and sound, as well as the actual construction of a piano. Indoor climates, which contain humidity, have the most noticeable effect, and regular maintenance of a piano is the best way to reduce the amount of tuning and adjustment necessary to keep the instrument sounding great. It is my understanding that maintaining consistent room temperatures, while avoiding any drastic temperature changes, is the most beneficial way of preserving the health of the piano; more important than is the humidity itself.
I have found that a room temperature of 70 degrees Fahrenheit with a relative humidity of 50% to be the most recommended environment. Being made mostly of wood, a noticeable change of tension and pitch can be observed in the piano when the relative humidity is over 55%, which is also known to cause the soundboard to shrink or swell, and this will change the amount of pressure that the soundboard exercises against the strings, making it sound sharp. On the other hand, if the relative humidity falls below about 38%, it forces the soundboard to expel moisture, shrink, and then fall flat. Each of the keys will issue forth its own version of the flat or sharp tuning; and the change in the sound does not effect each of the notes equally. This causes the piano to create out of tune and quite un-melodious sounding music .
To protect a piano from changes caused by temperature, I would firstly recommend to avoid placing it in direct sunlight. The sunlight will not only cause the fading of the finish, it will also cause it to eventually lose its tune. Other rays of light, such as stage lighting, can flatten the sound of the piano. Any type of lighting must always be balanced with any air conditioning or other temperature controls in the room, to protect the tuning during an entire performance for example. According to the experts, in general a 10 degree change in room temperature will cause a piano to either rise or drop in pitch at the rate of 1 cycle per second.
Overall, humidity is considered to be the greatest pressing danger to the longevity of a piano. Not only does it cause swelling, shrinking, and sporadic alteration of the notes, it also may eventually weaken the wood construction of the instrument to the point of cracking.