Pianos are wooden instruments and react to the relative humidity of their environment. Around here (Northeastern New England) they go flat in February and sharp in August. Their ideal state would be one of consistent temperature and humidity.
Sometimes whole-house approaches can work to stabilize the humidity. In our shop we use a dehumidifier to keep it below 65% (relative humidity) in the summer, and a whole-house humidifier to keep it above 35% in the winter. It gets stuffy and hot in the summer because I hate to open the windows and make the expensive dehumidifier run even more.
A preferred solution exists for individual pianos. We carry the Dampp-Chaser Piano Humidity Control System. This is a humidifier and a dryer we install in your piano. The controlling humidistat cycle one or the other to keep the humidity around 42%. It requires only to be kept plugged in year round and watered when the control light blinks. No piano disassembly is required to fill the water.
We have monitored vertical studio size pianos in the past and find that the use of a Dampp-Chaser System reduces the seasonal change in tuning to 25% of an unprotected piano. The subject piano, unprotected, would go 8 Hz flat of A-440 in February and 8 Hz. sharp of A-440 in September. With the protection of a Dampp-Chaser System the swing was 2 Hz. flat to 2 Hz. sharp. The subjective benefit to the customer is that the piano is tolerable to play through the majority of the year.
We strongly recommend Dampp-Chaser Systems in the pianos sold from our shop.