What effect does humidity have on your piano?

Humidity is the biggest factor in pianos going out of tune.

As the air dries out, the wood in the piano shrinks and the strings lose tension.

When the air becomes moist again, the wood swells and tends to increase tension.

This tension and release causes the string to gradually go down in pitch.

Some people have a special humidifier installed called a Damp Chaser which tries to control humidity around the piano. These can help, but require refilling with water periodically and many people end up not keeping up with that.

One tip is to try and avoid having the piano near a window where direct sunlight would hit it.