As the temperature drops during the winter season, so does the humidity in the air. You may notice gaps in your wood floors that weren’t there in the summer, or that old door that always sticks now opens freely. It’s all due to humidity. Wood is sort of like a sponge. It soaks up moisture from the air and expands and shrinks back when the humidity falls. The piano being primarily made of wood, does the exact same thing. If the relative humidity gets too low, the sensitive wood can crack and permanent damage can occur and the soundboard can crack.