Moisture and wood
In a comfortable home with slight humidity variations through the seasons, wood responds by expanding and contracting. These changes may be noticeable. During warm, humid weather, wood expands. During dry weather, wood contracts. This seasonal movement is a normal characteristic of wood, and it never stops, regardless of the age of the wood. One of the best ways to ensure that wood will give the performance homeowners expect is to install humidity controls and ensure that they are functioning before the wood steps are installed.
In winter, when homes are heated and the air is dry, wood gives up some of its moisture and therefore shrinks. When that happens, thin cracks appear. This is normal and homeowners should be forewarned of this. Once the indoor heat is turned off in the spring and the indoor environment regains moisture, most of these cracks will close.
The best way to minimize crack formation is to add moisture to the air during dry periods by installing a humidifier and run it throughout the winter.