The sun bakes one side, moist air from the inside of the house works on the other. Warped siding is the result. Cracking Logs, columns, and larger dimensional lumber usually crack radially as they dry. Very thick pieces of lumber take years to come to MC equilibrium with their environment. Because radial shrinkage cannot keep up with tangential shrinkage, a crack occurs where excessive tangential forces pull apart wood fibers.
Cracks form from the center to the outside.
Smaller pieces of stock and kiln-dried wood do not usually have this problem. It is often not practical to kiln dry massive timbers, so they commonly exhibit radial cracks. This picture shows cottonwood with radial cracks. Figure 3 also shows radial cracking in the T plane of the post. The quartersawn side of wood never exhibits these cracks, which may be another plus for the use of quartersawn lumber.