In climates where the year includes cold and warm or wet and dry seasons, the cells in annual rings vary in color due to the fact that the tree grows faster in one season than in the other. In most instances, the tree adds one light-colored ring during the time it grows fastest and a dark ring as growth slows.
The University of Arizona says a dendrochronologist is "a scientist who uses tree rings to answer questions about the natural world and the place of humans in its functioning." These scientists use tree rings to understand a proper historical context, the environment and conditions and to predict possible issues in the environmental future.
Depending on the tree rings of one tree often leads to errors in dating. Thus, dendrochronologists compare several trees, matching the growth rings to eliminate such errors. The thickness of the rings helps the scientist to establish information about rainfall or seasonal temperatures and how they affect the tree's growth.
Students often enjoy tree ring counting activities. They need a thin cross section of a tree. With age, the rings become difficult to see, but sandpaper helps to refresh the surface so the rings are more visible.