What happens during the natural growth of a tree each year?

During this century, archaeologists have discovered that trees can be a valuable tool for measuring time. In scientific terms, this method is called dendrochronology. Dendrochronology is based on the scientific discovery that each year a tree grows a new layer of wood. When you cut down a tree, for example, you can see and count the layers of concentric circles. The layers vary in color and in thickness. These variations are a result of differing climate conditions from year to year that affects the growth of the annual rings. Cold weather, for example, creates thinner and lighter rings while hot weather produces thicker and darker rings. All trees that grow in the same region will have roughly the same pattern of annual rings, and this can be used as a source of comparison and measurement. Early in this century, archaeologists began using the rings to date ruined structures and figures built in the southwestern United States. They did this by comparing the rings of the wood found in the ruins with the rings of recently cut trees. Slowly but surely, the system was extended backwards in time to measure older ruins. The heavy forestry of the United States, coupled with the relative youth of its history, led to a fairly uncomplicated study.