 Inside each tree is a recording of what it experienced during its life. Each growth ring usually represents one full year in the life of a tree. But the rings can also tell us about changes in the climate and even sunspot activity. Dendrochronology is the science of using tree rings to determine past climates, date old buildings, and even calibrate radiocarbon dating. Tree rings are the result of new growth in the vascular cambium of the wood. They are visible because trees grow quickly in the spring and early summer and slowly in the winter. Wide rings represent a year where conditions were favorable to growth, while a narrow ring may signal a drought occurred that year. Trees that grow in temperate zones usually have the most visible tree rings because seasons differ from each other markedly. While seemingly insignificant, tree rings have proven to be a valuable scientific tool. From the University of Wyoming Extension, I'm Barton Stamm, Exploring the Nature of Wyoming.