How much rain has fallen on the earth in the past﹖Man has not always kept weather records． Because scientists need a way to learn about past rainfall, they study the tree rings． A tree's trunk (树干) keeps growing year by year. Under its bark(树皮) , a tree adds a layer（分层） of new wood each year it lives．If you look at a tree stump (树根）, you can see the layers. They are called annual rings (年轮)． On some trees, all of the rings are of the same width. But the ponderosa (美国黄松) that grow in the Arican southwest have rings in different width. T