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   keeps growing year by year. Under its bark  , a tree adds a layer of new wood each year it lives. If you look at a tree stump, you can see the layers. They are called annual ring  . On some trees, all of the rings are of the same width. But the ponderosa  that grows in the American southwest have rings in different withes. The soil in he southwest is dry. The pines depend on rainfall for water. In a year of good rainfall, they form wide rings. In a dry year, they form narrow ones. Scientists do not have to cut down a pine to study its rings. With a special tool, they remove a narrow piece of wood from the trunk without harming the tree. Then they look at the width of each ring to see how much rain fell in the year it formed. Some pines are hundreds of years old and have hundreds of rings. These rings are from an annual record o0f past rainfall in the southwest. A tree grows a new layer of wood   _  .
each week whenever it rains every year every season