Introduction

The earth's atmosphere is an invisible envelope of gas we hardly notice except when the wind blows or when rain or snow falls. The atmosphere is also responsible for the blue of the sky, for the colors of sunrise and sunset, and for the rainbow, as we learned in Chap. 6. Less obvious but more important is the role of the atmosphere in the living world. Its oxygen, nitrogen, and carbon dioxide are essential for life. It screens out deadly ultraviolet and x-rays from the sun. It carries energy and water over the face of the earth. And, by weathering away rocks, it helps form the soil in which plants grow.

All the water of the earth's surface is included in the hydrosphere. Oceans, seas, rivers, and lakes cover about three-quarters of the surface area of our planet. The oceans, of course, make up by far the greatest part of the hydrosphere, and they are a major factor in shaping the environment of life on earth.