Water is such an integral part of human existence that it makes up nearly three-quarters of Earth’s surface and has a profound effect on all life on Earth. Water vapor in the atmosphere accounts for just one-thousandth of 1% of all the water on Earth, but it has a tremendous impact on Earth’s climate and weather patterns.

This Reading provides background information on the underlying concepts in the Activities included in the book, Project Earth Science: Astronomy, Revised 2nd Edition. The Readings go beyond what is found in the background information in the Activities. They can be used as supplementary information for the teacher, or to enhance classroom discussions.