59 The Water Cycle-High

The Water Cycle Written by Robert N. Knight

Introduction People call Earth the "water planet" because water covers three-fourths of its surface. We have lots of water, but most of it we can't use. The world population is growing, and as it grows, more people compete for this supply of water. Also, as we pollute more of our water supply, the water we need is getting scarcer. This book explores how Earth's water moves from place to place. It describes the changes water undergoes as it moves. It also describes how we use water and what we can do to conserve and protect it.

About 95 percent of Earth's water is contained in the oceans that cover most of the planet's surface. Ocean water contains many kinds of minerals. These minerals give ocean water its salty taste, which is why it is called salt water. Drinking salt water just makes you thirstier. People and many other animals can get sick or die if they drink too much salt water. About 3 percent of Earth's water is fresh water. Lakes, rivers, and streams all contain fresh water that comes from rain and melted snow. Fresh water also exists underground. But most of Earth's fresh water is contained in glaciers found in mountains as well as icebergs and ice sheets found around the North and South Poles.

The Movement of Water Matter is found in three states: liquid, solid, and gas. We are used to seeing Earth's water in its liquid state. Water also exists in its solid state as ice and in its gas state as invisible water vapor. Earth's water often changes from one state of matter to another as it moves from place to place. The changes and movement of Earth's water are all parts of the water cycle. The water cycle has been going on for billions of years, moving Earth's water from the oceans to land and back to the oceans.

Energy from the Sun powers the water cycle. This energy helps water change from one state to another. The water cycle begins with evaporation. During evaporation, liquid water changes into invisible water vapor. Heat and light from the Sun hit Earth's water in oceans, rivers, lakes, and even the smallest puddles. The tiny particles that make up water, called water molecules, absorb this energy. As they gain more and more energy, the molecules move faster and faster and begin to spread apart. Soon they absorb enough energy to escape Earth's surface water and enter the air as water vapor.

When water vapor in the air rises into the atmosphere, it begins to cool. Water molecules begin to collect on dust particles floating in the air. They gather to form tiny water droplets. This process, called condensation, happens whenever water changes from a gas back to a liquid. This is the next step in the water cycle. As more and more water vapor condenses, the droplets start to form clouds. As more droplets form, the clouds get thicker and darker. You can tell a lot about what weather might be coming by looking at clouds. High, wispy clouds mean there is ice and moisture above, and the weather may be changing. Fluffy, spread-out clouds may mean there is a little water above, but not enough for rain. Very tall clouds with dark bottoms usually mean that a thunderstorm with heavy rain is coming.