58 The Water Cycle-Mid

The Water Cycle Written by Robert N. Knight

Introduction Water covers three-fourths of Earth's surface. Other planets have only traces of water or no water at all This is why Earth is called the "water planet." As the number of people on Earth increases, more water is used. More people also mean more pollution. Earth's fresh water becomes more and more precious. In this book, you will learn about how Earth's water moves and changes. You will also learn how it is used and why it must be protected and conserved.

The majority of Earth's water is found in the oceans that cover most of Earth's surface. Ocean water has many minerals. These minerals make ocean water taste salty. For this reason, ocean water is called salt water. The water that is not found in the oceans is called fresh water. Only 3 percent of Earth's water is fresh water. Fresh water is found in lakes, rivers, and streams. It is also found under the ground. Most of Earth's fresh water is ice. It is found around the North and South Poles in ice sheets and icebergs. It is also found in mountain glaciers.

The Movement of Water Everything on Earth is made of matter. The three states of matter are solid, liquid, and gas. Most water on Earth is liquid. Some of it is solid ice and some is a gas we can't see. That gas is called water vapor. Earth's water moves in a cycle called the water cycle. As it moves from place to place, it changes from liquid to gas and gas to liquid. It also changes from liquid to solid and solid to liquid.

Water needs energy to change from liquid to gas or from solid to liquid. The energy it needs comes from the Sun. When water changes from liquid to water vapor, the process is called evaporation. This is the first step in the water cycle. Sunlight hits water on Earth's surface. Tiny particles that make up water, called water molecules, absorb some of the light energy and begin to move. As they get more energy from the Sun, they move even faster. When they get enough energy, they escape the liquid water and enter the air as water vapor.

When warm air near Earth's surface begins to rise, it cools. As the air cools, the water molecules in the air begin to slow down and lose energy. They collect on dust particles floating in the air. These molecules join other molecules and form tiny water droplets. As they lose energy, they change back to a liquid. This process is called condensation. That's the next step in the water cycle. As more and more water droplets gather on dust particles, clouds begin to form. As more droplets form, the clouds get thicker and thicker. The clouds can get so thick, that they form thick dark storm clouds.