57 The Water Cycle-Low

The Water Cycle Written by Robert N. Knight

Introduction Water covers three-fourths of Earth's surface. This is why Earth is called the "water planet." Each year, more and more people live on Earth. They use more and more water. They cause dirt to get into Earth's water. For all these reasons, clean water is more precious. This book will teach you about how Earth's water moves and changes. It will also teach you how we use water and how we can conserve it.

The world's oceans hold most of Earth's water. However, this water is salt water. Salt water is not good for drinking or watering plants. A very small amount of Earth's water is not salt water. Fresh water is in lakes, rivers, and streams. Some of it is found below the ground and high in mountains, too. But most fresh water is found in the form of ice around the North and South Poles.

The Movement of Water Everything on Earth is matter. The three states of matter are solid, liquid, and gas. Earth's water is found in all three of these states. Solid water is ice Water that is a gas is water vapor. You cannot see water vapor. Liquid water is the water you drink and see fall as rain. Earth's water moves all around Earth. As it moves, it changes states. This movement is called the water cycle.

The first step in the water cycle happens when the Sun shines on water. This sunlight is energy. Some of this energy passes to the tiny particles that make up water. The particles get more and more energy from the sunlight. They move faster and faster. When they move fast enough, they leave the water and move into the air above the water. This process is called evaporation. Evaporation happens when liquid water changes to water vapor.

Warm air near Earth's surface rises. As it rises, it cools. The tiny water particles in the cool air slow down and get closer together. They begin to gather on tiny bits of dust that are floating in the air. As water particles collect on the dust, they form water droplets. This process is condensation It's the next step in the water cycle. When millions of these water droplets come together, they form clouds. The more water droplets that come together, the thicker the clouds become. When clouds become very, very thick, they can form storm clouds.