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Reading Selection, Lesson 7

The Water Cycle: From the Sky to the Land and Back Again

Life as we know it could not exist without water. People need it. Animals need it. Plants need it. There’s a limited supply of it.

Think about the river that flows through your town. Unless you live in an area that has a very dry climate, the river usually has plenty of water—even though the water moves in only one direction. Why doesn’t the river run dry?

Water moves through the environment in a cycle. That is, it’s always changing form—freezing, melting, evaporating, condensing, falling back to earth as precipitation—over and over again. The water cycle is Nature’s perfect recycling project.

Let’s start in the sky. Water is present in air as water vapor—that’s water in its gas phase. The water particles—also called molecules—in water vapor are separate and invisible.

Illustration of a water molecule.

The smallest amount of water is one water molecule. Each water molecule is made up of two atoms of hydrogen(H) and one atom of Oxygen (O). Thus, the chemical symbol for water is H(sub2)O. Individual water molecules merge to form the small droplets that make up clouds or fog. The samll droplets merge to form the larger droplets that become rain, sleet, or snow.

When the water vapor is cooled, the water molecules join together in a process called condensation to form tiny droplets of water. These droplets combine with others to form larger droplets. As long as these droplets stay small they remain suspended—floating—in the air as clouds or, at ground level, as fog.

When the droplets become too heavy to float, they fall from the clouds as precipitation—as rain, or, under very cold conditions, as ice in the form of sleet, hail, or snow. Once the precipitation reaches the ground, it can take many paths:
 • It can fall directly onto water—streams, lakes, or oceans.
 • It can fall on land and flow into a body of water.
 • It can fall on land and soak into the soil. Once in the soil, it will be taken up by plants,     flow beneath the soil surface to eventually join surface water, or seep into the rock     underneath the soil.
 • It can be taken from surface or subsurface water supplies by people, who will use the     water for drinking, washing, preparing food, generating power, and many other     purposes.

Most water is eventually returned to the water cycle through evaporation—that is, it changes from its liquid phase to its gas phase. Water can also enter the atmosphere through transpiration. Transpiration is evaporation from the surface of plants. Plants take up water through their roots and then release it as water vapor mainly though their leaves. The warm air near the ground, which contains lots of water vapor, rises. The water vapor in the warm air condenses as the rising air cools. Clouds are formed from these newly formed water droplets. Then, the cycle repeats.

Water is the only compound that exists naturally in all three phases (or states) of matter on Earth: gas (steam or water vapor), liquid (water in, for example, lakes, oceans, and rivers), and solid (snow or ice). One phase can change into another. For example, water can evaporate to form water vapor or freeze to form ice.

Illustration of the water cycle.

The water cycle—from the sky to the land and back again.

No matter the phase of the water—solid, liquid or gas—the chemical composition of each molecule is always H2O. The water molecules do not break up. The difference is in way the water molecules are arranged and the speed at which they move. Keep in mind that water molecules are always in constant motion, even in ice.
• In the solid phase—as ice—the molecules are packed tightly together. They move slowly.
• In the gas phase—as water vapor—the H2O molecules are farther apart. They move very fast and spread quickly.
• In the liquid phase, they behave somewhere in between.

Dark clouds in the sky.

The water molecules in this large, dark cloud won't remain suspended in the air for long. This cloud is known as a thunderhead because it holds an electrical storm. Soon, rain will pour from this cloud, accompanied by lightning and thunder.

The speed at which the molecules move depends on the temperature. When heat is added to water, the molecules move faster. When heat is removed, the water molecules move slower. If enough heat is added the water may change phase. For example, on a hot summer day, some of the water evaporates from a lake. It changes from the liquid phase to water vapor, the gas phase. On a cold day the water loses heat; the motion of the water molecules in the lake slows. If enough cooling occurs, the phase of the water changes from liquid to solid. That same lake that was perfect for swimming in the summer might be perfect for ice skating in the winter.




It's Not the Heat, It's the Humidity

All air contains some water vapor. In humid areas, such as the Atlantic and Gulf Coast regions of the United States, the air usually contains a lot of water vapor. In arid areas, such as the deserts of northern Africa, there is very little.

The measure of water vapor in the air is called humidity. In general, the higher the humidity, the less comfortable you will be. High humidity combined with very high or very low temperatures produces the least comfortable environment. On very hot, very humid days, for example, the air can't hold much more water vapor. Your sweat stays on your skin and drips down into your eyes. Then when you wipe your brow you lose your balance; you swerve your bike into the ditch at the side of the road. So the next time someone says, "It's not the heat, it's the humidity," you can answer, "Actually, it's the heat and the humidity."

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