The water cycle is how we keep our precious water supply on Earth. It has no beginning and no end. It goes around and around and around and has for billions of years. In truth, the amount of water on Earth has stayed pretty much the same, but its location and form does change over time. To explore the water cycle, we'll start with one section, but it is not the beginning, it is just part of the ongoing cycle.

The Many Faces of Water

Water exists in three forms: liquid, solid (ice), and gas. All three forms of water can be seen in everyday life. Drinking a glass of ice water on a hot day can show you all three. The water in your glass is the liquid, the ice is the solid, and the wetness on the outside of your glass is the water vapor cooling back to liquid right before your very eyes! This is called “condensation.” In any of those forms, water is still water. It can take part in the water cycle. Water can be found in the ocean, polar ice caps, as rivers, lakes, wetlands, and snow on land, as underground aquifers, and even in the air as water vapor in the clouds. The sun drives the water cycle by evaporating water into its rising vapor form. Then it cycles back as rain or snow to start again!

Making Clouds

Clouds form when water vapor, heated by the sun, rises off the earth by evaporation. The water vapor condenses in the cooler air that is higher up in the atmosphere. It clings to small particles in the air like dust or pollen. When the water condenses into clouds it is changing back from vapor to liquid. You can see condensation all around you on the earth. Just look at dew-covered grass or your fogged-up ski goggles. Condensation happens when warm water vapor meets cooler air and turns back to a liquid.

When you drive through a patch of fog, you are actually driving through a cloud that has formed right on the surface of the earth. This happens when very moist, warm air meets cool air and the water condenses right on the spot as fog!

Making Rain

The clouds gather all the tiny water droplets and crash them together until they are big enough to fall as rain or snow. This is precipitation. Precipitation varies throughout the planet. It rains much more in warm, tropical places than in deserts. In colder places precipitation falls as snow instead of rain. When rain falls on land, it soaks into the ground and adds to the groundwater. This is the fresh drinking water for people throughout the world. Rain also runs into rivers and streams, which make their way to the ocean, our largest body of water.

Storing Water on Earth

The oceans store 95 percent of our planet's water. Oceans, rivers, and lakes together make up about 90 percent of all the water that evaporates into the atmosphere. The other 10 percent comes from plants losing water (transpiration), and a tiny amount from glacial ice evaporating (sublimation). With the process of evaporation, we have come back to where we started in the water cycle.

How Glaciers Form

In some places, like Antarctica and Greenland, more snow falls than melts. Over time the deep snow presses into ice. Water can be stored in glaciers for a long time until the earth's climate warms. Then the ice will melt and be released into the water cycle again. During the last ice age, glaciers covered much of North America, northern Europe, and Asia. Even now 10 percent of Earth's land is covered by glacial ice. Glacial ice is fresh water compared to the salt water of our oceans. Glaciers store more than half of all the fresh water on Earth. As global warming affects our planet we are seeing even Greenland and Antarctica's ice packs melting.

Global Warming

Global warming is a term used to describe an increase in Earth's temperature from, in part, humans releasing large amounts of carbon dioxide into the air. Scientists believe that this will lead to climate change (a warmer Earth) that will have many bad effects on living things on the planet, including us! Some changes scientists think may occur include changes in rainfall patterns, melting glaciers, rising sea levels, and numerous other impacts on animals and plants. More on this topic can be found in Chapter 4.