Teacher's Guide
Water Cycle

Overview: During this introductory lesson, students will be introduced to the Water Cycle. If you have already covered the water cycle, you may choose to omit this lesson and skip onto the next lesson, The San Francisco Bay.

Background Information:

The Earth is a dynamic system, spinning on its axis as it revolves around the Sun. The surface of the Earth includes the atmosphere (the air surrounding the Earth), the hydrosphere (the water on the planet), and the lithosphere (the land that makes up the Earth). The interaction of these spheres creates the life sustaining Water Cycle.

Where did water come from?

The oceans were created over eons of time by hydrogen and oxygen that united inside the earth to form water which was then trapped in the Earth. Volcanic events over a long span of time "out gassed" much of this water in the form of steam. The steam formed clouds in the atmosphere, and precipitation brought the water back to Earth, to continue the endless water cycle. The miracle of water then created conditions for life to develop and survive on this planet.

The Water Cycle

The water cycle is a major driving force on our planet. Water is in constant motion, evaporating into the atmosphere to later fall in the form of precipitation to form oceans, lakes, rivers, and streams. Water is not consumed, but is continuously recycled.

When water is heated, it changes from a liquid to a gas. This process is called evaporation. As the water vapor moves higher into the atmosphere, it cools. This cooling of water vapor results in condensation of the vapor into liquid. We can see water vapor condensing when we watch clouds. A cloud is nothing more than water vapor that has condensed back to a liquid form. A cloud is made of extremely tiny drops of water which can remain in the air. As a cloud grows, and more and more water condenses in the same place, the cloud droplets get larger. Eventually, these cloud droplets will be too large to remain in the air. The cloud is then said to be saturated. A saturated cloud will usually precipitate, or let fall, its excess water. Precipitation takes the form of rain, snow, sleet, or hail. The water flows on the surface of the ground and percolates through the Earth as groundwater. Eventually this water makes its way back to the rivers, streams, and oceans of the world where it is once again evaporated into the clouds, continuing the water cycle.

Pollution and the Water Cycle

Water is an important commodity to all organisms that live on Earth. Although it appears as if we will never run out of water, only 3% of all Earth's water is fresh. Of this 3%, 2/3 is locked up as ice and is therefore unusable. This means that only 1% of all the water on this planet is available for us to use. 97% of all the water on this planet is salt water which is unusable by most land organisms. The ocean water was formed as fresh water eroded the land and dissolved different salts that remain in the water. The oceans play important roles in the water cycle because they contain so much of our available water. Unfortunately we cannot use the ocean water as it is. Instead we must wait for it to be evaporated, leaving behind salt and other impurities, and then fall back to the Earth as precipitation.

Since only pure water evaporates, pollution will also be left behind in the ocean during evaporation. Even though this pollution does not enter our water cycle, it is still present in the waters of streams, rivers, and oceans. For this reason, we must be concerned with what we put into the water systems.

Student Activity:

Make an overhead copy of this worksheet, to use while you present the background information.
Trace out the path (on the overhead) of water as it travels through the water cycle.
Explain the importance of the water cycle in cleaning up our water supply. Be sure to explain that only pure water evaporates, leaving behind pollutants.
Print out a copy of the Water Cycle Worksheet for each student.
Review the directions with the students as they complete the worksheet.
If you choose to collect the worksheets, be sure to return them to the students for review.