Adopt-a-City in the USA The Water Cycle

What is a cycle? A cycle is a series of events (a process) that repeats itself endlessly. It is like a circle that goes round and round and never stops.

What is the water cycle? As you can see from the picture above, arrows show that the water on our planet travels in an endless circle. During the phases of this cycle, water can be found in all three of its common states: as a liquid (water), as a solid (snow, ice), and as a gas (water vapor). The water that is on our planet now is the same water that has been here for millions of years--the dinosaurs drank the same water we use today!

Definitions: the words in the picture above are important to knowing about the water cycle.

Condensation

the change of a substance from a gas to a liquid

Precipitation

water that falls from to earth from the sky as rain, snow, sleet, or hail--a direct result of condensation

Evaporation

the change of a substance from a liquid to a gas

Runoff

water that has fallen as rain and has stayed on the surface of the earth. It moves from higher to lower elevations as "runoff"

Groundwater

water beneath the earth's surface between saturated soil and rock that supplies wells and springs

The water cycle can be described in three phases. Learn how to explain and demonstrate each part of the water cycle:

condensation	Water vapor in the atmosphere changes to liquid water. Air pressure and temperature have a big part in this process. Since warmer air can hold more water vapor than colder air, the water vapor condenses as the temperature decreases. Cooling temperatures sometimes bring rain, and dew appears on the ground when the air cools in the evening.
precipitation	As the air condenses, gravity helps this liquid fall to the ground. Depending on the air temperature and wind patterns, precipitation can be in the form of rain, sleet, snow, or hail.
evaporation	Water in its liquid form can change to gas with a rise in the temperature and with the effect of wind blowing across the water. This gas rises into the atmosphere and can take the form of clouds and fog.

Demonstrate condensation:

Materials: an empty tin can, water, ice, food coloring

Procedures: Remove the label from the tin can and fill it with ice. Add water and a few drops of food coloring. Let it stand on a table for a few minutes.

Results: The can seems to be "sweating." Drops of water form on the outside.

Explanation: The ice drops are not colored, so they can't be coming from the ice water leaking out of the can. The water must have come from the air. Water vapor--water in the form of gas--in the air around the can has been cooled by the ice.

Demonstrate precipitation:

Materials: a double-boiler (or two pots one larger than the other), water, ice cubes, heat source (hotplate), hot pad

Procedures: Boil water in the bottom pan (the larger one). Place cold water and ice cubes in the top pan and hold it slightly above the boiling water.

Results: You see rain!

Explanation: The cold surfaces of the upper pot cool the steam from the boiling water. The steam changes back into water, collecting in drops. As the drops get bigger and heavier, it "rains." The boiling water is like the water heated by the Sun. The steam is like the water that evaporates into the air as water vapor. As the vapor rises, it cools. You see clouds when droplets form. As these droplets collect more moisture, they become heavy enough to fall to earth as rain.

Demonstrate evaporation:

Materials: a large flat dish, water, a deep narrow jar

Procedures: Place an equal amount of water in the dish and the jar. Place both, uncovered, on the table to stand overnght. Check them in the morning.

Results: Less water remains in the flat dish than in the narrow jar.

Explanation: Water molecules can escape only from the surface. So water will evaporate faster when the surface is large. A wide shallow puddle will dry up more quickly than a deep, narrow one.

Alternate demonstration: Place two dishes filled equally with water in different parts of a room--one in a sunny window, the other in a shady corner. The dish in the sun dries first. Explanation: the warmer the air, the faster the molecules move into the air and the faster they evaporate.