Water Cycle Teacher Guide

NSES Earth and space science – structure of the earth system

Water, which covers the majority of the earth’s surface, circulates through the crust, oceans, and atmosphere in what is known as the "water cycle." Water evaporates from the earth’s surface, rises and cools as it moves to higher elevations, condenses as rain or snow, and falls to the surface where it collects in lakes, oceans, soil, and in rocks underground.

NSES Earth and space science – structure of the earth system. 

Clouds, formed by the condensation of water vapor, affect weather and climate.

Discussion: Setting the Stage

1. How does water circulate through the crust, oceans, and atmosphere?  Prior to running the models, let students ponder this question in learning groups without much direction. Provide sticky notes for them to write 1-2 word responses (rainfall, water runoff, snow melting). On your whiteboard, provide a rough drawing of the Earth's surface with a coastline, mountain, lake, river, and clouds. It could look like the image BELOW, but without labels:  

2. There is water in the air, though we can't see it. How does it get there?  Students should be allowed to freely brainstorm. A few students may respond with the word "evaporation", but it's unlikely they will know much about the process. Explain that they will do an experiment to gather evidence that there is water in the air. 

3. Where do clouds come from?  Again, any response is accepted and should be recorded in their science journals. Students will be learning about the process of condensation in the activity "Water From the Air" and may be quite surprised by how clouds are formed. Take advantage of the discrepant event to have them reflect on their original ideas in their journals. 

Formative Questions

Water Moving Around the Earth

Collect Data I:  From where does the water in the air come?  Acceptable responses: Many students will respond "clouds". Ask them to slow down the model and look at it closely. What is moving downward? (raindrops) What is moving upward? (blue dots from the lake and ocean)

Where does the water eventually go when it falls on the ground?  Response: Some of it is absorbed into the ground, some falls into the lake, some runs off out of the lake to the ocean, and some falls directly into the ocean. After rain falls in lakes and oceans, some of it evaporates back up into the atmosphere. Some of it infiltrates into the ground where it can find its way to underground aquifers. 

Collect Data II: Explain how water can go from the ocean to the top of a mountain.  Response: Expect some students to have difficulty with this. Ask them to consider the opposite: "How does water go from the top of a mountain to the ocean? (melting snow runs off into streams that feed into the ocean)  Then ask them to change wind speed to see how this affects cloud movement. They should all be able to see that when clouds move over the mountains, it can rain. They may not yet perceive that water evaporates from the ocean to travel upwards, where it can eventually form more clouds. Wind can then carry the clouds over the mountains. Don't give them the answer!

Can you make it rain just over the mountains?  Answer: Yes, if you set ocean temperature to its lowest setting and move wind speed over to the left. 

Can you make it rain just over the ocean?  Answer: No, not in this model (see image above). Even if you set the highest ocean temperature and set wind speed to blow clouds over the ocean, there will still be rain falling along the coastline over the forested area. **NOTE: Ask students to run the model at least 60 seconds before changing settings. The model is designed to show the gradual change when cloud formations no longer appear in certain areas or when ocean temperatures change.  

Collect Data III:  How does ocean temperature affect the movement of water in the water cycle?  Responses will vary, but expect students to see that the higher the temperature, the more water evaporates out of the ocean. If they don't already know the word "evaporation", this is a good time to introduce it (see Vocabulary above). Also, at the lowest ocean temperature, no evaporation occurs and cloud formation stops over the ocean. 

Water Into the Air Activity

Collect Data I:  

Question: Which bag had a higher humidity level: the room air bag or the bag with your breath? Why?  Answer: if done correctly, the baggie with only exhaled breath will have a significantly greater relative humidity, if it is compared with air-conditioned room air. Responses to the "why" question can vary, but should include the idea that the human body has a way of adding water to the oxygen we take in to breathe. Teachers: Here's a more technical answer to why exhaled air is moister -- In the respiration process, when inhaled air reaches our lungs, it becomes humidified with water. Inside the lungs are tiny sponge-like sacs (alveoli) where the exchange of oxygen, CO2, and water between air and body occur. Typically, exhaled air is completely saturated with water vapor. This experiment won't reflect a 100% relative humidity because it's not really possible to transfer pure exhaled air to a baggie so that you keep out all surrounding air. The idea is for kids to see that exhaled air is MUCH moister than room air.

Collect Data II:  Task -- Fill one bowl halfway with warm water and a second bowl halfway with cold water. Cover both tightly with plastic wrap and wait one minute. Insert Relative Humidity sensor under plastic wrap and measure humidity levels in each bowl.  

Question:  Which had a higher humidity level: the air above the warm water or the air above the cold water?  Answer: The air above the warm water will have significantly higher relative humidity than the bowl filled with cold water. Teachers: To make it really interesting, try using a fairly deep bowl for the cold water and asking students to take a reading with the sensor just under the plastic wrap, then move the sensor closer to the cold water. Their graph should look similar to the one at above left (the relative humidity will go down as the sensor moves closer to the surface of the cold water).  

Collect Data III:  Task -- Notice that one of the plastic covers has water droplets forming on it. Take off the plastic wrap and wave it back & forth. 

Question:  Which plastic wrap has water droplets on it: the one covering the warm water or the cold water?  Answer: The plastic wrap covering the warm water.

Question 2: What happens to the water droplets when you wave the plastic back and forth in the air?  Answer: The water droplets roll off and land on the floor, your hands, or other nearby surfaces.

Water From the Air Activity

Collect Data I:  Task -- Compare the temperature of the outside surface of a WARM cup of water with the outside surface of a COLD cup of water.  See graph below:

Question 1: Was the outside surface of the COLD cup colder or warmer than the air?  Answer: colder, probably significantly so if the water was iced. In the graph above, the temperature of the cold cup surface dropped from 22 to 7 degrees Celsius (or from about 78 to 44 degrees Fahrenheit). 

Question 2: Was the outside surface of the WARM cup colder or warmer than the air?  Answers can vary a bit, depending on how hot the classroom tap will heat water and depending upon the material of the cup. In general, the temperature will be warmer on the warm cup surface. In this case, our water was warm, but not as hot as boiling water. The temperature increased from 20 to 29 degrees Celsius (or from about 68 to 84 degrees Celsius). If you boil the water, the temperature will be much higher. Teachers: We recommend plastic cups. Styrofoam won't conduct the heat well and ceramic cups might get too hot.

Question 3: Is there condensation on the warm cup or the cold cup?  Answer:  The cold cup (see image below).

Collect Data II:  Task -- Bring out a 3rd empty cup that was placed in the freezer. Immdiately measure the surface temperature by touching the sensor to the surface of the cup.

Question 2: Did the condensation appear different on the freezing cold cup compared to the ice water cup and the warm water cup? How was it different?  Answer: The freezing cold cup WITHOUT water no condensation, while the cup with ice water had condensation. 

Question 2: Record the temperature of the surface of the frozen cup.  Expect students to be surprised! The freezing cold cup warms up VERY quickly when exposed to air at room temperature. The graph above was generated only about 15 seconds after the cup came out of the freezer and its temperature measured at about 17 degrees Celsius (or about 62.5 degrees Fahrenheit). It was quite a bit warmer than the cup filled with ice water (which measured about 7 degrees C). For fun, you might try using glass instead of plastic to see if it retains the cold better.

Collect Data III

Question 1: In the model, what happened to the molecules on the left side when heat was added?  Answer: They began to move faster, break apart from each other, and have greater energy. See image above right.

Question 2: What happened to the molecules when the heat was removed?  Answer: They began to cluster together again and move more slowly and sluggishly. 

Analysis

Water Moving Around the Earth

1. A cycle means something that repeats over and over again, like traveling in a circle. Why do you think the water cycle is called a cycle?  Responses will vary, but in general students should recognize that water moves in a predictable way through rainfall, runoff, and evaporation. The movement of water repeats itself in a pattern, but the pattern can be interrupted by shifting wind and temperature changes. You may want to reinforce this idea with the definition for "water cycle" -- the continuous movement of water on, above, and below the surface of Earth through processes of rainfall, runoff, evaporation, condensation, transportation by wind, and infiltration into the ground. 

2. List at least 5 different forms water has in the different places during the water cycle.  Answers can include "rainfall (or precipitation), snow (represented by the white square in the model), water vapor (represented by the blue circles in the model), melted snow, clouds, underground water, lake water, ocean water."  

Water Into the Air

1. Why do you think the air from your breath has a high humidity level?  Responses will be varied, but expect students to respond that inhaled air undergoes some change inside our bodies that adds moisture to it. In the respiration process, when inhaled air reaches our lungs, it becomes humidified with water. Inside the lungs are tiny sponge-like sacs (alveoli) where the exchange of oxygen, CO2, and water between air and body occur. Typically, exhaled air is completely saturated with water vapor. This experiment won't reflect a 100% relative humidity because it's not really possible to transfer pure exhaled air to a baggie so that you keep out all surrounding air. The idea is for kids to see that, through respiration, our bodies transform the air we breathe and we exhale air that is almost saturated with water vapor. Other chemical processes occur during respiration, such as exhalation of carbon dioxide as a waste product. 

2. Is evaporation greater from warm water or from cold water?  Answer: From warm water, as the second experiment with water bowls confirms. 

3. Where else have you seen evaporation?  List examples.  Responses can include sweat dripping off your body on a hot day, water level decreasing in a glass you leave out, clothes drying on a line (water evaporates into the air), splashes drying up at the edge of a swimming pool. At this age, students may not realize that water is evaporating out of lakes, ponds, and oceans. They often need to see simulations to form an understanding of this process in the Water Cycle.

4. If you didn't have a humidity sensor, how could you tell that there was a lot of water in the air (i.e., the humidity is high)?  Again, responses will vary. Typical responses -- foggy windows of an air-conditioned house or car, a musty smell in the air, excessive dew on grass and plants, outside painted surfaces feel damp to the touch, your skin gets damp very quickly outside, the air feels "heavy", cold beverages get "sweaty" quickly.

Water From the Air - Analysis

1. If it's cold enough, water can condense from the air as snow, instead of water droplets. Draw what you think a water droplet and a snowflake look like.  Any work produce should be accepted here, but most students at this age are aware that snow flakes are a crystalline form that has a regular pattern. 

2. Describe what happened to the water molecules in the model when they were warmed up and when they were cooled down. Under what circumstances did they form droplets?  In general, responses should mention that as heat was added, the water molecules moved around much faster and with more energy. When they were cooled, they began to cluster together, which can represent the formation of droplets. This is the process of condensation (the opposite of evaporation).

3. Where else have you seen condensation?  Responses will vary. Common examples are dew forming on grass in early morning, glasses fogging up when you enter a warm building on a cold winter day, a car windshield fogging up on a cold day.  Clouds are a large-scale example of condensation. They form when water vapor in warm air rises to meet cold air higher in the atmosphere.

4. When you walk through fog, what does it feel like?  Responses will vary, depending on whether students have had much experience with foggy weather. In dry climates, students will have difficulty answering the question. In Chicago, on the other hand, fog often rolls in from Lake Michigan and on a cold day, the fog makes the air feel colder. 

Further Investigation

Water Moving Around the Earth

Task: Study the model again and draw your own interpretation of the water cycle. Include all the forms of water. Draw arrows to show how the water moves, then take a snapshot of your drawing.  Teachers: The important takeaway here is for students to realize that a cycle is a repeating pattern, and to create drawings that reflect this understanding. Ideally, their drawings will include ocean, lake, groundwater, surface runoff, snow on mountains, precipitation as rainfall, and evaporation. Since this is the first model they work with, their representation of clouds and condensation will likely be rudimentary or incomplete. This is not problematic since the 3rd activity deals with condensation. 

Water Into the Air

Task: Gently wave temperature sensor around in the air and note air temperature. Dip the sensor into water at room temperature. Wave it around again and note temperature of the sensor.

 Question: What was the temperature of the wet sensor compared to the dry sensor when you waved them around in the air?  Answer: It depends on the temperature of your environment. For indoor air-conditioned spaces, the temperate will range from ~ 19.5-26 degrees Celsius (68-80 degrees Fahrenheit). For this experiment, the temperature of the dry sensor was 19.5 C (about 68 degrees Fahrenheit). See graph above for data. When the dry sensor was waved around, its temperature went from 19.5 to 20.5 Celsius. When the sensor was dipped in water, the temperature immediately went up about 3 degrees Celsius, but began to go back down when the wet sensor was waved in the air, which caused evaporation. After about 60 seconds, the temperature sensor read 22 degrees Celsius. Eventually, the wet sensor will cool down (due to evaporation) back to the same temperature as the dry sensor being waved in the air (or about 20.5 C). This could take some time. Results could vary because it may be difficult to get the water at exactly room temperature. 

Water From the Air:  Task -- conduct an experiment to make a plastic "cloud in a bottle".  

Question: Were you able to make a cloud in a bottle?  The answer will depend on whether you could create enough particles of smoke from the match used to create smoke in the bottle. One key thing to remember: cover the bottle very quickly with plastic wrap after dropping the match in. Otherwise, the smoke will dissipate.