Water Cycle Teacher Guide: Middle School

Activity Names

Thinking About Discovery Questions

This lesson module will address the following essential questions:

1. How humid is the air around us and how could you measure water vapor in the air?

2. Where does humidity come from and what environmental factors affect humidity levels?

3. What would happen to the water cycle if this "invisible" water did not exist?




The water cycle is of such profound importance to life on Earth that students should have meaningful experiences to promote understanding of precipitation, evaporation, transportation of water around the Earth, and conservation of matter. Recognizing how water becomes a gas can be a difficult leap for students at this age. Young students often believe that when water evaporates, it ceases to exist, a belief that can persist into middle school years. This misconception is specifically addressed by Activity 1 and Activity 3. A second area of persistent misunderstanding involves the process of condensation in which water vapor changes into liquid water. In the middle grades, students may require significant support to grasp the physical process involved in condensation. Activities 2 and 3 help students form correct ideas about condensation. The following misconceptions pertaining to the water cycle, published by the American Association for the Advancement of Science, were derived from large-scale questionnaires of students in Grades 6-8:

As a unit, the three activities in this module address each of the misconceptions above.

Learning Objectives

Background Information

The information below was published by the U.S. Geological Survey on its web tutorial, The Water Cycle --  http://water.usgs.gov/edu/watercycleevaporation.html

Evaporation is the process by which water changes from a liquid to a gas or vapor. Evaporation is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor. Oceans, seas, lakes, and rivers provide nearly 90 percent of the moisture in the atmosphere via evaporation, with the remaining 10 percent being contributed by plant transpiration. Heat (energy) is necessary for evaporation to occur. Energy is used to break the bonds that hold water molecules together, which is why water easily evaporates at the boiling point (212° F, 100° C) but evaporates much more slowly at the freezing point. Net evaporation occurs when the rate of evaporation exceeds the rate of condensation. A state of saturation exists when these two process rates are equal, at which point the relative humidity of the air is 100 percent. Condensation, the opposite of evaporation, occurs when saturated air is cooled below the dew point (the temperature to which air must be cooled at a constant pressure for it to become fully saturated with water), such as on the outside of a glass of ice water. In fact, the process of evaporation removes heat from the environment, which is why water evaporating from your skin cools you. Evaporation from the oceans is the primary mechanism supporting the surface-to-atmosphere portion of the water cycle. After all, the large surface area of the oceans (over 70 percent of the Earth's surface is covered by the oceans) provides the opportunity for large-scale evaporation to occur. On a global scale, the amount of water evaporating is about the same as the amount of water delivered to the Earth as precipitation. This does vary geographically, though. Evaporation is more prevalent over the oceans than precipitation, while over the land, precipitation routinely exceeds evaporation. Most of the water that evaporates from the oceans falls back into the oceans as precipitation. Only about 10 percent of the water evaporated from the oceans is transported over land and falls as precipitation.

Heat is removed from the environment during evaporation, leading to a net cooling; notice how cold your arm gets when a physician rubs it with alcohol before pulling out a syringe with that scary-looking needle attached. In climates where the humidity is low and the temperatures are hot, an evaporator cooler, such as a "swamp cooler" can lower the air temperature by 20 degrees F., while it increases humidity. As this map shows, evaporative coolers work best in the dry areas of the United States (red areas marked A) and can work somewhat in the blue areas marked B. In the humid eastern U.S., normal air conditioners must be used.

Materials Needed

Suggested Timeline: Four Class Periods

Day One: Perform Wet Bulb/Dry Bulb experiment to determine how much water vapor is in the surrounding air. Allow one full class period to collect data and store graphs to digital album.

Day Two: Perform experiment with the 2-liter soda bottle and a relative humidity sensor to determine where humidity comes from and whether it can be controlled by changing environmental factors.

Day Three: Perform experiments with both relative humidity sensor and temperature sensore to measure the amount of water air can hold at different temperatures (the maximum water density). This activity may take more than one class period because students will do calculations in addition to their data collection.

Day Four:  Finish any experiments not completed, arrange digital albums for submitting to teacher, and work in groups to interpret different sets of data.

Discussion: Setting the Stage

Students should have some prior exposure to the Water Cycle and its importance in many Earth processes. Before introducing this module, you may want to review the basics of the Water Cycle, especially the concepts of evaporation and condensation. The poster (image above) is a good jumping-off point to review how water circulates through the crust, oceans, and atmosphere. In elementary school, the NGSS calls for students to understand precipitation, runoff, and the basic mechanism of evaporation. By middle school, students will be expected to greatly expand and refine their understanding of the Water Cycle. They will be asked to master the following things by the end of Grade 8:  1) Understanding how water continually cycles among Earth systems,including evaporation, condensation, transpiration, and crystallization (MS-ESS2.C.1), 2) How movement of water is a major determinant of local weather patterns (MS-ESS2.C.2), 3) The way in which water movement is driven by sunlight and gravity (MS-ESS2.C.3), 4) How differences in temperature and salinity cause density variations that drive global ocean currents (MS-ESS2.C.4), and 5) The major role played by Earth's oceans in redistributing water across throughout the Earth (MS-ESS2.D.3). This is a big change from prior standards requirements, and will bring about a shift in how we teach middle school Earth Science.  

Teasing Out Prior Knowledge -- Here are some example questions to elicit what your students know:

Explain that students will do three investigations with sensor devices that are connected to digital graphing tools. Their experiments will help them answer all the questions above. In Activity 1, they will use an old-school method of measuring relative humidity called the Wet Bulb/Dry Bulb Experiment. In Activity 2, they will do an experiment to figure out how environmental factors contribute to raising or lowering the humidity level. In Activity 3, they will work on teams to measure the water vapor in their classroom.

http://itsi.portal.concord.org/system/images/1548/attributed/tempsensor-vernier.jpg?1436645077          Relhumiditysensor

Left:   Temp sensor ("dry bulb")     Center:  Temp sensor in wet gauze ("wet bulb")     Right:  Relative Humidity sensor

Before starting, show students the sensors they will work with. The temperature sensor is very sensitive -- DON'T touch the end while running your experiment or you will skew the results! The classroom temperature sensor kits are quite accurate. Classroom relative humidity sensors with USB connectors are easy to use, but not as accurate as commercial models. 

NSES Standards:

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.

NSES Earth and Space Science – Structure of the Earth System
The atmosphere is a mixture of nitrogen, oxygen, and trace gases that include water vapor. The atmosphere has different properties at different elevations.

Formative Questions

Activity 1: Relative Humidity Measurement

Students will first use a digital graphing tool to predict the graph of Temperature Vs. Time for a dry temperature sensor being waved around in the air, then repeat the prediction for a wet sensor. Depending on the browser, students may need to create two different graphs to save their data. Any prediction should be validated, as long as students provide an explanation when annotating their Snapshot Albums.

Collect Data I:  Measure the air temperature with a dry sensor, waving the sensor back and forth to be sure it reaches room temperature. Next, dip the sensor in warm tap water, then wave it back & forth in the air. What happened to the temperature? Responses should state that the temperature changed only a little when you wave the dry sensor around in the air, but changed a lot when you wet the sensor with warm or tepid water and THEN waved it around. Why? The cooling effects of evaporation! (See graphs below)

Ms-drybulb-airwave        http://itsi.portal.concord.org/system/images/1551/attributed/ms-drybulb-waterdipped.jpg?1436647960


 Collect Data II:  First, measure the air temperature with the dry temperature sensor (do 3 trials for at least 60 seconds). Wrap gauze around the dry temperature sensor, then dip the device in room-temperature water. Record the temperature to get a baseline, then shake the sensor vigorously. Repeat the experiment at least 3 times. What is the lowest "wet bulb" temperature you produced? What is the lowest "dry bulb" temperature you produced?  (See graphs below. Responses will vary, but should generally reveal that the "dry bulb" produces readings with little fluctuation, while the "wet bulb" will have a significant temperature drop when you wave it around to create evaporation.)  Student takeaway -- evaporation causes cooling.)  ***NOTE: Astute students will recognize that when you wrap the temperature sensor, the gauze acts like a coat and raises the temperature produced by the sensor. You see this in the difference in baseline temperature readings below. 

Ms-drybulb-airwave        Ms-wetbulb-waterdipped

Task:  Use the chart in the Student Guide to determine the relative humidity.  What is the relative humidity of the room, based on the readings from Data Collection II?

Responses: In the case above, the experiment was done on a rainy day with no air conditioning running in the room. Students would round the mean temperature from the dry sensor to ~21.5. They must wait until the "wet sensor" temperature settles to a steady level. In the graph above, this produces a "wet bulb" mean temperature of about 18.5. The difference between the two is 3 degrees Celsius. Use the table and find the values. For the data above, this produces a relative humidity of 75%. In this case, that was probably close because it was a rainy day with outside temperatures of about 23 degrees Celsius. Due to the mild temperature, the air conditioning in the building was not running. This would account for the high humidity level in the building. It would be optimal to take readings on days when air conditioning or heat is running inside the building (which should lower humidity levels). 

Real-Life Connection:  When humidity and heat get high, people and animals can become stressed by the heat. The reason meteorologists talk about relative humidity is because it affects our comfort level both indoors and outdoors. (See chart below)


Data Collection III:  Move to a different room or (preferably) outside and repeat the data collection from Part II. See Graphs below. On this date, the outdoor temperature was a pleasant 78.5 degrees Fahrenheit, with sunny skies. The mean "dry bulb" temperature was 26 degrees Celsius. After wetting the gauze-wrapped sensor, the "wet bulb" temperature settled at a mean of ~19.65.  This gives a difference of about 6.35, which we rounded to 6.5.  Based on the chart in the Student Guide, the relative humidity outside was approximately 54%.  See Below

Outdoors-drybulb2        Outdoors-wetbulb2


1. Did you get the same temperature values in each location each time?  Responses will vary, but should show evidence that students understand the readings should be fairly close, though will be subject to fluctuation.   How do you account for the different relative humidities in different locations?  Correct responses will include: "It's more humid outside where there is no air conditioning",  "It's less humid on a bright, sunny day," "There's more humidity on a hot, cloudy day than a sunny day", "There's lots more humidity on a rainy day", "There's less humidity above ground than in a basement room," "It's more humid inside when you don't run the air conditioner, "Air conditioning takes humidity out of the air". 

2. In your own words, explain why the evaporation of water causes cooling.  After doing the experiments, students should have a better understanding that evaporation is removing heat from one object and transferring it somewhere else (in this case, to the surrounding air). However, students often erroneously think that the rapid air flow from a fan or waving a sensor in the air is transferring cool air to the hot object. It's the other way around!  Energy is spontaneously transferred out of hotter regions or objects into cooler ones (NGSS Disciplinary Core Idea MS-PS3.B.3). If students seem confused, take a moment to clarify this key idea.

3. Why do you think it is important to wave the wet bulb sensor back and forth?  Responses will vary. The accurate answer is that the rapid movement speeds up the rate of evaporation, which increases heat loss from the object (in this case a sensor). The same principle applies with clothes hanging on a clothesline. A gust of wind will increase their rate of evaporation and help them dry more quickly. 

4. Why do you think it is not correct to blow on the wet bulb sensor to cause evaporation?  Answer: Because the air we exhale is highly humidified by biological processes that occur in our lungs. It is warmer than the surrounding air temperature. Blowing on the sensor will produce inaccurate results.

5. Why do you think the cooling effect of evaporation is greater for dry air than for humid air?  Answer: On dry days, evaporation occurs more quickly because the air carries away the heat faster. You can readily see this in the way sweat evaporates quickly off your body on a dry day, but much more slowly on a humid day. On humid days, the air is closer to its saturation level, so sweat evaporates more slowly. This is why we feel much hotter on a high-humidity day -- the body's cooling system just doesn't work efficiently. 

Relative Humidity in Micro-Environments Activity

Teachers: Before students collect data, check to be sure each of their relative humidity testers have a secure clay seal so ambient air will not seep into their experimental humidity chamber. See below.

Collect Data I:  Measure the relative humidity of the air using a relative humidity sensor. Select three different surfaces and use the relative humidity chamber you constructed to test the moisture put into the air by the 3 surfaces you chose.  Did your results match your predictions?  Responses will be varied, but allow students to choose their own surfaces. Some of the students are likely to choose a wet surface, some may choose extremely porous surfaces, some may choose polished surfaces. After this data collection, share graphs with all students.  Below are examples of graphs created from different types of surfaces.

Rh-baselinerh        Rh-wetwashcloth2

Rh-densecarpet     Rh-terracottaplate     Rh-woodslab

Collect Data II:  Repeat Task for Data I. Choose three different surfaces and measure relative humidity with your sensor/humidity chamber device. Teachers: Expect students to cash in on the ideas of peers. This is a great way to promote teamwork and compare results later. Student Takeaways from this activity:  Students will be surprised to learn that materials like wood and clay have moisture in them. They should begin to realize that moisture on the surface of objects can be transmitted as humidity to the air. This is a very important connection for understanding evaporation and the water cycle. 

Collect Data III:  Pick one of your materials and do something to it that you think will increase the relative humidity above it. Test it before and after the change. Did it work? Why or why not?  Responses will be quite varied. Students might try putting water on a surface or even rapidly breathing onto a porous surface, then quickly testing the humidity. What would happen if they put moisturizing lotion or oil on the surface? Ask students to really use their imagination!

Next, pick another material and do something to it that you think will decrease the relative humidity above it. Test it before and after the change. Did it work?  Why or why not?  Responses will be varied. You might bring a hair dryer for students to use in drying a wet object. You could also bring talcum powder or corn starch to see if its absorptive property will reduce humidity of an object. Students can brainstorm other ways to decrease humidity, such as putting fingernail polish on an object. What if they coated the surface with a layer of clay?

Analysis: Relative Humidity in Micro-Environments

1. Did your measurements match your predictions or were there surprises in your results?  Students will probably be surprised that things like a block of wood might have moisture in them. They might be very surprised at the amount the relative humidity rose when measuring a wet cloth. 

2. What are some environmental factors which determine relative humidity inside a building?  Students should recognize that certain surfaces promote increases in humidity, while others help control humidity. For example, a non-porous and polished surface (tile) will help maintain humidity levels. Wood has a porosity that will allow more moisture than ceramic tile, but it won't be a big difference unless the wood is wet. Any spots where water can leak cause a room to become much more humid. Carpet in itself does not raise humidity levels, but if students track in rainwater, the carpet will absorb it and release it into the air as humidity. Astute students may understand that humidity in a building will rise when lots of people are inside because the breath we exhale is very high in humidity. So when the dismissal bell rings....humidity levels will go down!

3. What are some environmental factors which determine relative humidity outdoors?  After doing this experiment, students should understand that objects on Earth's surface hold moisture that can be released into the atmosphere as water vapor. Different objects, depending on their properties, can hold different amounts of moisture. In the next experiment, they will look more closely at how much water vapor the air can hold before it reaches saturation. 

4. Picture a natural environment. Based on your observations, where would the relative humidity be high and where would it be low?  There will be numerous acceptable responses. Expect students to understand that rainfall causes ground surfaces to act like the wet washcloth. They absorb moisture, then release it back into the air as water vapor. Some environments have surfaces that promote excessive water retention, like swamps or bogs. Some will allow it to be more quickly absorbed, like sandy soil. Environments with lots of plant growth will have higher humidity due to plant transpiration. Obviously, relative humidity will be higher in places where it rains more. But the surface features also play a big role in humidity level. 

5. Think of some examples in which organisms have adapted to and made use of the differences in micro-environments in nature.  Wide variety of responses will be acceptable.  

Water in Classroom Air Activity

Collect Data I:  Measure the air temperature in your classroom, then use the graph to figure out the maximum amount of water that one cubic meter of your classroom air can hold (maximum water density). This would be the water density if the relative humidity were 100%.  In the graph below, you can see that the temperature reading was 22.4 degrees Celsius. Using the Water Density Calculator in the student activity, this corresponds to a Maximum Water Density of 20 grams per cubic meter. Obtaining a correct answer is a matter of using the Water Density Table accurately (which shouldn't be difficult for students).

Waterinair-temperature          Waterinair-relhumidity

Collect Data II: Now measure the relative humidity of your classroom air, using the relative humidity sensor. Record the relative humidity and calculate how much water is in a cubic meter of air in your classroom.  Teachers: This task will be more complex. In the example above, the room temperature was 22.4 degrees Celsius. The relative humidity was measured at 60%. The maximum water density for this temperature is 20 grams per cubic meter (as obtained from the chart in the student activity). To determine how much water is actually in a cubic meter of air in this room at this point in time, you must do the following:

IF:  the saturation level for air at this temperature is 20 grams per cubic meter, and the relative humidity is 60%         

THEN:  Amount of water in 1 cubic meter of air  =  60% of 20 grams/meter cubed     

SOLUTION:  20 x .6  =  12 grams of water per cubic meter

Try to let students determine the solution for themselves without giving them a formula. For further testing, you may want to use an automated Relative Humidity Calculator. A very good one can be found at the Hyperphysics website:  http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/relhum.html#c4

Collect Data III:  Figure out a way to calculate the approximate volume of air in your entire classroom.   Answer:  Students need to measure the three-dimensional area of the room in meters. They will use the formula for finding area of a rectangular prism: L x W x H. They should use the metric side of the measuring tape. This will give them cubic meters. Example:  A room is 3 meters high by 7.5 meters long by 9.5 meters wide.   3 x 7.5 x 9.5 =  213.75 cubic meters.  This is the approximate volume of air in the classroom.  

Next, figure out the amount of water in your classroom's air.  Answer:  Using the example in Data II, the calculated water density in the air was 12 grams of water per cubic meter. Students should be able to readily figure out that you multiply the total cubic area of the classroom by the water density:  213.75 x 12  =  2,565 grams of water.  To convert to grams, remember that 1 gallon  = ~3,800 grams of water. Divide 2565 by 3800 (the value given in the student activity), which gives you 0.675.  So, on this particular day, there was a volume of slightly more than 2/3 gallon of water in the classroom air.  NOTE**  The actual conversion for grams-to-gallon is calibrated at 1 gallon  =  3,785.41 grams. The student activity rounds it to 3,800.  

Analysis:  Water in Classroom Air

1. How many liters of water are in your classroom air?  Answer: There are two ways to figure this out. One gallon = ~3.80 liters (as rounded in student activity guide). One gram = .001 liters.  Encourage students to use resources to find the conversion themselves. In the example above, there are 2,565 grams of water in the air. To convert grams to liters, multiply 2,565 x .001 to get the answer (2.565 liters of water in the air). To convert gallons to liters:  .675 gallons x 3.80 = 2.565 liters  

2. Do you think the humidity is the same at night as it is during the day?  Why?  Responses can be varied. This could be a stumper. People have misconceptions that high humidity means it is hot, sultry, and muggy. However, as temperatures fall (which happens at night) the relative humidity usually rises because air at lower temperatures holds less water vapor. This can be seen in the Water Density chart in this activity. "Relative humidity" is the RATIO of water vapor in the air compared to the water vapor the air can hold at saturation. So on a cool night, it's not unusual to have a relative humidity between 90 and 100%. As the sun warms the Earth, the air gets warmer and is able to hold more water vapor. Relative humidity would drop lower during the day. There is also a device to measure "absolute humidity", which measures the quantity of water vapor in the air, but it's rarely used.

3. What classroom activities might change the humidity in the room?  Acceptable responses could include:  "Lots of physical activity causes kids to breathe faster, and exhaled breath has higher humidity."  "Experiments with boiling liquids will raise the humidity in the room."  "Opening the window on a muggy day can bring moister air into the air conditioned room."  

4. Whenever the humidity is 80% or more, mold will grow on a surface. Think of places in buildings where humidity might be high enough for long enough for mold to grow. Make a list of places you have seem mold growing. Was the humidity high? What caused the humidity to be so high?  Acceptable responses could be:  "Bathrooms are high-humidity areas because of hot water and steam introduced into the closed-off room." "Basements can have high humidity because rainwater flows right next to the walls and can seep in cracks in the foundation."  "Places with a leaky roof are more humid."  "Laundry rooms can be very humid because of the steam from hot water and from wet clothes drying."  "Kitchens are high-humidity places when anything is being boiled or cooked in a pan."  "Mold could grow around a dishwasher, shower stall, bathtub, drains, or basement corners because these areas are constantly exposed to moisture."  

5. Is the humidity outdoors the same as the humidity indoors?  Expect students to struggle some with this question. If they are comparing air conditioned rooms to outdoors, expect them to respond that air conditioning removes humidity from the air. (They would not be expected to know the mechanism by which air conditioners accomplish this.) On a cool night, on the other hand, the relative humidity outdoors may be lower than indoors (see #2 above).  

Further Investigation

Activity I: Relative Humidity Measurement

Task: Find several places in your room where you think the relative humidity might be different. Use the wet-bulb/dry-bulb apparatus to measure the differences in relative humidity.  Appropriate responses will contain computer-generated graphs that show a steeper curve for evaporation off the wet-bulb apparatus in a dry vs. a humid room. For example, the relative humidity should measure greater in a basement room than in an above-ground room because basement retain humidity.

Activity II:  Relative Humidity in Micro-Environments

Task: Think of two micro-environments, one arid and one very humid. Create models of these two environments in a plastic container or aquarium in your classroom Test the local humidity with the RH meter.  Teachers: Ideas for the microcosm environments could include:  1) A boggy environment with peaty soil and swamp plants, 2) Arid environment with sand or very sandy soil and cacti, 3) A loamy soil with a tiny area for a contained pond and rainforest plants, 4) A combination sandy/rocky soil with plants suitable for dry climates. See below for examples:

Desertterrarium          Rainforestterrarium

Photo at Left:  Desert Terrarium          Photo at Right:  Rainforest Terrarium

Activity III:  Water in the Classroom

Task: Choose another room in your school and repeat the same experiment you did in your classroom. Compare the water content to your classroom's water content.  We suggest choosing a high-ceilinged room such as the gym, a room where you might expect high humidity (the food preparation area), the principal's office (which is air conditioned and experiences less humidification from exhaled breath), and the entry hall where a front door is continuously opened to let in outdoor air.